Audio system, method for generating an audio signal, computer program and audio signal

Abstract

The invention relates to an audio system for generating an audio signal. More specifically the invention relates to an audio system, especially an audio alarm system, for generating an audio signal comprising means for generating a component of the audio signal at a base frequency and means for generating further components of the audio signal at other frequencies than the base frequency, whereby the base and the other frequencies are separated from each other by separating frequency bands in order to enhance the loudness of the audio signal. The invention furthermore relates to a method for generating an audio signal, a computer program and an audio signal.

Claims

1. An audio system (1) for generating an audio signal comprising: a first module configured to generate a component of the audio signal at a base frequency, a second module configured to generate further components of the audio signal at other frequencies than the base frequency, whereby the base and the other frequencies are separated from each other by separating frequency bands in order to enhance the loudness of the audio signal, characterized by a controlling module configured to select the phase relations of the components of the audio signal at each of the base frequency and other frequencies to decrease a Crest factor of the audio signal based on a Crest factor of a reference audio signal wherein all components of the reference audio signal have equal phase.

2. The audio system (1) according to claim 1, characterized in that the decreased Crest factor is less than 80% of the Crest factor of the reference audio signal.

3. The audio system (1) according to claim 1, characterized in that the further frequencies are harmonics of the base frequency.

4. The audio system (1) according to claim 1, characterized in that the separating frequency bands are determined by evaluating an auditory filter at the base frequency and the other frequencies, whereby the auditory filter assigns a bandwidth to each frequency, turning each frequency into a frequency band, whereby the frequency bands are arranged non-overlapping.

5. The audio system (1) according to claim 4, characterised in that the auditory filter is realised as an equivalent rectangular bandwidth (ERB) filter.

6. The audio system (1) according to claim 1, characterised in that all frequencies are arranged in the audible spectrum.

7. The audio system (1) according to claim 1, characterised in that the value of the base frequency is time dependent.

8. The audio system (1) according to claim 7, characterised in that the highest frequency value of the time dependent base frequency defines the maximum amount of harmonic numbers for the further components of the audio signal.

9. A method for generating an audio signal, the method comprising: generating a component of the audio signal at a base frequency, generating further components of the audio signal at frequencies other than the base frequency, whereby the base and the other frequencies are separated from each other by separating frequency bands in order to enhance the loudness of the audio signal, selecting phase relations of the components of the audio signal at the base frequency and at the other frequencies, and minimizing the Crest factor of the audio signal based on a Crest factor of a reference audio signal, whereby all components of the reference audio signal have the equal phase, while keeping the RMS value of the audio signal on a constant level.

10. A non-transient computer program comprising program-code means enabling to carry out the method according to claim 9, when the computer program is carried out on a computer.

11. An audio signal, embodied on a memory, the audio signal comprising: a component of the audio signal at a base frequency, further components of the audio signal at other frequencies than the base frequency, whereby the base frequency and the other frequencies are separated from each other by separating frequency bands in order to enhance the loudness of the audio signal, characterised in that the phase relations of the components of the audio signal at the base frequency and at the other frequencies are selected such that the Crest factor is less than 80% of a Crest factor of a reference audio signal, whereby all components of the reference audio signal have the equal phase.

12. The audio system (10) according to claim 1, wherein the audio system is an audio alarm system.

13. The audio system (1) according to claim 1, characterized in that the decreased Crest factor is less than 60% of a Crest factor of a reference audio signal, whereby all components of the reference audio signal have the equal phase.

14. The audio system (1) according to claim 1, characterized in that the decreased Crest factor is less than 40% of a Crest factor of a reference audio signal, whereby all components of the reference audio signal have the equal phase.

15. The audio system (1) according to claim 1, characterized in that the further frequencies are harmonics of a selection of the base frequency.

16. The audio system (1) according to claim 1, characterised in that all frequencies are arranged below 8 kHz.

17. A non-transitory computer readable medium comprising program-code for carrying out the method according to claim 9, when the computer program is executed out on an audio system.

Description

(1) Further features, effects and advantages will become apparent by the detailed description and the figures of embodiments of the invention. The figures show:

(2) FIG. 1 a block diagram illustrating an audio system as an embodiment of the invention;

(3) FIG. 2 a flow diagram illustrating a method for generating an audio signal on basis of a fixed frequency tone according to the invention;

(4) FIG. 3 a flow diagram illustrating a method for generating an audio signal on basis of a frequency sweep tone frequency tone according to the invention;

(5) FIG. 4 a graph showing the distribution of valid harmonics versus possible base frequencies.

