METHOD AND A SYSTEM FOR DECOMPOSITION OF ACOUSTIC SIGNAL INTO SOUND OBJECTS, A SOUND OBJECT AND ITS USE

Abstract

A method and a system for decomposition of acoustic signal into sound objects having the form of signals with slowly-varying amplitude and frequency, as well as sound objects and their use. The object is achieved by a method for decomposing an acoustic signal into digital sound objects, a digital sound object representing a component of the acoustic signal, the component having a waveform, comprising the steps of converting the analogue acoustic signal into a digital input signal (PIN); determining an instantaneous frequency component of the digital input signal, using a digital filter bank; determining an instantaneous amplitude of the instantaneous frequency component; determining an instantaneous phase of the digital input signal associated with the instantaneous frequency; creating at least one digital sound object, based on the determined instantaneous frequency, phase and amplitude; and storing the digital sound object in a sound object database.

Claims

1. A method for decomposing an acoustic signal into digital sound objects, a digital sound object representing a component of the acoustic signal, the component having a waveform, the method comprising: converting the analogue acoustic signal into a digital input signal (PIN), wherein the digital signal comprises samples of the acoustic signal; determining, for each sample, an instantaneous frequency component of the digital input signal, using a digital filter bank comprising digital filters(n); determining, for each sample, an instantaneous amplitude of the instantaneous frequency component; determining, for each sample, an instantaneous phase of the digital input signal associated with the instantaneous frequency; creating at least one digital sound object, wherein the digital sound object includes the determined instantaneous frequency, phase and amplitude; and storing the digital sound object in a sound object database, characterized in that, for each sample, for each filter(n), locations of frequencies present in the acoustic signal are determined based on an intersection of a value of an angular frequency at the output of each filter(n) and its nominal angular frequency.

2. The method of claim 1, wherein a digital filter in the digital filter bank has a window length proportional to its central frequency.

3. The method of claim 2, wherein central frequencies of the filter bank are distributed according to a logarithmic scale.

4. The method of claim 1, characterized in that an operation improving the frequency-domain resolution of said filtered signal is executed sample by sample.

5. The method of claim 1, wherein the step of determining an instantaneous frequency component takes into account one or more instantaneous frequency components determined using adjacent digital filters of the digital filter bank.

6. The method of claim 1, wherein the instantaneous frequency is tracked over subsequent samples of the digital input signal.

7. The method of claim 6, characterized in that values of the envelope of amplitude and values of frequency and their corresponding time instants are determined in order to create characteristic points with coordinates in time-frequency-amplitude space describing the waveform of said sound object.

8. The method of claim 7, characterized in that the values are determined not less frequently than once per period of duration of a given filter's window W(n).

9. The method of claim 6, further comprising the step of correcting an amplitude and/or frequency of selected sound objects as to reduce an expected distortion in said sound objects, the distortion being introduced by said digital filter bank.

10. The method of claim 3, characterized in that improving the frequency-domain resolution of said filtered signal further comprises a step of increasing the window length of selected filters.

11. The method of claim 4, characterized in that the operation of improving the frequency-domain resolution of said filtered signal further comprises the step of subtracting an expected spectrum of located adjacent sound objects from the spectrum at the output of the filters.

12. The method of claim 4, characterized in that the operation of improving the frequency-domain resolution of said filtered signal further comprises a step of subtracting an audio signal generated based on located adjacent sound objects from said input signal.

13. A digital sound object, the digital sound object comprising at least one parameter set representing a waveform of at least one component of an acoustic signal, generated by a method according to claim 1.

14. Non-volatile, non-transient computer-readable medium, storing a sound object generated according to claim 1.

15. A method for generating an audio signal, comprising the steps of: receiving a digital sound object according to claim 13; decoding the digital sound object in order to extract at least one parameter set describing a waveform of at least one component of the audio signal; generating the waveform from the parameter set; synthesizing the audio signal, based on the generated waveform; and outputting the audio signal.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0037] The invention has been depicted in an embodiment with reference to the drawings, wherein:

[0038] FIG. 1 is a block diagram of a system for decomposition of audio signal into sound objects,

[0039] FIG. 2a is a parallel structure of a filter bank according to the first embodiment of the invention,

[0040] FIG. 2b is a tree structure of the filter bank according to the second embodiment of the invention, FIG. 2c shows the tone spectrum of a piano, FIG. 2d shows an example of a filter structure using 48 filters/octave, i.e. four filters for each semitone,

[0041] FIG. 3 shows a general principle of operation of a passive filter bank system,

[0042] FIG. 4 shows exemplary parameters of filters,

[0043] FIG. 5 is the impulse response of a filter F(n) having the Blackman window,

[0044] FIG. 6 is a flowchart of a single filter,

[0045] FIGS. 7a and 7c show a part of a spectrum of the filter bank output signal, comprising the real component FC(n), the imaginary component FS(n) and the resulting amplitude of the spectrum FA(n) and the phase FF(n)

[0046] FIGS. 7b and 7d show the nominal angular frequency F#(n) of a corresponding filter group and angular frequency of the spectrum FQ(n).

