Background noise reduction device in an electroacoustic line-up

The present invention relates to a noise reduction device in an electroacoustic system of recording or reproducing.

We already know the different types of noise-reducing devices to mitigate noise due to transmission or storage of information (see eg US-A 3,784,749).

In a first type of noise reduction device, such as for example one of the systems known under the names Dolby, .dbx, it is necessary to perform coding information from, registration information and perform decoding information on arrival at the return of information. The encoding can be achieved in the form of a signal compression while the decoding corresponds to an expansion of the signal. In such a system, it is essential to have two additional devices to achieve improved signal / noise ratio. Moreover, this system is ineffective to reduce, during reproduction, the background due to poor quality or altered records that have not been subject to an appropriate coding when recording, for example because they are too old.

According to another type of noise reduction device, it is used bandpass filters followed by noise removal doors, in the absence of useful signal, cut all available information as input. This system does not realize a reduction in noise suppression but can also be detrimental to the information signal itself.

Other types of noise reduction, use variable gain amplifiers that provide a comprehensive information processing, which leads to poorly performing or delicate devices to achieve.

They also offered to make a noise reduction system in which the audio or video frequency band considered is sliced ​​with a bandpass filter bank arranged in parallel and each receiving the signal input, the signal / noise ratio is improved. Each filter is associated with a programmable threshold compressor limiter for passing a useful signal that if in the bandwidth consideration, the signal level exceeds a predetermined threshold. Such a system, however, involves having a large number of filters whose pass-band overlap accurately, which increases the cost and complexity of the system.

The present invention aims to overcome the above drawbacks and to allow the implementation of a noise reduction device that is both cheap, efficient, usable without coding Signal and modular design.

These aims are achieved with a noise reduction device in an electroacoustic system of recording or reproducing including at least one correction circuit interposed in the electroacoustic system and comprising a bandpass filter receiving an input signal, a compressor limiter circuit connected to the output of the bandpass filter, a threshold detector circuit connected to the output of the compressor-limiter circuit and looped back to said compressor-limiter circuit to only allow compression by said compressor limiter circuit, of the signal from the bandpass filter if the input signal exceeds a predetermined threshold, and a summing-inverter circuit to which is applied on the one hand the signal from the compressor limiter circuit, and secondly, by means of an inverter circuit, the input signal aplliqué to the input of bandpass filter.

The device of the invention may comprise only one floor to eliminate noise associated with a band of specific frequencies.

However, the device can advantageously include several correction circuits cascaded in the electroacoustic system and comprising band-pass filters whose pasantes bands are different but not necessarily intersect.

In this case, it is preferable that the pass-band filters of the correction circuits bandpass cascaded successively be increasing.

According to one feature of the present invention, the threshold detection circuit comprises a variable gain linear amplifier connected to the output of the compressor limiter circuit, and a voltage reference circuit.

More specifically, the threshold detection circuit further comprises a delay circuit for controlling the compressor limiter circuit, with a predetermined time constant adapted to the pass-band of filter.

According to a particular embodiment, the threshold detection circuit comprises a voltage doubler circuit connected to the output of the variable gain linear amplifier and the voltage dereferenced circuit comprises a Zener diode to the mons.

According to another advantageous feature, the compressor-limiter circuit comprises an operational amplifier and a V-MOS field effect transistor controlled by the threshold detector circuit for selectively short-circuiting the input of the operational amplifier when the circuit threshold detection détacte exceeding a predetermined voltage threshold.

Other characteristics and advantages of the invention appear from the following description of a particular embodiment given by way of example, with r eference to the accompanying drawings, in which:<ul><li>Figure 1 is a block diagram showing a correction circuit incorporated in a noise reducer according to the invention,</li><li>Figure 2 is a more detailed diagram of an exemplary embodiment of the correction circuit of Figure 1,</li><li>3 shows diagrams showing the evolution of a compensation voltage V_L depending on the input voltage V_e for the maximum and minimum values ​​of the linear gain of the variable gain amplifier of the correction circuit of Figures 1 and 2.</li><li>4 shows charts showing changes in the output voltage V_S depending on the input voltage V_e for the maximum and minimum values ​​of the gain of the variable gain amplifier of the correction circuit of Figures 1 and 2, and</li><li>Figure 5 shows curves giving the change in the output voltage V_S in function of the frequency f for different levels of input signal.</li></ul>

Shown in Figure 1 the general diagram of a correction circuit 100 that provides dynamic filtering an input signal V_e applied to the input 11 of the circuit 100 to provide the output 12 of the correction circuit 100 an output voltage signal V_S whose signal / noise ratio is improved in a predetermined frequency band.

The correction circuit 100 thus acts in a limited frequency band centered on a frequency f_not predetermined. A plurality of correction circuits 100 each acting in a limited frequency band which may or may not overlap with the band of other frequency correction circuits may be cascaded one after each other to improve the signal / noise over a wider frequency range. In this case, it is preferable that the correction circuitry acting in the low frequencies and having the closest frequency bands are arranged upstream of the correction circuits operating in the high frequencies and having strips of wider frequencies.

