FR2474736A1 - NOISE REDUCING CIRCUIT FOR A RECORDING APPARATUS - Google Patents

Abstract

A. NOISE REDUCTION CIRCUIT, ESPECIALLY FOR VCRs. B. CIRCUIT CHARACTERIZED IN THAT IT CONSISTS OF A SUBTRACTOR 2 RECEIVING THE SIGNAL GIVEN 1, TO APPLY IT TO A VARIABLE GAIN AMPLIFIER 3 THEN TO A LOW PASS FILTER 4 AND TO THE OUTPUT TERMINAL 5 AS WELL AS 'A REACTION CIRCUIT IN THE FORM OF A LOW PASS FILTER 6 WHOSE OUTPUT IS CONNECTED TO THE INPUT - OF THE SUBTRACTOR 2. C. THE INVENTION CONCERNS MAGNETIC RECORDING TECHNIQUES.

Description

The present invention relates to a noise reduction circuit for a

  apparatus for recording and reproducing information signals and in particular a noise reducing circuit for reducing the noise generally associated with an information signal reproduced by a recording apparatus

and signal reproduction.

  Noise reducing circuits to reduce

  the noises and distortions accompanying a signal of

  reproduced are well known in general. Such cir-

  Cooked noise reducers are intended to increase the dynamic range of the signal that can be recorded and reproduced on a recording medium such as a magnetic tape. A noise reducing circuit is known, in which there is provided an encoder for the signals to be recorded and a complementary decoder for the signals to be reproduced. The encoder generally comprises a level compression circuit and a high frequency pre-emphasis circuit which emphasize the higher frequency components of the information signal to be recorded, this being done by linking the level of reverse at the information signal level. The decoder generally comprises

  a level expansion circuit and a de-emphasis circuit

  high frequency to perform a complementary operation.

  on the information signals to be reproduced.

  In the Dolby noise reduction system (registered mark), the low level input signal is amplified with a substantially constant gain until the input signal reaches a predetermined level. Then the amplification of the input signal is reduced until one reaches another high level and the amplification is done again with

  an essentially constant gain. In addition to this amplification

  input signal before recording, an emphasis circuit is used to pre-emphasize the high frequency components of the input signal. This operation is generally called signal compression. After properly compressing the input signal, it is recorded. Complementary expansion of the signal during reproduction

  of the signal above, that is to say that we de-emphasize the

  pre-accented high frequency speakers and the signal thus de-emphasized is amplified with a gain less than unity. This gain is practically constant in a predetermined range of

  relatively low signal levels; when the signal reproduces

  duced exceeds a predetermined level, the gain is

  than to reach an ever higher level.

  The Dolby noise reduction system mentioned above has a relatively simple construction and has been used extensively for home-use installations such as tape recorders or tape recorders.

  like. However, although the Dolby system ensures

  improvement of the dynamic range of the tape recorder, this improvement is generally limited to about 10 dB; this improvement first appears in the frequency range exceeding 1 KHz. In addition, the aforementioned variations of the gain of the compression amplifiers and the level expander are not linear; because of this non-linearity of the gains, it is difficult to ensure the adaptation between the coding and the decoding. Distortions can appear in

  signals whose levels are intermediate. -

  Another noise reduction system called

  "DBX system" (trade mark) is described in U.S. Patent 3,789,143.

  One of the advantages of this DBX system compared to the system

  Dolby is that the gains of the amplifiers ensuring the compres-

  The expansion and expansion of the signals, ie the compression and expansion coefficients, are virtually constant regardless of the levels of the input information signals. By way of example before recording the information signal on a magnetic tape or the like, the input information signal is compressed at a constant compression ratio k. When the compressed signal is subsequently reproduced, the reproduced signal is dilated in a constant ratio 1 / k

  that is to say, an expansion ratio which is the opposite of the

  Compression port. Since constant expansion and compression ratios are used throughout the entire range of

  signals, the non-linearity of the Dolby system is avoided and

  lite the adaptation of the levels between the recorded signals and

  the reproduced signals. Moreover, in the DBX system, the improvement

  apparent range of the dynamic range of the tape recorder is of the order of about 40 dB and provides an interesting noise reduction practically throughout the range of

  audio frequencies between 20 Hz and 20 KHz.