[0047] FIG. 8 is a block diagram of a system for tracking sound objects, FIG. 8a shows a relationship between four individual frequency components and their sum, FIG. 8b shows another example of a signal having four different frequency components (tones),

[0048] FIGS. 9a and 9b show exemplary results of operation of a voting system, FIG. 9c shows instantaneous values calculated and analyzed by the spectrum analyzing a system 31 according to an embodiment of the invention,

[0049] FIG. 10 is a flowchart of a sound system for associating objects, FIG. 10a is an illustration of the element detection and object creation process according to an embodiment of the invention, FIG. 10b illustrates the application of a matching function according to an embodiment of the invention,

[0050] FIG. 11 shows the operation of a frequency resolution improvement system according to an embodiment,

[0051] FIG. 12 shows the operation of a frequency resolution improvement system according to another embodiment, FIG. 12/2a shows a spectrum of the signal according to FIG. 7c, FIG. 12/2b shows the determined parameters of the well localized objects 284 and 312, FIG. 12/2c shows the spectrum of well localized objects, FIG. 12/2d shows the difference between the signal spectrum and the calculated spectrum of well localized objects, FIG. 12/2e shows the determined parameters of the objects 276 and 304 located in the spectrum of differential,

[0052] FIG. 13 shows the operation of a frequency resolution improvement system according to yet another embodiment,

[0053] FIGS. 14a, 14b, 14c, 14d show examples of representation of sound objects, FIG. 14e shows an example of a multi-level description of an audio signal according to an embodiment of the invention,

[0054] FIG. 15 shows an exemplary format of notation of information about sound objects, FIG. 15a shows an audio signal composed of two frequencies (dashed lines) and a signal obtained from the decomposition, without correction,

[0055] FIG. 16 shows a first example of a sound object requiring correction,

[0056] FIG. 17 shows a second example of a sound object requiring correction,

[0057] FIGS. 18a to 18c show further examples of sound objects requiring correction; FIG. 18d shows an audio signal composed of two frequencies (dashed line) and a signal obtained from the decomposition, with enabled correction system,

[0058] FIGS. 19a, 19b, 19c, 19d, 19e, 19f, 19g, 19h show the process of extracting sound objects from an audio signal and synthesis of an audio signal from sound objects.

DETAILED DESCRIPTION OF EMBODIMENTS

[0059] In the present patent application the term connected, in the context of a connection between any two systems, should be understood in the broadest possible sense as any possible single or multipath, as well as direct or indirect physical or operational connection.

[0060] A system 1 for decomposition of acoustic signal into sound objects according to the invention is shown schematically in FIG. 1. An audio signal in digital form is fed to its input. A digital form of said audio signal is obtained as a result of the application of typical and known A/D conversion techniques. The elements used to convert the acoustic signal from analogue to digital form have not been shown herein. The system 1 comprises a filter bank 2 with an output connected to a system for tracking objects 3, which is further connected with a correcting system 4. Between the system for tracking objects 3 and the filter bank there exists a feedback connection, used to control the parameters of the filter bank 2. Furthermore, the system for tracking objects 3 is connected to the input of the filter bank 2 via a differential system 5, which is an integral component of a frequency resolution improvement system 36 in FIG. 8.

[0061] In order to extract sound objects from an acoustic signal, a time-domain and frequency-domain signal analysis has been used. Said digital input signal is input to the filter bank 2 sample by sample. Preferably, said filters are SOI filters. It is shown in FIG. 2a a typical structure of the filter bank 2, in which individual filters 20 process in parallel the same signal with a given sampling rate. Typically, the sampling rate is at least two times higher than the highest expected audio signal's component, preferably 44.1 kHz. Since such a number of samples to be processed per 1 second requires large computational expense, preferably a filter bank tree structure of FIG. 2b can be used. In the filter bank tree structure 2 the filters 20 are grouped according to the input signal sampling rate. For example, the splitting in the tree structure can be done at first for the whole octaves. For individual sub-bands with lower frequencies it is possible to cut off high frequency components using a low-pass filter and to sample them with a smaller rate. As a consequence, due to reduction of the number of samples a significant increase in processing speed is achieved. Preferably, for the interval up to 300 Hz the signal is sampled with fp=600 Hz, up to 2.5 kHz with fp=5 kHz.

[0062] Since the main task of the method and the system according to the invention is to localize all sound objects in the spectrum, an important issue is possible accuracy of determination of signal's parameters and a resolution of simultaneously appearing sounds. The filter bank should provide a high frequency-domain resolution, i.e. greater than 2 filters per semitone, making it possible to separate two adjacent semitones. In the presented examples 4 filters per semitone are used.

[0063] Preferably, in the method and the system according to the invention a scale corresponding to human ear's parameters has been adopted, with logarithmic distribution, however a person skilled in the art will know that other distributions of filters' central frequencies are allowed within the scope of the invention. Preferably, a pattern for the distribution of filters' central frequencies is the musical scale, wherein the subsequent octaves begin with a tone 2 times higher than the previous octave. Each octave is divided into 12 semitones, i.e. the frequency of two adjacent semitones differs by 5.94% (e.g. e1=329.62 Hz, f1=349.20 Hz). To increase accuracy, there are four filters for each semitone in the method and the system according to the invention, wherein each filter listens to its own frequency, differing from an adjacent frequency by 1.45%. It has been assumed that the lowest audible frequency is C2=16.35 Hz. Preferably, the number of filters is greater than 300. A particular number of filters for a given embodiment depends on the sampling rate. With sampling at 22050 samples per second the highest frequency is e6=10548 Hz, 450 filters being in this range. With sampling at 44100 samples per second the highest frequency is e7=21096 Hz, 498 filters being in this range.

[0064] A general principle of operation of a passive filter bank is shown in FIG. 3. The input signal which is fed to each filter 20 of the filter bank 2 is transformed as a result of relevant mathematical operations from the time domain into the frequency domain. In practice, a response to an excitation signal appears at the output of each filter 20, and the signal's spectrum jointly appears at the filter bank's output.

[0065] FIG. 4 shows exemplary parameters of selected filters 20 in the filter bank 2. As can be seen in the table, central frequencies correspond to tones to which a particular music note symbol can be attributed. The window width of each filter 20 is given by the relation:

W(n)=K*fp/FN(n) (1) [0066] where: W(n)window width of a filter n [0067] fpsampling rate (e.g. 44100 Hz) [0068] FN(n)nominal (central) frequency of a filter n [0069] Kwindow width coefficient (e.g. 16)

[0070] Since a higher frequency-domain resolution is necessary in the lower range of the musical scale, therefore for this range of frequencies the filter windows will be the widest. Thanks to an introduction of coefficient K and a normalization to the filter nominal frequency FN there is provided an identical amplitude and phase characteristic for all the filters.

