Abstract
A method for reproducing an acoustic signal from a digital signal, the digital signal being formed of successive bit sequences each having bits representative of the amplitude of an acoustic signal at a time sample, the method including (i) providing a plurality of transducers configured to emit acoustic signals that are wave trains having each a duration lower or equal to the duration of one time sample and including at least one oscillation per time sample, and (ii) successively, for each bit sequence, having each bit associated to at least one of the transducers and independently govern, depending on its value, amplitudes of the acoustic signals emitted by its associated transducers.
Claims
1. Method for reproducing an audible acoustic signal from a digital signal, the digital signal being formed of successive bit sequences each comprising bits representative of the amplitude of the audible acoustic signal at a time sample, the bits of each bit sequence including at least one quantification bit, the method comprising: providing a plurality of transducers configured to emit acoustic signals that are wave trains having each a duration lower or equal to the duration of one time sample and comprising at least one oscillation per time sample, successively, for each bit sequence, having the or each quantification bit associated to at least one of the transducers and independently govern, depending on a value and a rank of the bit, at least two different amplitudes of the acoustic signals emitted by the transducer or each of the transducers associated to the quantification bit, in a manner so as to obtain a carrier wave of the audible acoustic signal that is modulated in at least two different amplitudes according to the digital signal.
2. Method according to claim 1, wherein the acoustic signal emitted by the transducers comprises at least several successive oscillations.
3. Method according to claim 1, wherein the acoustic signal emitted by the transducers is a periodic and alternating signal.
4. Method according to claim 1, wherein the acoustic signal emitted by the transducers is of sinusoidal shape.
5. Method according to claim 1, wherein a fundamental frequency of the acoustic signal emitted by the transducers is in the range of ultrasonic frequencies.
6. Method according to claim 1, wherein, in each bit sequence, the quantification bit or bits define a binary number which indicates a quantified absolute value of the amplitude of the acoustic signal to be reproduced at the time sample associated with the bit sequence.
7. Method according to claim 6, wherein, for each successive bit sequence, one of the value of each quantification bit does not induce any variation of the amplitudes of the acoustic signals emitted by its associated transducers, the other one of the value of each quantification bit induces a variation of the amplitudes of the acoustic signal or signals emitted by its associated transducer or transducers.
8. Method according to claim 7, wherein the transducers whose operations are not modified by any quantification bit are at an off default state.
9. Method according to claim 7, wherein, for each bit sequence, the bit or bits of a higher rank of the binary number induce either a greater variation in amplitude for each transducer, a variation in amplitude for more associated transducers, or both, than the bit or bits of lower rank of the binary number.
10. Method according to claim 7, wherein each transducer has the amplitude of its acoustic signal determined by at most one bit value of each bit sequence.
11. Method according to claim 1, wherein each transducer has only two different operating states, one of which is a default state.
12. Method according to claim 1, wherein each transducer has only three different operating states, one of which is a default state.
13. Method according to claim 1, wherein the digital signal is an electromagnetic signal, and wherein the transducers are electro-acoustic transducers driven by an alternative current or an alternative voltage.
14. Method according to claim 1, wherein the transducers are arranged so that the acoustic signals converge at a same focus in phase.
15. Method according to claim 1, wherein the transducers are all arranged facing outwards on a plane surface orthogonal to the axis of a parabolic surface, or are all arranged facing outwards on the internal surface of a sphere.
16. Method according to claim 1, wherein the amplitudes and the shapes of the acoustic signals emitted by the transducers are adapted to produce nonlinear demodulation of the acoustic signals into a lower frequency signal, as in an acoustic parametric array.
17. Method according to claim 1, wherein means of redirection of the signal are arranged at a focus.
18. A loudspeaker comprising a plurality of transducers configured to emit acoustic signals at a predetermined frequency which is equal or greater than any frequency of sampling of an audible acoustic signal, and arranged so that the acoustic signals converge at a same focus in phase, wherein the loudspeaker has a configuration in which a digital signal, being formed of successive bit sequences each comprising bits representative of the amplitude of the audible acoustic signal at a time sample, the bits of each bit sequence including at least one quantification bit, is used in a manner so that successively, for each bit sequence, the or each quantification bit is associated to at least one of the transducers and independently governs, depending on a value and a rank of the quantification bit, at least two different amplitudes of the acoustic signals emitted by the transducer or each of the transducers associated to the quantification bit, in a manner so as to obtain a carrier wave of the audible acoustic signal that is modulated in at least two different amplitudes according to the digital signal.