  However, the particular compression and expansion characteristics of the noise reduction system described above are mainly obtained for constant levels of the input signals, ie signal levels which do not undergo sudden transitions. . In others

  In short, the benefits of noise abatement systems depend on

  mainly tooth static characteristics. However, there are difficulties in the transient characteristics

  dynamics of these systems. For example, the information signal

  Since the recording rate has a relatively low signal level, the gain or compression ratio of the coding amplifier may be relatively high. If, under these conditions, the information signal undergoes a sharp increase in the level

  signal, that is, whether there is a significant positive transition

  aunt, the gain of the amplifier or compression ratio will not be reduced as fast as the ratio of increase

  the signal level. Thus the gain or the ratio of compres-

  should be reduced for the processing of the information signal.

  high level, in fact this ratio remains at the previous high level. As a result, the strong transition is amplified with a relatively high gain, resulting in a compression signal exhibiting an "overshoot", i.e.

  that the level of the compressed signal is much too important.

  This important level, when recorded, causes saturation

  ration of the magnetic medium and as a result of the deformations in

  the recorded signal and in the signal reproduced later.

  Another disadvantage of the noise reduction systems described above is that they can be modulated by noise. In the case of noise modulation, the noise components vary as a function of the variations of the level of the input signal. Such variations of the noise components or noise modulations are very strongly perceptible and are

  very annoying when they accompany a reproduced audio signal

  Duit. This phenomenon is accentuated when the components of

  frequency of the input signal are very different from the

  noise frequency. For example, if the information signal

  mation is an audio signal representing the sound of a piano, the modulation of noise is distinct and distinct and is not masked even if the signal level is increased

of information.

  U.S. Patent 4,162,462 discloses a solution for reducing noise modulation in a noise reduction circuit. According to this solution, the components are pre-emphasized

  high frequency of the information signal before recording them

  when the information signal has low and medium signal levels and a slight pre-emphasis is

  when the information signal has high levels.

  When the information signal processed as described above is reproduced, the high frequency components experience a relatively high deemphasis when the reproduced signal has low and medium signal levels; components

  high frequency are subject to a relative de-emphasis

  low when the playback signal is at a high level.

  Although this solution reduces the inconvenience of

  noise, there nevertheless remains a saturation of the magnetic medium as a result of exceeding the compressed signal. To overcome the disadvantages of exceeding described above, it has also been proposed to increase the response speed of the level compression circuit. However, if the speed of response is increased, the relative improvement in

  removal of the overrun is accompanied by a deterioration

  ration of the noise modulation characteristic.

  There is also known another noise reduction circuit which minimizes the overshoot by using several substantially identical noise reduction circuits which are connected in parallel. Each noise reduction circuit works on a selected part of the frequency spectrum of the input information signal. The output signals of these different noise reduction circuits are combined or mixed to provide a combined, global level information signal suitable for recording. However

  - the use of several noise reduction circuits

  parallel is a relatively complex solution and

  expensive. For example, if n reduction circuits are used

  noise, we multiply by n the total cost of the noise reduction system compared to a reduction circuit of

unique noise.

  The present invention aims to create a noise reduction circuit that overcomes the disadvantages of the solutions

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  known, and which can be used in a recording installation.

  and / or reproduction of information signals,

  cable to an encoder to compress the level of an information signal.

  before it is recorded as well as in a decoder to ensure the expansion of the reproduced signal level, to dilate the apparent dynamic range of the recording and playback system.

  reproduction according to a coefficient of the order of 20 to 30 dB.

  The invention also aims to create a noise reduction circuit giving a pre-emphasis and a

  deemphasis without external manual adjustment, which is used

  ble with a kernel system without reducing its operation

  and avoiding the transient saturation of the recording medium

  which could result from overtaking.

  It is also an object of the invention to provide a level compression circuit whose transfer characteristic is more frequency-sensitive for the low-level input signals than for the high-level input signals, to arrive at a low-level input signal. stronger pre-emphasis of the low level input signals, and similarly to achieve a stronger deemphasis for the relatively low level signals than for the relatively high level signals at the

  reproduction of the signals of the recording medium.