[0071] With regard to the implementation of said filter banka skilled person will know that one of possible ways of obtaining the coefficients of a SOI type band-pass filter is to determine the impulse response of the filter. An exemplary impulse response of a filter 20 according to the invention is shown in FIG. 5. An impulse response in FIG. 5 is the impulse response of a filter with a cosine window, which is defined by the relation:

y(i)(n)=cos((n)*i)*AB*cos(2i/W(n))+C*cos(5i/W(n)) (2)

[0072] where: (n)=2n*FN(n)/fp

[0073] W(n), FN(n), fpare defined above

TABLE-US-00001 Window type A B C Hann 0.5 0.5 0 (Hanning) Hamming 0.53836 0.46164 0 Blackman 0.42 0.5 0.08

[0074] The operations performed by each of the filters 20 have been shown in FIG. 6. The task of the filter bank 2 is to enable the determination of an audio signal's frequency spectrum in the range of frequencies from the lowest audible by human (e.g. C2=16.35 Hz) to 1/2 fpsampling rate (e.g. e7=21096 Hz at 44100 samples per second). Before each filter begins its operation, parameters of the filter 20 are initiated, the exemplary parameters being the coefficients of particular components of time window function. Then, the current sample P.sub.IN of the input signal, having only a real value, is fed to the input of the filter bank 2. Each filter 2, using a recursive algorithm, calculates a new value of components FC(n) and FS(n) based on the previous values of the real component FC(n) and the imaginary component FS(n), and calculates also values of the sample P.sub.IN input to the filter and the sample P.sub.OUT leaving the filter's window and which is stored in an internal shift register. Thanks to the use of a recursive algorithm the number of calculations for each of the filters is constant and does not depend on the filter's window length. The executed operations for a cosine window are defined by the formula:

[00001]$\begin{array}{cc}\mathrm{FC}\left(n\right)=\underset{i=-W\left(n\right)}{\overset{0}{.Math.}}.Math..Math.y_1*\cos \left(\left(n\right)*i\right)*\left(A-B*\cos \left(\frac{2.Math..Math..Math..Math.i}{W\left(n\right)}\right)\right)+C*\cos \left(\frac{4.Math..Math..Math..Math.i}{W\left(n\right)}\right)& \left(3\right)\\ \mathrm{FS}\left(n\right)=\underset{i=-W\left(n\right)}{\overset{0}{.Math.}}.Math..Math.y_1*\sin \left(\left(n\right)*i\right)*\left(A-B*\cos \left(\frac{2.Math..Math..Math..Math.i}{W\left(n\right)}\right)\right)+C*\cos \left(\frac{4.Math..Math..Math..Math.i}{W\left(n\right)}\right)& \left(4\right)\end{array}$

By using trigonometric equations relating to products of trigonometric functions for equations (3) and (4) one obtains a dependence of the components FC(n) and FS(n) on the values of these components for the previous sample of the audio signal and a value of the sample inputted to the filter P.sub.IN, and the one outputted from the filter P.sub.OUT, according to the equation shown in FIG. 6. In the case of each filter 20 the calculation of the equation for each subsequent sample requires 15 multiplications and 17 additions for Hann or Hamming type windows, or 25 multiplications and 24 additions for a Blackman window. The process of the filter 20 is finished when there are no more audio signal samples at the filter's input.

[0075] Values of the real component FC(n) and the imaginary component FS(n) of the sample obtained after each subsequent sample of the input signal are forwarded from each filter's 20 output to a system for tracking sound objects 3, and in particular to a spectrum analyzing system 31 comprised therein (as shown in FIG. 8). Because the spectrum of the filter bank 2 is calculated after each sample of the input signal, the spectrum analyzing system 31 except of the amplitude characteristic can utilize the phase characteristic at the filter bank's 2 output. In particular, in the method and the system according to the invention the change of phase of the current sample of the output signal in relation to the phase for the previous sample is used for precise separation of the frequencies present in the spectrum, what will be described further with reference to FIGS. 7a, 7b, 7c and 7d, and FIG. 8.

[0076] A spectrum analyzing system 31, being a component of the system for tracking objects 3 (as shown in FIG. 8) calculates individual components of the signal's spectrum at the filter bank output. To illustrate the operation of this system, an acoustic signal with the following components has been subjected to analysis:

TABLE-US-00002 Tone No. FN Note 276 880.0 Hz a2 288 1046 Hz c3 304 1318 Hz e3 324 1760 Hz a3

[0077] There are shown in FIGS. 7a and 7b plots of instantaneous values of quantities obtained at the output of selected group of filters 20 for said signal and values of quantities calculated and analyzed by the spectrum analyzing system 31. For filters with number n from 266 to 336 with a window having the window width coefficient K=16 there have been represented: the instantaneous value of the real component FC[n], the instantaneous value of the imaginary component FS[n], which are fed to the input of the spectrum analysis system 31, and the instantaneous value of the spectrum's amplitude FA[n] and the spectrum's phase FF[n], which are calculated by the spectrum analyzing system 31. As already mentioned, the spectrum analyzing system 31 collects all the possible information necessary to determine the actual frequency of the sound objects present at a given time instant in the signal, including the information about the angular frequency. The correct location of the tone of component frequencies has been shown in FIG. 7b, and it is at the intersection of the nominal angular frequency of the filters FQ[n] and the value of the angular frequency at the output of the filters FQ[n], calculated as a derivative of the phase of the spectrum at the output of a particular filter n. Thus, according to the invention, in order to detect a sound object, the spectrum analyzing system 31 analyses also the plot of angular frequency F#[n] and FQ[n]. In the case of a signal comprising components which are distant from each other, points which are determined as a result of analysis of the angular frequency correspond to locations of maxima of the amplitude in FIG. 7a.