(6) FIG. 5 is a graph of a zero phase signal having a high Crest factor and a graph of an optimized phase signal having a low Crest factor.

(7) FIG. 1 shows a block diagram of an audio system 1 for generating an alarm signal. Such alarm signals are for example used in connection with public address systems. The audio system 1 comprises a module 2 for generating a component of the alarm signal at a base frequency and a module 3 for generating further components of the alarm signal at other frequencies. A controlling module 4 is operable to control the modules 2 and 3, so that the resulting alarm signal has specific properties leading to advantages, when the alarm signal is fed into an amplifier 5.

(8) FIG. 2 illustrates the method of generating the alarm signal with the specific properties. The resulting audio signal is constructed starting at step 6 with a signal with a fixed base frequency f, which represents a first component of the alarm signal.

(9) In a next step 7, harmonics to the base frequency f are determined, which form further components of the alarm signal. The harmonics are integer multiples of the base frequency f, as this will not change the perceived pitch of the alarm sound and will not introduce beatings.

(10) As the auditory filter of the listeners of the alarm signal broadens the harmonics to harmonic bands, the harmonics are selected, so that the said harmonics bands are non-overlapping in order to allow a high loudness of the resulting alarm signal The auditory filter may be represented as the ERB-Filter as explained before and may additionally have the correction term for the decreasing hearing ability of the listeners with age.

(11) A next point is that the audible spectrum is also restricted concerning the frequency range, so it is preferred to use an upper frequency fmax limit which cuts components, which cannot be heard at all or only ineffectively heard by the listeners. A possible upper frequency fmax is 8 kHz.

(12) After the steps 6 and 7 the alarm signal is defined concerning the RMS and the loudness. In step 8 the phase relations of the components, i.e. the signal at base frequency and at the harmonics, are set. As already disclosed in the description, the phase relations are set so that the Crest factor of the alarm signal is decreased or minimised. This variable Crest factor enables an optimal match for the alarm signal and the applied amplifier, as the power efficiency of a certain type of amplifier depends on the level of the signal and its Crest factor. The new alarm signal now generates a higher perceived loudness for the same RMS power consumption as the pure tone alarm; for arbitrary phase values of its components it will sound the same and has the same perceived loudness and it can have certain Crest factor (within certain bounds) by manipulating the phase values for its components, which helps to fit the signal in the specifications of the amplifier.

(13) A further advantage of using multiple frequency components over a pure tone is that people with hearing disabilities in a certain frequency range will still notice the alarm tone or attention signal if not all of the frequency components fall within that problematic frequency range.

(14) Also, in case a masking background noise is present in a certain frequency range, e.g. from machines in operation, the alarm signal might still be heard, while a pure tone within that frequency range could go unnoticed.

(15) Decreasing the Crest factor is especially beneficial in connection with class AB, class G and class H amplifiers as the amplifier 5 in FIG. 1. The selection of the harmonics and of the phase relations is performed by the controlling module 4.

(16) FIG. 3 illustrates a method for generating the alarm signal for a base signal with a time-dependent frequency f(t) as the base frequency. Again in step 6 a base signal is defined, representing for example a frequency sweep or a siren.

(17) In a next step 9, the maximum or a critical frequency of the base signal is determined and the harmonics are calculated in a similar manner as described in connection with the last figure. As a difference to the last figure, the harmonics are calculated for the maximum or critical frequency of the base signal, as this frequency defines the maximum amount of harmonics. The rationale for this solution is given in FIG. 4 where the selected harmonics are given as a function of base frequency. In this figure, the upper frequency limit for the harmonic components is 8 kHz. For all considered frequencies, the harmonic content is quite constant over considerably wide bandwidths. In some embodiments, the number of harmonics is restricted to less than 10. After the selection of the harmonics, the phase relations are adapted in step 8 as described in connection with FIG. 2.

(18) Another way of generating this tone or sweep signal is not to generate harmonics in a separate stage at the moment of playback, but to carefully define a signal consisting of a base frequency with selected harmonics with the optimum amplitude and phase relations. Then generate a samples wavetable for this artificial signal that is read from memory with a fixed or variable speed.

(19) The way that an amplifier copes with signals with various Crest factors depends on the working principle of the amplifier.