  It is also an object of the invention to provide a compression and expansion circuit which can be switchably connected to provide level compression in combination with signal recording and level expansion in combination with the signal reader, while leading to a

  simple and inexpensive construction circuit.

  For this purpose, the invention relates to a noise reduction circuit comprising a first signal path having

  a variable gain amplifier which receives a sub-signal

  traction to amplify this signal with a controlled gain and a means to ensure a significant disconnection of the components

  high frequency of the subtraction signal amplified by the ampli-

  variable gain factor relative to the low frequency components to provide an output signal, a second signal path to provide at least a relatively weak deemphasis of the high frequency components of the output signal by

  to low-frequency components, a means of subtracting

  to subtract the output signal from the second signal path with respect to an input information signal and to provide the retracted signal in response thereto as well as means for controlling the gain of the variable gain amplifier for

  that the gain is higher when the level of the signal of in-

  entry training is relatively high and a lower gain

  when the level of the input information signal is relatively

weak.

  The present invention will be described in more detail with the aid of the accompanying drawings, in which: FIGS. 1 and 2 are graphs of the compression and expansion characteristics of two circuits

  noise reduction according to the prior art.

  FIG. 3 is a block diagram of a decoder

  according to a basic embodiment of the invention.

  FIG. 4 is a more detailed block diagram of a

  practical embodiment of the decoder of FIG.

  FIG. 5 is a graph of the level / frequency characteristics of the decoder of FIG. 4 for different

entry levels.

  FIG. 6 is a graph of the characteristic

  input / output of the decoder of Figure 4 for various frequencies

  quences. FIG. 7 is a wiring diagram of a mode of

  realization of the decoder circuit of FIG.

  FIG. 8 is a partially wired block diagram of the decoder of FIG. 4 used as an encoder or as

  decoder in a noise reduction system.

  DETAIL DESCRIPTION OF THE DIFFERENT EMBODIMENTS

PREFERRED:

  Figure 1 gives the graph of the compression / expansion characteristic of a Dolby noise reducer whose input and output signal levels are given in decibels (dB). Curve R represents the input / output level compression characteristic and curve P represents the input / output level expansion characteristic of the Dolby noise reduction system. We see that for signals

  relatively low level of entry, we arrive at a noticeable

  uniformly greater than unity until it reaches an intermediate level, for example between -30 dB and

  0 dB, then the level compression linearity of the

  The input / output risk is no longer true. It should be noted that this characteristic of non-linearity makes it difficult to adapt the levels during coding and decoding as indicated. The curve in phantom in Figure 1 represents the "low flat", for which the levels of the

  input and output signals are constant for compres-

expansion and expansion.

  Fig. 2 is a graphical representation of the level compression input / output characteristic R and the level expansion input / output characteristic P of the DBX noise reducer described above. It should be noted that in this system, the compression and expansion coefficients are substantially constant throughout the range of the input signal levels. In this graph, the dashed line curve also represents the "low flat" response. However

  as shown, Dolby and DBX systems have

  for their dynamic transient characteristics.

  FIG. 3 schematically shows a noise reduction circuit 10 according to a basic embodiment of FIG.

  the invention to reduce the noise contained in a signal of

  reproduced training. The noise reduction circuit 10 consists of an input terminal 1 which receives the reproduced information signal such as, for example, the audio signal read on a magnetic tape and which in turn provides the information signal reproduced at the same time. addition input of a subtractor 2. A first signal path consists of a variable gain amplifier 3 which receives the output signal of the subtractor 2 and whose gain is controlled by the reproduced information signal applied

  at input terminal 1, after straightening and smoothing (not shown

  shown in Figure 3). The output of the variable gain amplifier 3 is coupled to a low-pass filter 4 as well as to the first signal path in which the high-level components

  frequency of the signal provided to it are de-emphasized.

  The signal resulting from the low-pass filter 4 is then applied to the output terminal 5 as well as to the subtraction input of the subtractor 2 via a second signal path

which has a low pass filter 6.