[0078] Due to some typical phenomena in the signal processing domain basing only on maxima of amplitude of the spectrum is not effective. The presence of a given tone in the input signal affects the value of the amplitude spectrum at adjacent frequencies, leading in consequence to a severely distorted spectrum when the signal comprises two tones close to each other. To illustrate this phenomenon, and to illustrate the functionality of the spectrum analyzing system 31 according to the invention, a signal has been subjected also to the analysis, comprising sounds of frequencies:

TABLE-US-00003 Tone No. FN Note 276 880.0 Hz a2 284 987.8 Hz h2 304 1318 Hz e3 312 1480 Hz #f3
As shown in FIGS. 7c and 7d, in the case of a signal with closely located components, the correct location of a tone determined based on the analysis of angular frequency plots does not correspond to the maximum of amplitude in FIG. 7c. Thus, for such a case, thanks to various parameters analyzed by the spectrum analyzing system 31 it is possible to detect situations which are critical for decomposition of an acoustic signal. In consequence, it is possible to apply specific procedures leading to correct recognition of components, what will be described further with reference to FIG. 8 and FIG. 9a, and FIG. 9b.

[0079] The fundamental task of the system for tracking objects 3, a block diagram of which is shown in FIG. 8, is to detect at a given time instant all frequency components present in an input signal. As shown in FIG. 7b and FIG. 7d, the filters adjacent to the input tone have very similar angular frequencies, different from the nominal angular frequencies of those filters. This property is used by another subsystem of the system for tracking objects 3, namely the voting system 32. To prevent incorrect detection of frequency components, the values of the amplitude spectrum FA(n) and angular frequency at the output of filters FQ(n), calculated by the spectrum analyzing system 31, are forwarded to the voting system 32 for calculation of their weighted value and detection of its maxima in function of the filter's number(n). In this way, one obtains a voting system, which takes into account the frequency at the outputs of all the filters 20 adjacent to it in order to determine frequencies present in the input signal for a given frequency at the filter's 2 output. The operation of this system is shown in FIGS. 9a and 9b. FIG. 9a illustrates a relevant case shown in FIGS. 7a and 7b, while FIG. 9b illustrates a relevant case shown in FIGS. 7c and 7d. As it can be seen, the plot of the signal FG(n) (the weighted value calculated by the voting system 32) has distinct peaks in locations corresponding to tones of frequency components present in the input signal. In the case of an input signal comprising components distinctly separated from each other (as shown in FIG. 9a) these locations correspond to a maximum of amplitude of the spectrum FA(n). In the case of a signal comprising components situated too close to each other (as shown in FIG. 9b), without the voting system 32 tones reflected in maximum of amplitude of the spectrum would have been detected, which are located in places other than the mentioned peaks in the weighted signal FG(n).

[0080] In other words, said voting system performs an operation of calculating votes, namely an operation of collecting votes of each filter(n) on a specific nominal angular frequency which votes by outputting its angular frequency close to the one on which said vote is given. Said votes are shown as a curved line FQ[n]. An exemplary implementation of said voting system 32 could be a register into which certain calculated values are collected under specific cell. The consecutive number of filter, namely the number of a cell in the register under which a certain value should be collected would be determined based on specific angular frequency outputted by a specific filter, said outputted angular frequency being an index to the register. The person skilled in the art will know that the value of outputted angular frequency is rarely an integer thus said index should be determined based on certain assumption, for example that said value of instant angular frequency should be round up or round down. Next the value to be collected under a determined index can be for example a value equal to 1 multiplied by the amplitude outputted by said voting filter or a value equal to a difference between the outputted angular frequency and the closest nominal frequency multiplied by the amplitude outputted by said voting filter. Such values can be collected in a consecutive cell of the register by addition or subtraction or multiplication or by any other mathematical operation reflecting the number of voting filters. In this way the voting system 31 calculates a weighted value for a specific nominal frequency based on parameters acquired from the spectrum analyzing system. This operation of calculating votes takes into account three sets of input values, the first one being values of nominal angular frequencies of filters, the second one being values of instant angular frequencies of filters, third ones being values of the amplitude spectrum FA(n) for each filter

[0081] As is shown in FIG. 8, the spectrum analyzing system 31 and the voting system 32 are connected at their output with a system for associating objects 33. Having at its disposal the list of frequencies detected by the voting system 32 which composes the input signal, and additional parameters, such as amplitude, phase and angular frequency associated to each detected frequency, the system for associating objects 33 combines these parameters in elements and next builds sound objects out of them. Preferably, in the system and the method according to the invention, the frequencies (angular frequencies) detected by the voting system 32, and thus elements, are identified by the filter number n. The system for associating objects 33 is connected to an active objects database 34. The active objects database 34 comprises objects arranged in order depending on the frequency value, wherein the objects have not yet been terminated. The term a terminated object is to be understood as an object such that at a given time instant no element detected by the spectrum analyzing system 31 and the voting system 32 can be associated with it. The operation of the system for associating objects 33 has been shown in FIG. 10. Subsequent elements of the input signal detected by the voting system 32 are associated with selected active objects in the database 34. To limit the number of required operations, preferably, detected objects of a given frequency are compared only with the corresponding active objects located in a predefined frequency range. At first, the comparison takes into account the angular frequency of an element and an active object. If there is no object sufficiently close to said element (e.g. in the range of distances in frequency corresponding to 0.2 tone) this means that a new object has appeared and it should be added to the active objects 34. If, once associating objects with current elements has been finished, there is no element sufficiently close for an active sound object (e.g. in the range of distances in frequency corresponding to 0.2 tone) this means that no further parameters for the object are detected and it should be terminated. Said terminated object is taken into account in the association process still for 1 period of its frequency to avoid an accidental termination caused by a temporary interference. During this time it can return to active sound objects in the database 34. After 1 period the object's final point is determined. If the object lasted for a sufficiently long time (e.g. its length was not shorter than the width of the corresponding window W[n]), then this object is transferred to a sound objects database 35.