(20) A class D amplifier, for instance, has a high efficiency (typically above 90%) and the efficiency will not change much for an output signal with a low Crest factor or a high Crest factor. But a low Crest factor allows for a higher level output signal before clipping occurs when the output voltage peaks are close to the supply voltage(s).

(21) The situation is different for a class AB or class B amplifier. If the idle current of a class AB amplifier is neglected and it is just considered as a class B amplifier, the class B amplifier has an efficiency that is a function of the output voltage as a fraction of the maximum output voltage. Theoretically, for a pure sine wave, the maximum efficiency is reached when the output voltage is equal to the maximum output voltage (clipping level). Then the efficiency is /4, or 78.5%. The efficiency decreases linearly with the modulation index of the output signal k=U.sub.out peak/U.sub.supply. So for efficiency reasons it is good to have a maximum modulation (k=1) and drive the amplifier close to clipping with a signal having a low Crest factor. This will produce the highest rms output power.

(22) But in many cases a class B amplifier is designed for having a high peak output power that can only be delivered for a short moment and a much lower output power that can be delivered continuously. The power supply and the heatsinks of the amplifier are scaled down for cost reasons. This is a valid design objective as the Crest factor of music and speech signal is typically quite high, around 15 dB. The power supply and the heatsinks of the amplifier are designed to match the maximum rms power of a typical music/speech signal, while the supply voltages of the amplifier are designed to a value that matches the peak output power that the amplifier should deliver. If such an amplifier is used, a continuous alarm tone can only be delivered at a level that matches the continuous rms output power of the amplifier, or lower. In such a case it might be useful to modify the wave shape of the alarm tone to have a high Crest factor in order to minimize the amplifier dissipation. Although the efficiency of class B amplifiers increases with the modulation index k to the already mentioned /4 for k=1, the dissipation of a class B amplifier reaches a maximum for k=2/=0.637. So, from the point of view of minimizing the amplifier dissipation for the same rms output power, it can be useful to keep the signal level of the alarm tone low (k<<0.637) for most of the time, with only short periodic peaks (k>0.637). In this way the signal traverses just briefly through the high dissipation area.

(23) Another class of amplifier that is often used is the class G amplifier. This type of amplifier uses multiple power supply voltages and the amplifier is designed in such a way that the lower supply voltage(s) are used as long as the output signal is small enough to avoid clipping and the higher supply voltage(s) are used for an output signal that exceed the limits of the lower supply voltage(s). Most class G amplifiers use two or three levels of supply voltage. In case of two levels the lower voltage is often or of the higher voltage (n= or n=). This type of amplifier typically has a significantly higher efficiency than a class B amplifier that would only have the highest supply voltages for the same maximum output power. For example see Highest Efficiency And Super Quality Audio Amplifier Using MOS Power Fets In Class G Operation, IEEE Transactions on Consumer Electronics, Vol. CE-24, No. 3, August 1978, and Average Efficiency Of Class-G Power Amplifiers, IEEE Transactions on Consumer Electronics, Vol. CE-32, No. 2, May 1986. For a class G amplifier it is very beneficial to make the Crest factor of an alarm signal low and keep the peak output voltage just below the level of the lower supply voltage. In this way a high efficiency can be achieved and low dissipation. Would the Crest factor be a little higher for the same rms level, then k increases and the amplifier would move to the higher power supply voltage and dissipation would increase rapidly, although the loudness of the signal would remain the same.

(24) As an example FIG. 5 shows two signals, whereby the signal with the low Crest factor (indicated as optimized phase) has a Crest factor of 3.2 dB, the signal with the high Crest factor (indicated as zero phase) has a Crest factor of 7.2 dB. This signal comprises 4 sine waves, one base frequency and the first 3 harmonics. Though the waveforms are clearly different and obviously have a different Crest factor these two complexes sound the same. The notation in dB is derived by the formula Crest.sub.dB=10*log(Crest.sup.2).

(25) A further point to mention is the use of a so-called A-weighting filter that is used when the sound pressure level (SPL) is measured during commissioning of the system. Compared to a single sine wave the more complex tones with harmonics will give a higher reading for the same rms level, because more emphasis is put on the harmonics between 800 Hz and 8 kHz, compared to the typical base frequency that is between 300 Hz and 800 Hz. This may be important because during commissioning the actual measured SPL level decides whether the system complies, not the perceived loudness of the alarm tone. Unfortunately the measurements do not fully reflect the actual gain of the complex alarm tones because they do not take into account the loudness models of the human hearing system.