  Variable gain amplifier 3 amplifies the signal

  output of the subtractor 2 according to a variable gain. In part-

  In particular, the gain G of amplifier 3 is determined by the

8 2474736-

  gain control voltage V derived from the information signal c

  applied to the input terminal 1 after straightening and smoothing.

  The gain G of the variable gain amplifier 3 increases when

  that the gain control voltage V increases for example according to the relation G = K. V or (G = e KVC) relations in which K is a constant. In this way, the gain of the amplifier 3 is directly related to the level of the information signal applied to the input terminal 1, so that the

  gain is relatively high when the signal level of

  the input terminal is relatively high and conversely the gain is relatively low when the

  information signal applied to the input terminal 1 is relatively

  low. The variable gain amplifier 3 ensures

  the expansion of the level of the signals it receives.

  The low-pass filter 4 which is also part of the first signal path gives high frequency components

  very de-emphasized in the output signal of the

  In other words, the low-pass filter 4 actually provides a significant de-emphasis of the low frequency components of the signal supplied to it with respect to the high frequency components. For example, the high frequency components are preferably de-emphasized

  compared to the low frequency components according to a coeffi-

  corresponding to approximately 20 dB.

  - The low-pass filter 6 which forms the second signal path ensures a relatively weak deemphasis of

  high frequency components of the output signal of the pass filter

  low 4 compared to that of the low frequency components. For example, the high frequency components are reduced or attenuated with respect to the low frequency components of the order of 6 dB. By connecting the low-pass filter 6 to the output of the low-pass filter 4, it has been found that a smoother overall de-emphasis characteristic is obtained than in the prior art and that it simplifies the circuit. Alternatively, the low-pass filter 6 may function primarily as an attenuator to provide uniform de-emphasis over the entire frequency range i.e. for both the high-frequency components and the low-frequency components. However, it is preferable that the low-pass filter 6 gives a low de-emphasis characteristic for the

9 2474736

  high frequency components of the signal applied to it.

  In this case, since the output of the low-pass filter 6 is

  information signal applied to the input terminal 1 of the

  subtractor 2, this subtractor actually provides a

  less pre-emphasis to higher components

frequency of the information signal.

  Thus, the input / output characteristic of the noise reduction circuit 10 depends on which of the first, second, dominant signal paths. In particular, it should be recalled that the input / output characteristic of the subtractor 2 gives a lower pre-emphasis of the high frequency components of the signal applied to the variable gain amplifier 3. For example, the level of the high frequency components may exceed by only a few dB the level of the low frequency components When the level of the information signal applied to the input terminal 1 is low, the gain of the variable amplifier 3

  is relatively low. This does not result in a significant change

  both levels of the high and low frequency components so that the high frequency components of the amplified signal applied to the low-pass filter 4 have a level which is only

  a few dB higher than that of the low frequency components.

  Since the low-pass filter 4 ensures a significant deemphasis

  of the high frequency components of the signal applied to this filter, for example of the order of 20 dB as indicated, the output signal of the low-pass filter 4 which is applied to the output terminal 5 has high components. frequency which are significantly de-emphasized with respect to the low-frequency components of this signal. In other words, the first signal path formed by the variable gain amplifier 3 and the low-pass filter 4 has a dominant effect and the influence of the second signal path constituted by the low-pass filter 6

  is minimal on the signal supplied to the output terminal 5.

  This significant deemphasis of the high frequency components of the signal supplied to the output terminal 5 decreases when the level of the information signal applied to the terminal

  input 1 increases. So when the level of the information signal

  The input provided at input terminal 1 is high, the gain of variable amplifier 3 is also high. Since the signal applied to the variable gain amplifier 3 has high frequency components which are slightly pre-emphasized by

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  compared to the low frequency components, the amplification provided by the variable gain amplifier 3 gives a pre-emphasis

  noticeable high-frequency components compared to

  low frequency, for example amplification greater than 20 dB. In other words, the variable gain amplifier 3 has an expansion characteristic for the signal applied thereto. Thus when the low-pass filter 4 has a

  important deemphasis feature for frequency

  these high frequency of the signal applied to it, for example of the order of 20 dB, the resulting signal obtained on the output terminal 5 has a high frequency pre-emphasis characteristic which is light or weak. It should be noted that for high level information signals, the second

  signal path plays an important or dominant role in determining

  undermine the signal output characteristic applied to the

output terminal 5.