[0082] In the case of associating with each other an active object and an object sufficiently close to, a matching function is further calculated in the system for associating objects 33, which comprises the following weighted values: amplitude matching, phase matching, objects duration time. Such a functionality of the system for associating objects 33 according to the invention is of essential importance in the situation when in a real input signal a component signal from one and the same source has changed frequency. This is because it happens that as a result of frequency changing a number of active objects become closer to each other. Therefore, after calculating the matching function the system for associating objects 33 checks if at a given time instant there is a second object sufficiently close to in the database 34. The system 33 decides which object will be a continuer of the objects which join together. The selection is decided by the result of the matching function comparison. The best matched active object will be continued, and an instruction to terminate will be issued for the remaining ones. Also a resolution improvement system 36 cooperates with the active objects database 34. It tracks the mutual frequency-domain distance of the objects present in the signal. If too close frequencies of active objects are detected the resolution improvement system 36 sends a control signal to start one of the three processes improving the frequency-domain resolution. As mentioned previously, in the case of presence of a few frequencies close to each other, their spectrum overlap. To distinguish them the system has to listen intently to the sound. It can achieve this by elongating the window in which the filter samples the signal. In this situation a window adjustment signal 301 is activated, informing the filter bank 2 that in the given range the windows should be elongated. Due to the window elongation the signal dynamics analysis is impeded, therefore if no close objects are detected the resolution improvement system 36 enforces a next shortening of the filter's 20 window.

[0083] In the solution according to the invention a window with length of 12 to 24 periods of nominal frequency of the filter 20 is assumed. The relation of the frequency-domain resolution with the window's width is shown in FIG. 11. The table below illustrates the ability of the system to detect and track at least 4 non-damaged objects subsequently present next to each other, with the minimal distance expressed in percentage, as a function of the window's width.

TABLE-US-00004 Detects objects Tracks objects Window width in the in the (in periods) distance of distance of 12 17.4% 23.2% 16 14.5% 17.4% 20 8.7% 14.5% 24 5.9% 11.6%

[0084] In another embodiment the system listens intently to a sound by modifying the filter bank's spectrum, what is schematically illustrated in FIG. 12. The frequency-domain resolution is improved by subtracting from a spectrum at the tracking system's 3 input the expected spectrum of well localized objects, which are localized in vicinity of new appearing objects. Well localized objects are considered as objects the amplitude of which does not vary too quickly (no more than one extreme per window's width) and the frequency of which does not drift too quickly (no more than 10% variation of frequency per window's width). An attempt to subtract a spectrum of objects varying quicker can lead to the phase inversion at the measurement system input and to a positive feedback resulting in generation of an interfering signal. In practice the resolution improvement system 36 calculates the expected spectrum 303 based on the known instantaneous frequency, amplitude and phase of an object by the following formula:

FS(n)=FA(n)*exp(=(xFX(n)/2/22(W(n)))*sin(FD(n)*(xFX(n))+FF(n))

FC(n)=FA(n)*exp((xFX(n))2/22(W(n)))*cos(FD(n)*(xFX(n))+FF(n))

where It is a function of the width of the window when width of the window=20 then 2=10, i.e. based on the known instantaneous frequency and subtracts them from the real spectrum, causing that the spectrum of adjacent elements will not be interfered so strongly. The spectrum analyzing system 31 and the voting system 32 perceive only adjacent elements and a variation of the subtracted object. However, the system for associating objects 33 further takes into account the subtracted parameters while comparing the detected elements with the active objects database 34. Unfortunately, to implement this frequency-domain resolution improvement method a very large number of computations is required and a risk of positive feedback exists.

[0085] In a yet another embodiment, the frequency-domain resolution can be improved by subtracting from the input signal an audio signal generated based on well localized (like in the previous embodiment) adjacent objects. Such operation is shown schematically in FIG. 13. In practice, this relies on the fact that the resolution improvement system 36 generates an audio signal 302 based on information about frequency, amplitude and phase of the active objects 34, which is forwarded to a differential system 5 at the filter bank's 2 input, as shown schematically in FIG. 13. The number of required calculations in an operation of this type is smaller than in the case of the embodiment in FIG. 12, however due to an additional delay introduced by the filter bank 2 the risk of system's instability and unintended generation increases. Similarly, also in this case the system for associating objects 33 takes into account the parameters of the subtracted active objects. Due to mechanisms which have been described the method and the system according to the invention provide the frequency-domain resolution of at least semitone (i.e. FN[n+1]/FN[n]=102.93%)

[0086] According to the invention, the information contained in the active objects database 34 is also used by a shape forming system 37. The expected result of the sound signal decomposition according to the invention is to obtain sound objects having the form of sinusoidal waveforms with slowly-varying amplitude envelope and frequency. Therefore, the shape forming system 37 tracks variations of the amplitude envelope and frequency of the active objects in the database 34 and calculates online subsequent characteristic points of amplitude and frequency, which are the local maximum, local minimum and inflection points. Such information allows to unambiguously describe sinusoidal waveforms. The shape forming system 37 forwards these characteristic information in the form of points describing an object online to the active objects database 34. It has been assumed that the distance between points to be determined should be no less than 20 periods of the object's frequency. Distances between points, which are proportional to frequency, are capable to effectively represent dynamics of the objects' variation. Exemplary sound objects have been shown in FIG. 14a. This figure illustrates four objects with frequency varying in function of time (sample number). The same objects have been shown in FIG. 14b in the space defined by amplitude and time (sample number). The illustrated points indicate local maxima and minima of the amplitude. The points are connected by a smooth curve, calculated with the use of third order polynomials. Having determined the function of frequency variation and the amplitude envelope it is possible to determine the audio signal. FIG. 14c illustrates an audio signal determined based on the shape of the objects defined in FIG. 14a and FIG. 14b. The object shown in the plots have been described in the form of the table FIG. 14d, wherein for each object there are described the parameters of its subsequent characteristic points, including the first point, the last point and the local extrema. Each point has three coordinates, i.e. the position in time expressed by the sample number, the amplitude and the frequency. Such set of points describes unambiguously a slowly-varying sinusoidal waveform

[0087] The description of sound objects shown in the table FIG. 14d can be written down in the form of a formalized protocol. Standardization of such notation will allow to develop applications using the properties of the sound objects according to the invention. FIG. 15 shows an exemplary format of sound objects notation.