19. Method according to claim 2, wherein the acoustic signal emitted by the transducers is a periodic and alternating signal.
20. Method according to claim 8, wherein, for each bit sequence, the bit or bits of a higher rank of the binary number induce either a greater variation in amplitude for each transducer, a variation in amplitude for more associated transducers, or both, than the bit or bits of lower rank of the binary number.
Description
(1) The invention can be better understood and other details, characteristics, and advantages of the present invention appear more clearly on reading the following description made by way of non-limiting example and with reference to the accompanying drawings, in which:
(2) FIGS. 1A, 1B and 1C are diagrams showing the transformation of an analogic signal into a digital signal,
(3) FIG. 2 is a diagram showing how transducers are associated to bits of a digital signal,
(4) FIG. 3 is a diagram showing an operation of a transducer during a little duration according to the value of its associated bits in the digital signal,
(5) FIG. 4 is a diagram showing a system used to drive the transducers,
(6) FIG. 5 shows an example of a sum of signals emitted by different transducers during a little duration,
(7) FIG. 6 is a diagrammatic example of arrangement of the transducers, coupled with acoustic propagation means,
(8) FIG. 7 is another diagrammatic example of arrangement of the transducers, coupled with acoustic propagation means,
(9) FIGS. 8A and 8B schematically illustrate source analogic signal quantified and sampled to a digital signal, then used as an entry signal in the method of the invention,
(10) FIG. 9 show the acoustic signal measured at the focus of the invention, when processing the digital signal of FIG. 8, and
(11) FIGS. 10 and 11 are Fast Fourier Transformations of the acoustic signal illustrated in FIG. 9.
(12) FIGS. 1A, 1B and 1C illustrates the transformation of an audible analogic signal into a digital signal. The source analogic signal 10 is here a sinusoidal wave. The analogic signal 10 is sampled in time at a chosen frequency. For each time sample, it is calculated an average of the amplitude of the analogic signal, which is chosen to transcribe the value of the amplitude at the associated time sample. As a consequence, from the continuous information given by the analogic signal, only discrete values of amplitude, exactly one by time sample, are conserved, as illustrated on the transformed wave 12 of FIG. 1B. Then, a finite number of bits are chosen. Those bits constitute together a bit sequence which is used to represent the value of the average amplitude corresponding to a time sample. The bit sequence usually contains one first bit which defines the polarity, i.e. the sign, of the amplitude of the signal, and a binary number which is used to define the absolute amplitude of the signal at the time sample. As a bit number can only define a finite number of values, the calculated average of the amplitude at the time sample is approximated towards the most proximate available value that can be defined by the binary number. It is said that the analogic signal has been quantified. Usually, amplitude steps separating two consecutive binary numbers are calculated by dividing the maximum amplitude of the analogic signal by the number of values that can be defined by the binary number, depending on the number of bits forming the binary number. A sequence of bits is then obtained for each time sample. All the bit sequences are joined consecutively, usually in the chronological order, to form the digital signal 14 shown in FIG. 1C.
(13) FIG. 2 illustrates a sampled and quantified audible signal 16, wherein each sample has been attributed a bit sequence 18 as defined above to define the average amplitude of the signal at the time sample. In this example, the bit sequence comprises one polarity bit and a binary number composed of three bits to define the signal. According to the invention, each bit of the binary number has been associated to one or more transducers 20 able to emit an ultrasonic signal of a fundamental frequency greater than the sampling frequency of the sampled and quantified signal 16, and not audible by a human being. More specifically, the bit of rank 1 (the unit) has been associated to one transducer, the bit of rank 2 has been associated to two transducers, and the bit of rank 3 has been associated to four transducers. For each bit sequence, when a bit of rank n of the binary number has its value equal to 1, its associated transducers are activated during the attributed time sample. All the transducers playing at a given time sample are emitting acoustic signals of same amplitude in phase. For example, when the bit sequence contains a binary number equal to 101, the bit of rank 1 is 1, which means that its unique associated transducer is activated; the bit of rank 2 is 0, which means that its two associated transducers are off; and the bit of rank 3 is 1, which means that its four associated transducers are activated. A total of five transducers are thus activated during the time sample. Depending on the binary number attributed to a time sample, the number of activated transducers varies. Consequently, the sum of the amplitudes of the acoustic signals emitted by the transducers varies in time according to the variation of the binary numbers in the digital signal.