  Figure 4 shows a practical embodiment of a noise reduction circuit 10; in this diagram, the elements corresponding to those described for the circuit of the

  Figure 3 bear the same numerical references. As a representative

  4, the variable gain amplifier 3 consists of a voltage controlled amplifier (VCA) 31 and an adder 33 receiving the output signal of the voltage controlled amplifier, on one of the inputs, the other input receiving the input signal of the voltage control amplifier 32, the latter signal being applied through the resistor 32. The combination of the VCA oscillator 31 and the adder 33 gives a gain for the signal which depends on the level of the information signal applied to the input 1. In other words, the low level signals are processed with a low gain and the high level signals are processed

with a high gain.

  The noise reduction circuit 10 of FIG. 4

  further comprises a gain control circuit 7 for

  to request the gain of the oscillator VCA 31; this circuit consists of a weighting circuit 71 coupled to the input terminal 1 and a rectifying and smoothing circuit 72 for rectifying and smoothing the output signal of the weighting circuit 71 and the

  provide the VCA oscillator 32 as a voltage signal

  gain. The weighting circuit 71 has a high-pass filter characteristic which, in one example,

  can be directly opposed to the characteristic of

  In other words, the weighting circuit 71 in this case gives a characteristic of pre-emphasis of the high components.

  frequency of the signal provided to it in relation to the

  low frequency. Alternatively, it is possible to connect the gain control circuit 7 to the input of the VCA oscillator 31 and not to the input terminal 1 although in this case it is necessary to

  make modifications to the weighting circuit 71.

  In addition, the noise reduction circuit 10 of FIG. 4 comprises a core or anti-limiter circuit 34 provided between the subtractor 2 and the VCA oscillator 31. When the level of the signal applied to the anti-limiter circuit 34 is low ,

  this circuit 34 does not theoretically have any effect on the signal.

  However, when the signal level is high and exceeds a predetermined level, the signal supplied to it is developed which results in an additional expansion of the signal applied to the variable gain amplifier 3. The circuit 34 will be described more detailed later; it is intended for a decoder or noise reduction circuit 10 and

  has a complementary coder to avoid or limit the

  transient signal to be recorded on the magnetic tape.

  that and that causes a distortion of the signal by the saturation

magnetic tape.

  As previously, the low-pass filter 4 ensures a significant deemphasis of the high frequency components with respect to the low frequency components contained in the signal supplied to it by the adder 34.

  Low-pass filter 6 described previously ensures a de-emphasis of

  relatively low frequency components of the

  signal provided in relation to the low frequency components.

  FIG. 5 is a graph of the level expansion characteristics of the noise reduction circuit 10 of FIGS. 3 and 4. The abscissa in FIG. 5 shows the frequency of the information signal applied to the input terminal. 1; the ordinates represent the level of the output signal at terminal 5 (dB). Each curve of Figure 5 corresponds to a particular signal level. We see that when the

  level of the information signal is relatively low, the

  it has the risk of high frequency deemphasis of the first signal path dominating to determine the characteristic of the output signal. In other words for such a low level signal, the gain of the VCA'31 oscillator is also low, so that the slightly pre-emphasized high frequency signals are supplied to this circuit so that the high-frequency components

  frequency are slightly pre-accentuated. However, as the low-pass filter 4 ensures a significant deemphasis on

  high frequencies for the signals applied to it, the -

  High frequency components of these low level signals are very attenuated. This is shown in particular by the three lower curves in FIG. 5 which correspond to input levels Vi of the information signal respectively equal to in

  -30, -40 and -50 dB. It should be noted that the anti-

  limiter 34 has virtually no effect on the signals of

  low level applied to this circuit and does not influence

  makes the frequency output characteristic of these signals.