[0088] 1) Header: The notation starts with a header having as an essential element a header tag comprising a four byte keyword, informing that we deal with the description of sound objects. Next, in two bytes an information about the number of channels (tracks) is specified and two bytes of time unit definition. The header occurs only once at the beginning of a file.

[0089] 2) Channel: Information about channels (tracks) from this field serves to separate the group of sound objects being in an essential relation, e.g. left or right channel in stereo, vocal track, percussion instruments track, recording from a defined microphone etc. The channel field comprises the channel identifier (number), the number of objects in the channel and the position of the channel from the beginning of an audio signal, measured in defined units.

[0090] 3) Object: An identifier contained in the first byte decides about the type of the object decides. Identifier 0 denotes a basic unit in the signal record which is the sound object. Value 1 can denote a folder containing a group of objects like, for example, basic tone and its harmonics. Other values can be used to define other elements related to objects. The description of the fundamental sound object includes the number of points. The number of points does not include the first point, which is defined by the object itself. Specifying maximal amplitude in object's parameters allows to control simultaneous amplification of all points of the object. In the case of a folder of objects, this affects the value of amplitude of all the objects contained in the folder. Analogically, specifying information about frequency (applying notation: number of tone*4 of a filter bank=notes*16) allows to simultaneously control the frequency of all the elements related to an object. Furthermore, defining the position of the beginning of an object in relation to a higher level element (e.g. a channel) allows to shift the object in time.

[0091] 4) Point: Points are used to describe the shape of the sound object in time-frequency-amplitude domain. They have relative value with respect to parameters defined by the sound object. One byte of amplitude defines which part of the maximal amplitude defined by the object the point has. Similarly, tone variation defines by what fraction of tone the frequency has changed. Position of point is defined as relative with respect to the previously defined point in the object.

[0092] The multilevel structure of recording and relative associations between the fields allow a very flexible operation on sound objects, making them effective tools for designing and modifying audio signals.

[0093] Condensed recording of information about sound objects according to the invention, in the format shown in FIG. 15, greatly affects in a positive way the size of registered and transferred files. Taking into account that an audio file can be readily played from this format, we can compare the size of the file shown in FIG. 14c, which in .WAV format would contain over 2000 bytes, and in the form of sound objects record UHO according to the invention, it would contain 132 bytes. A compression better than 15-fold is not an excellent achievement in this case. In the case of longer audio signals much better results can be achieved. The compression level depends on how much information is contained in the audio signal, i.e. how many and how composed objects can be read from the signal.

[0094] Identification of sound objects in an audio signal is not an unambiguous mathematical transformation. The audio signal created as a composition of objects obtained in the result of a decomposition differs from the input signal. The task of the system and the method according to the invention is to minimize this difference. Sources of differences are of two types. Part of them is expected and results from the applied technology, other can result from interference or unexpected properties of input audio signal. To reduce the difference between the audio signal composed of sound objects according to the invention and the input signal a correcting system 4, shown in FIG. 1, is used. The system takes parameters of objects from the sound objects database 35 already after terminating the object and performs the operation of modification of selected parameters of objects and points such as to minimize the expected differences or irregularities localized in these parameters.

[0095] The first type of correction of sound objects according to the invention, performed by the correcting system 4, is shown in FIG. 16. The distortion at the beginning and at the end of the object is caused by the fact that during transient states, when the signal with defined frequency appears or fades, filters with a shorter impulse response react to the change quicker. Therefore, at the beginning the object is bent in the direction of higher frequencies, and at the end it turns towards the lower frequencies. Correction of an object can be based on deforming the object's frequency at the beginning and at the end in the direction defined by the middle section of the object.

[0096] A further type of correction according to the invention, performed by the correcting system 4, has been shown in FIG. 17. The audio signal samples passing through a filter 20 of the filter bank 2 cause a change at the filter's output, which manifests as a signal shift. This shift has a regular character and is possible to be predicted. Its magnitude depends on the width of the window K of the filter n, the width being in accordance to the invention a function of frequency. This means that each frequency is shifted by a different value, what affects the sound of the signal perceivably. The magnitude of the shift is ca. filter window's width in the area of normal operation of the filter, window's width in the initial phase and ca. window's width in the case of the objects end. Because for each frequency the magnitude of the shift can be predicted, the task of the correcting system 4 is to properly shift all the points of the object in the opposite direction, so that the dynamics of the representation of the input signal improves.

[0097] Yet another type of correction according to the invention, performed by the correcting system 4, is shown in FIG. 18a, FIG. 18B and FIG. 18C. The distortion manifests itself as an object splitting into pieces which are independent objects. This splitting can be caused e.g. by a phase fluctuation in an input signal's component, an interference or mutual influence of closely adjacent objects. The correction of distortions of this type requires the correcting circuit 4 to perform an analysis of the functions of envelope and frequency and to demonstrate that said objects should form an entirety. The correction is simple and is based on combination of the identified objects into one object.

[0098] A task of the correcting system 4 is also to remove objects having an insignificant influence on the audio signal's sound. According to the invention it was decided, that such objects can be the ones having the maximal amplitude which is lower than 1% of the maximal amplitude present in the whole signal at a given time instant. Change in the signal at the level of 40 dB should not be audible.

[0099] The correcting system performs generally the removal of all irregularities in the shape of sound objects, which operations can be classified as: joining of discontinuous objects, removal of objects' oscillations near the adjacent ones, removal of insignificant objects, as well as the interfering ones, lasting too shortly or audible too weakly.

[0100] To illustrate the results of the use of the method and the system for sound signal decomposition a fragment of stereo audio signal sampled at 44100 samples per second has been tested. The signal is a musical composition including sound of guitar and singing. The plot shown in FIG. 19a illustrating two channel includes ca. 250000 samples (ca. 5.6 sec.) of the recording.

