System for processing an audio signal.

Auxiliary signaling systems for the transmission of an audio-visual signal for the transmission of an audio signal (1) for an audio signal in an electric form, for a filter group (2) for the input of an audio signal to a signal frequency band, (3) for shielding the subband signal for the detection of the audio signal signal and the compressor / expander (4) for shielding the subband signal for the detection of the audio signal signal and the compressor, och en andra filtergrupp (5, för combering av de komprimerade / expanderade subbandssignalerna för ästadkommande av en rekonstruerad audiosignal (6). In addition, the filter group (2.5) is connected to the filter group (2.5) and the signal is connected to the filter group, and the signal to the compressor / expander (4) is connected to the signal group. the detection detector (3) has a fixed signal in the audio frequency band.

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A system for processing an audio signal

The present invention relates to a system for processing an audio signal to reduce the noise contained therein, comprising a source of an audio signal in electronic form, dividing the audio signal of the first filter group into several subband signals occurring in different frequency bands, for each subband signal, a detector for detecting its audio signal level and compressing / expanding each subband signal of the compressor / expander based on its detected audio signal level and combining the compressed / expanded subband signals of the second filter group to generate a reconstructed audio signal; whose width allows signal processing in the region of the critical bands, and the second group of filters is adapted to combine the audio signals of the bands to generate a reconstructed audio signal.

One known system for reducing the noise of an audio signal is the so-called Dolby SR noise reduction system using five independently controlled noise reduction channels to improve the signal-to-noise ratio separately in five different frequency bands. The playback area of ​​the sound system is thus divided into five areas, each of which is dealt with separately. In a Dolby noise reduction system, the basic principle is to improve the signal-to-noise ratio so that when recording an audio signal, the lower the audio signal level in each case, the more it is amplified. This procedure of reducing the dynamic range of the audio signal is called compression. The audio signal compressed in this way has to be expanded during playback, whereby its dynamic range is restored to almost the same level. The method has the advantage of attenuating the noise of the transmission path or storage medium by about 10 dB.

The main disadvantage of the Dolby system is that it can only reduce noise when playing audio signals that have been compressed according to that system. In addition, the frequency division of that system, when used, is very rough.

U.S. Patent Nos. 4,896,362 and 5,105,463 disclose a system in which a digital audio signal is divided into subbands of approximately critical bandwidth to utilize the noise coverage curve of the human auditory system by removing the obscured portion of the signal. The ultimate goal of the system is to re-quantize the signal in each subband to a minimum number of bits so that quantization noise is not audible15. U.S. Pat. No. 4,882,762 further discloses a system in which an audio signal is frequency selectively compressed so that the compression ratio is independently selected for each frequency band. However, these known systems are not intended to attenuate the noise involved in the audio signal.

It is an object of the present invention to provide a noise reduction system which is also suitable for use with audio signals which have not been pre-filtered or processed, such as compressed. This is achieved by means of a system according to the invention, characterized in that each compressor / expander is adapted to attenuate the signal in its band when the associated detector has detected that the audio signal is missing from that frequency band.

One of the essential basic starting points of the system according to the invention is to divide the audio signal into frequency bands, which are so-called critical bands or portions thereof, by appropriate combination of which critical bands can be obtained. In this case, the basic features of the human hearing35 system, such as the masking effect and

ML, L Hill II I m the associated critical lane property. The masking phenomenon of the human auditory system is related to the ability of sound to attenuate a lower level signal as unrecognizable and unheard in the vicinity of sound. Broadly speaking, the spectral band in which lower-level sounds are attenuated is termed the critical band. A more precise definition of a critical band is the frequency range of noise signals in which the sounds present contribute to sound coverage centered on the frequency band in question. The masking phenomenon also occurs in the time plane. This is referred to as non-simultaneous coverage. The masking signal can then mask the sound that occurs before or after the actual signal. The duration of these masking effects is very short, less than 100 ms.

Normal audio signals (speech, music) are not narrowband signals. Thus, their ability to act as masking signals is greater than that of pure sound. When the sound power is in a certain frequency band, it acts as a masking signal for any noise in that band. This noise may be inherent in the recording or may be due to suboptimal operation of the sound system.

When no audio signal is present in the critical band as the filter groups of the system according to the invention divide the frequency band, that band can be attenuated without having an audible effect on the signal itself. The audible effect will be noise attenuation or loss. The damping frequency / time is determined by the pre- and post-coverage phenomenon. Since the actual signal content is not changed in the system according to the invention, the attenuation of the noise does not lead to any level changes in the signal itself, i.e. it does not involve any pumping effect of the sound level or noise level.

The system according to the invention will now be described in more detail with reference to the accompanying drawing, the figure of which shows a basic block diagram of the sound reproduction system 97758 according to the invention.

In the figure, reference numeral 1 denotes an audio source, which may be any conventional source of an audio signal in electronic form, such as a microphone, tuner, recorder, CD player or the like. From the audio signal source 1, the audio signal is fed to a filter group 2, which divides the audio signal into several frequency bands, the frequency bands of which either correspond to the so-called human auditory system. critical bands or can be combined into such bands. Each subband signal is applied to a detector 3, which detects the level of the audio signal in the respective frequency band and controls the compressor / expander 4, on the basis of which this subband signal is also routed, on the basis of this level. Thus, each subband signal has its own detector 3 and its own compressor / expander 4.

The effect of the control from the detector 3 on the compressor / expander 4 on the operation of the compressor / expander can be varied as required. For example, under optimal listening conditions, it is most advantageous for the detector 3 to control the compressor / expander 4 in such a way that, in the absence of an audio signal in that frequency band, the compressor / expander attenuates the signal in that band completely unheard. In this way, the noise in the audio signal from the audio signal source 1 in that frequency band can be completely attenuated without affecting the audio signal itself in any way. If, on the other hand, the detector 3 detects the presence of an audio signal in the frequency band in question, the compressor / expander 4 is controlled in such a way that it does not affect the audio signal in any way.

The signals from the compressor / expander 4 of all critical frequency bands are fed to a reconstruction filter group 5 which combines them into a reconstructed audio signal 6. In the above situation, this reconstructed audio signal 6 no longer contains noise other than those critical frequency bands which also have an audio signal. Since the audio signal in question is in most cases able to mask the noise occurring at the same frequency, the reconstructed audio signal appears to be almost noiseless. In this context, it should be borne in mind that the audio signal of the most modern audio signal sources, such as a CD player, may also contain a lot of noise if the recording from which the CD in question has been compressed has a lot of noise. This is, of course, particularly the case with older recordings.

The sound reproduction system according to the invention can also be applied in non-optimal listening conditions, such as vehicles, in such a way that the compressor / expander 4 is adapted to compress the audio signal when the level of the audio signal is low. Such a procedure naturally results in a distortion of the dynamics of the audio signal, but nevertheless represents an improvement in the overall impression that can be achieved in a noisy environment, such as a vehicle, because then low-level signals can also be heard.

The procedures described above can also be used together by compressing those critical bands that contain an audio signal and attenuating those critical bands that do not contain an audio signal to remove noise in those bands. The system according to the invention can thus be used to eliminate noise generated by the sound reproduction system, but also noise which is already present in the recording itself. In addition, noise can be removed despite compressing the dynamic range of the audio signal.

The system according to the invention for processing an audio signal has been described by means of only one exemplary embodiment and it is understood that the blocks included therein can be implemented by means of very different device solutions, without, however, departing from the scope defined by the appended claims. For example, a detector indicating the level or absence of an audio signal in that critical band can be implemented in a wide variety of ways, such as in ways applied in connection with known noise reduction systems.