For example, a noise reduction device according to the invention may comprise five or six floors of 100 cascaded correction the first two floors operating in the low frequency spectrum to reduce disturbance created in the supply frequency the network (50 or 60 Hz) or corresponding to harmonic thereof, and the three or four correction stages following acting at the top of the frequency spectrum in a range of approximately 3 to 15 kHz to eliminate the superimposed breath the useful signal

The noise reducing device according to the invention can naturally operate with a greater number of correction circuits cascade or otherwise with only one correction circuit in the case of performing correction around a given frequency . The device has the advantage of being easily expandable because of its modular design, and be relatively inexpensive since the different correction circuits or modules have the same overall structure. Especially, the device of the invention can reduce more or less the noise level over the entire spectrum according to the number of floors of correction, but, whatever the number of stages, the useful signal is fully retained.

We will now describe more particularly the correction circuit 100 of Figure 1. The input signal V_e available on the right input 11 is applied firstly to a band pass filter 2 and on the other hand, via a resistor 71 to the inverting input of an operational amplifier 70 which constitutes, with the resistor 71 and a feedback resistor 72 an inverter circuit 7. the output of the inverting amplifier 7 is applied to one input of a summing amplifier 9.

The output of the bandpass filter 2 is connected to the input of a compressor-limiter circuit 3, the output 4 is itself connected to a second input of the adder circuit 9 by means of an element 8 of adjusting the level of voltage V_L, The same sign as the input voltage V_e, Available at the output 4 of the compressor-limiter circuit 3. The output 4 of the compressor-limiter circuit 3 is further connected to the input of a linear amplifier 5 adjustable gain whose output is applied to a detection circuit 6 threshold that controls itself the compressor-limiter circuit 3.

The summing amplifier 9 comprises an operational amplifier 90 whose inverting input is connected to the output by a feedback resistor 93 and receives the signals from the output 4 of the compressor limiter 3 and the output of the inverting amplifier 7 by via resistors 91, 92 respectively.

FUNCTIONING The correction circuit 100 of FIG 1 is as follows.

The linear variable gain amplifier 5 looped by the threshold detection circuit and the time constant 6 of the compressor-limiter 3 shows an adjustable gain between a minimum value G_vmin and a maximum value G_vmax. The threshold detector circuit 6 itself being set to respond when the output voltage of the variable gain amplifier 5 exceeds a predetermined level or threshold, the operation of the compressor-limiter 3 will be determined by the value of the gain G_v the amplifier 5, which define a trigger threshold of the effective operation of the compressor-limiter 3. In practice, if V_b represents the voltage of the background noise at the input of compressor limiter 3, the gain G_v of the amplifier 3 is set such that the compressor-limiter 5 plays a role of compression that if the voltage at the output of filter 2 exceeds the value V_b. Thus, for all the signals V_e input such that, in the passband of the filter 2, the output level of this filter is less than a predetermined threshold V_bThe compressor-limiter maintains a gain equal to 1 and outputs in point 4 a voltage signal V_L substantially corresponding to the input voltage of the compressor-limiter 3 and the same sign as the latter. The summing circuit 9 then performs the addition of the input signal V_e reversed and the signal V_L, Reversing the result of this addition is to say provides an output signal V_S equivalent to the input signal Ve but decreased the signal V_Lthe bandwidth of the filter 2. There has been much less noise in this bandwidth.

In contrast, when the voltage at the dulimiteur compressor inlet 3 exceeds the level V_bThe limiteurcompresseur 3 shows a significant attenuation coefficient and no longer issues in point 4 a signal V_L negligible level capped at level V_G. The signal V_S the output of the summing amplifier 9 corresponds substantially to the signal Ve applied to the eentrée 11 terminal of the correction circuit 100, minus a correction value V_L which represents only a small proportion of the value V_e.

Figures 3 and 4 show the curves representing the one hand the correction voltage V_L at the outlet 4 of the compressor-limiter 3 and secondly the output voltage V_S at the output 12 of the correction device 100 according to the input voltage V G for two adjustment values_vmax (Curves 101 and 201) and G_vmin (Curves 102 and 202) of the variable gain amplifier 5 looped back to the compressor-limiter 3, and the filter bandwidth-2.

Figure 5 shows the shape of the output voltage V_S depending on the frequency f with a correction device 100 equipped with a bandpass filter 2 centered on a frequency fn, for different decreasing values ​​of the input voltage V_e (Curves 301 to 305). It is noted that for high level input voltage higher than the threshold voltage, the compressor-limiter 3 outputs a signal V_L very low compared to V_e, So that the level of the output voltage V_S is practically not changed around the frequency fn (curve 301). In contrast, when the level of the input voltage V_e decreases, resulting in an overall decline in the level of output voltage V_SThe compressor-limiter 3 produces a less strong attenuation in the bandwidth of the filter 2 centered on the frequency f and voltage V_Lthe output of the compressor limiter 3 represents a higher fraction of all the input voltage V_e the latter is lower (curves 302, 303, 304). For very low level of the input voltage V_e (Curve 305), the output signal is reduced to zero even around the frequency f, the voltage V_L delivered by the compressor-limiter 3 totally offsetting the input voltage V_e.