(14) FIG. 3 illustrates the behavior of a transducer depending on the value of its associated bit in the binary number, for successive bit sequences. When decoding the digital signal, for the first bit sequence, the value of its associated bit is 0, which means that the transducer is off during the duration corresponding to the time sample of the bit sequence. For the second bit sequence, the value of its associated bit is 1, which means that the transducer is activated, i.e. on, and emits an acoustic signal 21, during the duration corresponding to the time sample of the bit sequence. Etc.
(15) FIG. 4 illustrates an example of system adapted to operate the method described above. A computer 22 has as many outputs 22a, 22b as bits composing the binary number of each bit sequence of the digital signal, each output being particularly associated to one of those bits. For each bit sequence, when a bit has its value equal to 1, the computer uses its associated output to emit a low continuous electrical signal during the attributed time sample. Such signal is amplified by an amplifier 24a, 24b, and then modulated by a modulator 26a, 26b into a sinusoidal electrical signal of frequency corresponding to the frequency of operation of the transducers. Such sinusoidal signal is again amplified by an amplifier 28a, 28b to the desired amplitude of drive of each transducer 20a, 20b. When a bit has a value equal to 0, the computer does not emit any signal through its associated output, and the transducers 20a, 20b are consequently not driven by any electrical signal.
(16) FIG. 5 illustrates the sum 29 of the amplitudes of the acoustic signals emitted by transducers thanks to the above described method, for a digital signal containing bit sequences with binary numbers increasing incrementally from 000 to 111, and then decreasing incrementally from 111 to 000. The resulting acoustic signal shows a carrier wave with the frequency of the transducers, and which is modulated in amplitude, in a quantified manner, by the number of activated transducers, according to the digital signal.
(17) FIGS. 6 and 7 illustrate preferred arrangements of the transducers 20. The objective is to dispose all the transducers so that at a particular point in space, called focus, all the acoustic signals 21 coming from the transducers converge synchronously in phase. At the focus, the resulting acoustic signal is optimal for self-demodulation. In FIG. 6, the transducers are arranged facing inwards from the inner surface of a portion of a sphere 30. The focus is the center 32 of the sphere. In FIG. 7, the transducers 20 are arranged on a flat surface 34 which is perpendicular to the axis 36a of a parabola 36. The transducers 20 are also facing the parabola 36 parallel to the axis of the parabola 36. The focus point is then the focus 38 of the parabola.
(18) In order to widely propagate the particular acoustic signal obtained at the focus, in FIG. 6, a slot 40 is arranged at the focus, which diffracts the acoustic signal such as obtained at the focus. In FIG. 7, a horn 42 is disposed with its inlet at the focus point, and spreads the acoustic signal.
(19) FIGS. 8 to 11 are related to an experiment featuring the invention. The digital signal 44 of FIG. 8B is used as the source to be reproduced by the transducers. The digital signal 44 is composed of bit sequence 46 each comprising one polarity bit 48 and a bit number 50 of 2 bits. The digital signal is a sampling and quantification of an analogic signal 52, shown in FIG. 1A, comprising a single frequency sinusoidal wave. Only one wavelength is shown in FIG. 8, even though the signal is periodic and continuous. This digital signal is used as input in the method of the invention, such as it has been described in reference to the proceedings figures. However, it has not been used here any kind of focalization means or sound spread means along with the transducers. The transducers are only aligned parallel to each other.
(20) FIG. 9 shows the measured effective received acoustic signal 54 at two meters from the transducers, in the time dimension. As explained above, it is observed a carrier wave of ultrasonic frequency, whose amplitude is modulated according to the transducers effectively activated across time, i.e. according to the exploitation made of the digital signal. The amplitude is given with an arbitrary unit. It can already be observed that the modulation of amplitude has a regular period of 0.5 milliseconds.
(21) When transposing the measured signal of FIG. 9 into the frequency dimension, using a Fast Fourier Transformation, it is obtained the graphs 56, 58 of FIGS. 10 and 11. FIG. 10 shows the Fast Fourier Transformation of the signal between 36.5 and 41 kHz, and FIG. 11 shows the Fast Fourier Transformation of the signal between 0 and 20 kHz. As expected, a main amplitude peak 60 is observed at a frequency of 39 kHz, which is the operating frequency of the transducers. Thanks to the self demodulation phenomenon, a relatively important amplitude peak 62 is also observed at 2 kHz, which is the amplitude of modulation of the carrier signal. This peak represents an audible acoustic signal obtained from the ultrasonic signals of the transducers. Such acoustic signal obtained thanks to the invention is relatively faithful to the information originally contained in the digital signal.