[0101] FIG. 19b shows a spectrogram resulting from the operation of the filter bank 2 for the audio signal's left channel (upper plot in FIG. 19a). The spectrogram includes the amplitude at the output of 450 filters having frequency from C2=16.35 Hz up to e6=10548 Hz. On the left side of the spectrogram a piano keyboard has been shown as reference points defining the frequency. Furthermore, staffs with bass clef and a staff with treble clef above have been marked. The horizontal axis of the spectrogram corresponds to time instants during a composition, while the darker color in the spectrogram indicates a higher value of the filtered signal's amplitude.

[0102] FIG. 19c shows the result of operation of the voting system 32. Comparing the spectrogram in FIG. 19b with the spectrogram in FIG. 19C it can be seen that wide spots representing signal composing elements have been replaced by distinct lines indicating precise localization of said composing elements of the input signal.

[0103] FIG. 19d shows a cross-section of the spectrogram along the A-A line for the 149008th sample and presents the amplitude in function of frequency. The vertical axis in the middle indicates the real component and the imaginary component and the amplitude of the spectrum. The vertical axis at the right side shows peaks of the voting signal, indicating the temporary localization of audio signal composing elements.

[0104] FIG. 19e is a cross-section of the spectrogram along the line BB at the frequency of 226.4 Hz. The plot shows the amplitude of the spectrum at the output of the filter 2 with the number n=182.

[0105] In FIG. 19f sound objects are shown (without operation of the correcting system 4). The vertical axis indicates the frequency, while the horizontal axis indicates time expressed by the number of the sample. In the tested fragment of the signal 578 objects have been localized, which are described by 578+995=1573 points. To store these objects ca. 9780 bytes are required. The audio signal in FIG. 19a comprising 250000 samples in the left channel requires 500 000 bytes for direct storing, which in the case of using the signal decomposition method and sound objects according to the invention leads to a compression at the level of 49. The use of correcting system 4 further improves the compression level, due to removal of objects having a negligible influence on the signal's sound.

[0106] In FIG. 19g there are shown amplitudes of selected sound objects, shaped with the use of already determined characteristic points by means of smooth curves created of third order polynomials. In the figure there are shown objects with amplitude higher than 10% of the amplitude of the object with the highest amplitude.

[0107] As a result of using the method and the system for signal decomposition according to the invention one obtains sound objects according to the invention, which can serve for an acoustic signal synthesis.

[0108] More specifically, a sound object comprises an identifier indicating the object's location relative to the beginning of the track and the number of points included in the object. Each point contains the position of the object in relation to the previous point, the change of the amplitude with respect to the previous point, and a change of pulsation (expressed on a logarithmic scale) against the pulsation of the previous point. In a properly built object amplitude of the first and last point should be zero. If it is not, then in the acoustic signal such amplitude jump can be perceived as a crack. An important assumption is that objects begin with a phase equaling zero. If not, the starting point should be moved to the location in which the phase is zero, otherwise the whole object will be out of phase.

[0109] Such information is sufficient to construct an audio signal represented by an object. In the simplest case, by using parameters included in the points it is possible to determine a polygonal line of an amplitude's envelope and a polygonal line of pulsation changes. To improve the sound signal and remove high frequency generated in places of the breaks of the curves one can generate a smooth curve in the form of a polynomial of second or higher order, whose subsequent derivatives are equal in the peaks of the polygonal line (e.g. cubic spline).

[0110] In the case of linear interpolation, the equation describing the section of the audio signal from one to the next point may be in the form:

AudioSignalP.sub.i(t)=(A.sub.(i)+t*A.sub.i+1)/P.sub.(i+1))*(cos*.sub.i+t*(.sub.i+.sub.(i+1)/P.sub.(i+1)))

Where: A.sub.iamplitude of point i [0111] P.sub.iposition of point i [0112] .sub.iangular frequency of point i [0113] .sub.iphase of point i, .sub.0=0

[0114] Object's audio signal composed of the P points is the sum of offset segments described above. In the same way, the complete audio signal is the sum of offset signals of objects.

[0115] A synthesized test signal in FIG. 19a is shown in FIG. 19h.

[0116] The sound objects according to the invention have a number of properties enabling their multiple applications, in particular in processing, analysis and synthesis of sound signals. Sound objects can be acquired with the use of the method for signal decomposition according to the invention as a result of an audio signal decomposition. Sound objects can be also formed analytically, by defining values of parameters shown in FIG. 14d. A sound object database can be formed by sounds taken from the surrounding environment or created artificially. Below some advantageous properties of sound objects described by points having three coordinates are listed:

[0117] 1) Based on parameters describing sound objects it is possible to determine the function of amplitude and frequency variation, and to determine location in respect to other objects, so that an audio signal can be composed of them.

[0118] 2) One of the parameters which describe sound objects is the time, thanks to which the objects can be shifted, shortened and lengthened in the time domain.

[0119] 3) A second parameter of sound objects is the frequency, thanks to which the objects can be shifted and modified in the frequency domain.

[0120] 4) A next parameter of sound objects is the amplitude, thanks to which envelopes of sound objects can be modified.

[0121] 5) Sound objects can be grouped, by selecting e.g. the ones present in the same time or/and the ones with frequencies being harmonics.

[0122] 6) Grouped objects can be separated from or appended to an audio signal. This allows to create a new signal from a number of other signals or to split a single signal into a number of independent signals.

[0123] 7) Grouped objects can be amplified (by increasing their amplitude) or silenced (by decreasing their amplitude).

[0124] 8) By modifying proportions of harmonic amplitude included in a group of objects it is possible to modify the timbre of the grouped objects.

[0125] 9) It is possible to modify the value of all grouped frequencies by increasing or decreasing frequencies of harmonics.

[0126] 10) It is possible to modify audible emotions contained in sound objects, by modifying the slope (falling or raising) of component frequencies.