Referring now to Figure 2 which relates to a particular embodiment of corection circuit 100 of Figure 1.

The compressor-limiter circuit 3 is connected via a coupling capacitor 21 to the bandpass filter 2 whose input is connected to the input terminal 11 of the correction circuit 100. Lecircuit compressor limiter 3 comprises an operational amplifier 30 whose the inverting input is connected firstly to ground via a resistor 37 and secondly to the output of the operational amplifier 30 via a resistor 38. the non-inverting input of the operational amplifier 30 is it -even connected via a resistor 31 to the coupling capacitor 21 and a resistor 33 to ground. The drain-source space of a field effect transistor 34 of the V-MOS is connected between the non-inverting input of the operational amplifier 30, which receives signals from the filter 2, and ground. The gate of transistor V-MOS 34 is itself connected firstly to ground via a capacitor 36 and on the other hand via a resistor 35 to the circuit 6 sensing threshold and time constant. A capacitor 32 in parallel with the resistor 31 allows a compensation of stray capacitance due to the presence of the V-MOS transistor 34.

The outlet 4 of the compressor-limiter circuit 3 is connected via a coupling capacitor 41 to the non-inverting input of an operational amplifier 50 which, together with associated resistors 51-54 a linear amplifier 5 with adjustable gain. A resistor 51 is connected between the noninverting input of the operational amplifier 50 and ground, while a resistor 52 in series with an adjustable resistor 53 is connected between the inverting input of the operational amplifier 50 and ground. A resistor 54 is connected between the output and the inverting input of the operational amplifier 50.

The threshold detection circuit 6 connected to the output of the linear amplifier 5 comprises a voltage doubler rectifier circuit 61 to 65, a voltage reference element 67 and an RC 68 circuit 69 providing a time constant. The voltage doubling circuit comprises an operational amplifier 63 whose noninverting input is connected to earth, whose inverting input is connected via a resistor 65 to the output of the linear amplifier 5, and whose output is looped back to the inverting input via a resistor 64. the output of the operational amplifier 63 is connected to the anode of a diode 62 whose cathode is connected to the cathode of a diode 61 whose anode receives the output signals the linear amplifier 5. the cathodes of diodes 61 and 62 are connected by a resistor 66 to the output 66A of the threshold detection circuit 6 connected by the resistor 35 to the gate of transistor V-MOS 34 for controlling the limiter circuit 3-compressor.

Within the threshold detection circuit 6, the voltage level of reference is given by a Zener diode 67 connected between the positive and ground 66A. A time constant circuit consisting of the capacitor 69 in parallel with a resistor 68 is further connected between the positive and ground 66A. This time constant circuit is adapted to the bandwidth of the filter 2. In the case of a filter 2 centered on a high frequency, the time constant introduced by the RC circuit 68, 69 is indeed smaller than in the case of a filter 2 centered on a low frequency corresponding to the grave, to prevent a hunting phenomenon in the compressor limiter circuit 3.

The V-MOS transistor 34 of the compressor limiter circuit 3 is held clamped by the circuits 5 and 6 when the input signal V_e is too low so that the transistor V-MOS 34 is equivalent to a resistance of several megohms and does not cause attenuation of the input signal in the limiter circuit 3.

The signal available at point 4 can then be transmitted by the dividing bridge 8 contitué a resistor 81 and an adjustable resistor 82 to the input of summing circuit 9 connected to the resistor 91. The summing circuit 9 then contributes to subtract the bandwidth of the filter 2, a voltage V_L close to V_e the input signal V_e applied by the inverter circuit 7 on the second input of the adder circuit 9, thereby reducing the background noise.

However, when the signal V_e contains a useful signal exceeding a predetermined threshold by the circuits 5 and 6, the V-MOS transistor 34 is rendered conductive and plays the role of limiter now the output signal of the operational amplifier 30 to said predetermined threshold value; sorteque of the signal applied to resistor 91 of the summing circuit 9 serves only to mask the noise of residual background while the useful signal is not altered. practically not modified from the input signal V_e.

The correction device of the invention can greatly improve the dynamics of an audio or video source. Thus, the variable gain amplifier 5 can be set for example so as to have a gain between 0 and 60 dB. The compressor limiter may itself have an effective range extending over 80 dB.

Furthermore, the invention is applicable regardless of the origin of the noise superimposed on the useful signal, that noise comes from recording on a medium, eg magnetic, for transmission over the air, or breath electronic circuits upstream such as preamplifiers.