  When the level of the information signal applied to the input terminal 1 is high, the subtractor 2 provides a high level signal whose high frequency components are slightly pre-accentuated with respect to the low frequency components for the circuit 34. indicated for signals

  of high level, circuit 34 functions as an expander circuit

  to ensure the development of the signals applied to it. In other words, the difference in levels between the high frequency and low frequency components is further increased. The accented signal is then supplied to the VCA oscillator 31. As the gain of the VCA oscillator 31 is controlled by the control circuit 7 to be at a high level at this time, the accented signal provided by the circuit 34 is emphasized by complementary way by the variable gain amplifier 3 to give a very strongly accentuated signal whose high frequency components are at a much higher level than the low frequency components. This strongly accentuated signal is applied to the low-pass filter 4 which ensures a significant deemphasis of the high frequency components. For levels

  Vin 'high input of the information signal, and which correspon-

  At 0 and 10 dB, the circuit 34 and the variable gain amplifier 3 give a signal whose high frequency components are accentuated with respect to the low frequency components of dB.

  As the low-pass filter 4 ensures a de-emphasis

  frequency of the signal applied to it, but only on the order of about 20 dB, the signal applied to the output terminal 5 (Figure 5) has high frequency components slightly pre-emphasized relative to

  to the low frequency components of this signal. The line of separation

  ration between the pre-emphasis and the deemphasis of the high frequency components of the signal applied to the output terminal 5 is shown in FIG. 5; this line corresponds to a Vin input signal level equal to -10 dB. In other words, for Vin input levels below -10 dB, the first signal path and in particular the low-pass filter 4 of this path has a dominant effect on the signal applied to the output terminal 5 However, for high levels of the information signal, ie for Vin signals greater than -10 dB, the influence of the low-pass filter 4 is considerably reduced so that the slight pre-emphasis coming from the low-pass filter 6 and which is significantly amplified or accentuated by the variable gain amplifier 3 provides a dominant effect

  on the signal applied to the output terminal 5.

  The expansion or level enhancement characteristics of the noise reduction circuit 10 for signals whose frequencies are 100 Hz, 1 KHz and 10 KHz are represented by the respective curves in FIG. 6. This figure shows that the level of expansion corresponds to a wider range and a higher degree for high frequency components (usually 10 KHz) than for low frequency components (usually 100 Hz and 1 KHz). The dashed line in Figure 6 represents the answer "low flat"

usual.

  The previous remarks show that the circuit

  of noise reduction according to the invention ensures a de-emphasis of

  variable which is to say that this circuit gives different de-emphasis curves for different levels of the input signal. Given this variable deemphasis, we arrive at a markedly greater deemphasis for the beach.

  high frequencies when the input signal is relative-

  low to reduce the effect of noise modulation which is more noticeable at this time. We arrive at a relatively flat deemphasis characteristic or a characteristic

14 2474736

  low emphasis for high frequencies when the input signal level is relatively high. This solution is preferable if the level of the input signal is relatively high because it can be recorded on the magnetic medium without requiring any pre-emphasis. The invention described in the previous embodiments is of relatively simple construction and therefore inexpensive. Nevertheless, this invention makes it possible to provide a variable deemphasis as described above without requiring any manual or external adjustment. Ensuring significant deemphasis in a high frequency range when

  level of the input signal is low,

  the previous phenomenon of noise modulation and in many cases

  case cut it can be effectively removed. In addition, the

  The present invention makes it easy to use an anti-tamper circuit.

  limiter 34 as a limiter circuit in the corresponding coding section to avoid transient saturation of the magnetic recording medium due to overshoot during a sudden increase in the level of the signal which could not be

compensated sufficiently quickly.

  FIG. 7 shows an embodiment of a diagram of a noise reduction circuit 10 according to FIG. 4; in this figure, the elements that correspond to those of Figure 4 bear the same references. According to FIG. 7, the subtractor 2 consists of two addition resistors 21, 22 and an operational amplifier 24. In particular, a terminal of the resistor 21 is connected to the input terminal 20

  and the other terminal is connected to one end of the resistor 22.

  The other end of the resistor 22 is connected to the output of the operational amplifier 24 which plays the role of an inverter to invert the output signal of the second signal path and

  give the necessary subtraction signal.

  The junction point of the resistors 21, 22 is connected to the inverted input of an operational amplifier 23 which supplies the subtraction signal to the first signal path

  and in particular at the inverted input of the operational amplifier

The first signal path also includes a feedback resistor 36 connected between the output and

the input of the amplifier 35.