[0127] 11) By presenting an audio signal in the form of objects described by points with three coordinates it is possible to significantly reduce the number of required data bytes without loss of information contained in the signal.

[0128] Considering the properties of sound objects, a great deal of applications can be defined for them. The exemplary ones include: [0129] 1) Separation of audio signal sources such as instruments or speakers, based on proper grouping of sound objects present in the signal. [0130] 2) Automatic generation of musical notation for individual instruments from an audio signal. [0131] 3) Devices for automatic tuning of musical instruments during ongoing musical performance. [0132] 4) Forwarding the voice of separated speakers to speech recognition systems. [0133] 5) Recognition of emotion contained in separated voices. [0134] 6) Identification of separated speakers. [0135] 7) Modification of the timbre of recognized instruments. [0136] 8) Swapping the instruments (e.g. a guitar playing instead of a piano); [0137] 9) Modification of a voice of a speaker (raising, lowering, conversion of emotion, intonation). [0138] 10) Swapping of voices of speakers. [0139] 11) Synthesis of a voice with the possibility of emotion and intonation control. [0140] 12) Smooth joining of speeches. [0141] 13) Voice control of devices, even in an environment with interference. [0142] 14) Generation of new sounds, samples, unusual sounds. [0143] 15) New musical instruments. [0144] 16) Spatial management of sound. [0145] 17) Additional possibilities of data compression.

Further Embodiments

[0146] According to an embodiment of the invention, a method for decomposition of acoustic signal into sound objects having the form of sinusoidal wave with slowly-varying amplitude and frequency, comprises a step of determining parameters of short term signal model and a step of determining parameters of long term signal model based on said short term parameters, wherein a step of determining parameters of a short term signal model comprises a conversion of the analogue acoustic signal into a digital input signal P.sub.IN and wherein in said step of determining parameters of short term signal model the input signal P.sub.IN is then split into adjacent sub-bands with central frequencies distributed according to logarithmic scale by feeding samples of the acoustic signal to the digital filter bank's input, each digital filter having a window length proportionally to the nominal central frequency [0147] at each filter's (20) output the real value FC(n) and the imaginary value FS(n) of the filtered signal is determined sample by sample, and then based on this [0148] the frequency, the amplitude and the phase of all detected constituent elements of said acoustic signal are determined sample by sample, [0149] an operation improving the frequency-domain resolution of said filtered signal is executed sample by sample and involves at least a step of determining the frequency of all detected constituent elements based on maximum values of the function FG(n) resulting from a mathematical operation reflecting the number of neighboring filters (20) outputting an angular frequency value substantially similar to an angular frequency value of each consecutive filter (20), [0150] and in that in said step of determining parameters of long term signal model: [0151] for each detected element of said acoustic signal an active object in an active objects database (34 ) is created for its tracking [0152] subsequent detected elements of said acoustic signal are associated sample by sample with at least selected active objects in said active objects database (34) to create a new active object or to append said detected element to an active object, or to close an active object [0153] for each active object in the database (34) values of the envelope of amplitude and values of frequency and their corresponding time instants are determined not less frequently than once per period of duration of a given filter's (20) window W(n) so as to create characteristic points describing slowly-varying sinusoidal waveform of said sound object [0154] at least one selected closed active object is transferred to a database of sound objects (35) to obtain at least one decomposed sound object, defined by a set of characteristic points with coordinates in time-frequency-amplitude space.

[0155] The method may further comprise a step of correcting selected sound objects which involves a step of correcting of amplitude and/or frequency of selected sound objects as to reduce an expected distortion in said sound objects, the distortion being introduced by said digital filter bank.

[0156] Improving the frequency-domain resolution of said filtered signal may further comprise a step of increasing window length of selected filters.

[0157] The operation of improving the frequency-domain resolution of said filtered signal may further comprise a step of subtracting an expected spectrum of assuredly located adjacent sound objects from the spectrum at the output of the filters.

[0158] The operation of improving the frequency-domain resolution of said filtered signal may further comprise a step of subtracting an audio signal generated based on assuredly located adjacent sound objects from said input signal.

[0159] A system for decomposition of acoustic signal into sound objects having the form of sinusoidal waveforms with slowly-varying amplitude and frequency according to a further embodiment of the invention comprises a sub-system for determining parameters of a short term signal model and a sub-system for determining parameters of a long term signal model based on said parameters, wherein said subsystem for determining short term parameters comprises a converter system for conversion of the analogue acoustic signal into a digital input signal P.sub.IN wherein said subsystem for determining short term parameters further comprises a filter bank (20) with filter central frequencies distributed according to logarithmic distribution, each digital filter having a window length proportionally to the central frequency wherein each filter (20) is adapted to determine a real value FC(n) and an imaginary value FS(n) of said filtered signal, said filter bank (2) being connected to a system for tracking objects (3), wherein said system for tracking objects (3) comprises a spectrum analyzing system (31) adapted to detect all constituent elements of the input signal P.sub.IN, a voting system (32) adapted to determine the frequency of all detected constituent elements based on maximum values of the function FG(n) resulting from a mathematical operation reflecting the number of neighboring filters (20) which output an angular frequency value substantially similar to an angular frequency value of each consecutive filter (20), and in that said subsystem for determining long term parameters comprises

[0160] a system for associating objects (33), a shape forming system (37) adapted to determine characteristic points describing slowly-varying sinusoidal waveforms, an active objects database (34) and a sound objects database (35).

[0161] The system for tracking objects (3) may further be connected with a correcting system (4) adapted to correct the amplitude and/or the frequency of individual selected sound objects so as to reduce an expected distortion in said sound objects introduced by said digital filter bank and/or adapted to combine discontinuous objects and/or to remove selected sound objects.

[0162] The system may further comprise a resolution improvement system (36) adapted to increase window length of selected filter and/or to subtract an expected spectrum of assuredly located adjacent sound objects from the spectrum at the output of the filters and/or to subtract an audio signal generated based on assuredly located adjacent sound objects from said input signal.