Amplifiers – With control of power supply or bias voltage – With control of input electrode or gain control electrode bias
Patent
1995-03-22
1996-09-17
Mottola, Steven
Amplifiers
With control of power supply or bias voltage
With control of input electrode or gain control electrode bias
330279, 381107, H03G 330
Patent
active
055572375
DESCRIPTION:
BRIEF SUMMARY
The invention concerns automatic control of the gain of a variable gain amplifier in an electroacoustic system as a function of the signal/background noise ratio.
It finds a particularly advantageous application in compensating background noise in audio frequency electroacoustic systems including a variable gain amplifier, in particular multichannel systems, for example systems incorporated in automobile vehicles.
In this field, diverse solutions or attempted solutions have been proposed for a very serious problem inherent to this kind of control device, namely identifying the nature of the signal picked up by the sensor of the device. This signal can be noise produced by the automobile, the signal from the loudspeakers or a combination of the two in varying proportions.
Especially if the sensor is a microphone, the smallest error in discriminating between the signal and the noise causes immediate positive feedback reaction and the system rapidly reaches its maximum gain through a process comparable to the LARSEN effect.
To circumvent this problem it was proposed at the 1988 AES congress to use a sensor in the form of a vibration detector fixed to the automobile bodywork, after establishing some degree of correlation between the noise inside the passenger compartment and the vibration of the bodywork. Apart from the fact that this solution is not ideal because there is only partial correlation, it does not completely eliminate the problem of discriminating the signal from the noise because the sound from the loudspeakers causes vibration in the bodywork which excites the detector.
Other devices use a high order low-pass filter in the circuit for processing the signal picked up by the microphone. This filter has a cut-off frequency of about 15 Hz which effectively eliminates output from the loudspeakers, which are always at a higher frequency. These devices are ineffective in that they also eliminate components of the noise spectrum in the audible frequency domain and retain only inaudible components which are not a problem. For these systems to operate correctly the level of the noise frequencies constituting a problem would have to be proportional to the level of the frequencies below 15 Hz, which is not the case.
A third method widely used in sound reinforcement in public places entails measuring the noise level in the absence of any signal. These systems operate very well if the signal is "chopped" like the human voice, for example, in which there are frequent periods of silence during which the noise level can be measured without error. In the case of music, however, several minutes can go by without any period of silence, and this rules out this type of device for the type of application intended.
In devices intended for use in automobiles the best results are currently obtained in the following manner: a microphone picks up a complex signal made up of the noise produced by the car and the sound from the loudspeakers. This signal is filtered by a high-pass filter to eliminate very low noise frequencies, which are not a problem for the listener, and is then rectified for comparison with the wanted or pure signal sampled on the output side of the variable gain amplifier, filtered by an identical filter and rectified. The comparator is calibrated using pink noise and in the absence of any background noise to obtain comparable voltages from the respective rectifiers of the pure signal channel and the complex signal channel.
The problem encountered with these devices is the non-linear nature of the electroacoustic system comprising the loudspeaker, the listening environment and the microphone. The response curve of the microphone commonly shows 15 dB of variation around a mean level. This 15 dB represents the uncertainty with which these devices distinguish signal from noise. If they are not to confuse the signal and noise, a safety margin of 15 dB must be adopted during setting up, i.e. the comparator which commands an increase in the gain must operate only if the voltage from the microphone channel reach
REFERENCES:
patent: 4684899 (1987-08-01), Carpentier
patent: 4864246 (1989-09-01), Kato et al.
Mottola Steven
S L X
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