  In general, the gain of an amplification circuit

  cation formed by an operational amplifier depends on the

  reaction, that is to say the impedance connected between the output and the input of the operational amplifier, divided by the input impedance that is to say the impedance connected to the input of this amplifier. The gain of the amplification circuit can thus be adjusted by modifying either the reaction impedance or the input impedance. In the embodiment of FIG. 7, the reaction impedance is determined by the feedback resistor 36 while the input impedance which will be examined later is adjusted according to a control signal applied to it for change the gain of

  the variable amplifier 3 formed by the operation amplifier

35.

  In particular the input impedance of the amplifier

  Operator 35 is controlled by three impedance paths

  connected in parallel between the output of the op-

  rational 23 and the inverted input of the operational amplifier

  The first impedance path consists of a resistor 37 of fixed value; the second impedance path is constituted by a variable resistor 38; the third impedance path consists of an anti-limiting circuit 34 connected in series with a fixed value resistor. In particular, the value of the variable resistor 38 changes according to the control signal of the control circuit 7. For example, the variable resistor 38 may be constituted

  by a photosensitive component such as a photoconductive cell

  Cds, a photoresistor or the like have an imperative

  or a resistance varying according to the intensity of

  the light that this component receives. For example, the photo element

  The sensitive sensor may cooperate with a photodiode or other light source capable of transmitting light to a photosensitive member according to a control voltage applied thereto. This control voltage is supplied by a control circuit 7 formed of the weighting circuit 71

  and the straightening and smoothing circuit 72. When the

  As the control voltage increases, the intensity of the light emitted by the light emitting source increases in the same manner to reduce the resistance or the impedance of the photosensitive element.

  and thus increase the gain of the variable gain amplifier 3.

  Conversely, when the control voltage decreases, the intensity

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  light emitted by the light emitting source decreases correspondingly to increase the impedance of the photosensitive element and thus reduce the gain of the amplifier to

variable gain 3.

  As an alternative to the photosensitive element mentioned above, it is possible to envisage a variable resistor element 38 formed of a field effect transistor (FET), a bipolar junction transistor or the like whose impedance is controlled as a function of the control voltage applied to it by

  the control circuit 7. Thus when the impedance of the transis-

  tor FET or bipolar transistor varies, the gain of the amplifier

  variable gain indicator also changes.

  In the embodiment of FIG. 7t the anti-limiter circuit 34 comprises a pair of reverse-bias diode circuits connected in series on

  a fixed resistance forming part of the third imperative path

  entrance dance. In this example, each diode circuit se-

  consists of two diodes in series. It should be noted that the cir-

  cooked 34 functions as a core circuit. In other words, if each of the diodes has a voltage or cutoff level Vbe equal to 0.7 volts (silicon diode case) then the third impedance path is only conductive when the level of the signal that is applied is greater than 1.4 volts or less than -1.4 volts, that is to say only for signals given high levels either positive or negative. Thus, when the level of the input signal is low, the third impedance path is advantageously removed from the circuit for

  that the impedance of the signals applied to the amplifier

  35 is determined by the parallel connection of the resistor 37 and the variable resistor 38. However, as soon as the input level exceeds the cut-off voltage of the

  diode, the resistance of the third impedance path is

  In the input impedance circuit, this is in parallel with the resistor 37 and the variable resistor 38, which reduces the effective input impedance and increases the gain of the amplifier. In other words, circuit 34 accentuates

  the high levels of input information signal.

  The low-pass filter 4 is obtained by connecting a

  high-pass filter between the input and the output of the ampli-

  In particular, the low-pass filter 4 consists of a resistor 41 and a capacitor 42 which are connected to one another.

  series between the input and output of the amplifier-

  35 and in parallel on the feedback resistance 36.

  The output of the operational amplifier 35 is coupled to the input terminal 5 as well as to the second signal path formed by the low-pass filter 6. In particular the low-pass filter 6 is formed of a pair of series resistors 61 , 62 connected between the output of the operational amplifier 35

  and the inverting input of the operational amplifier of

  version 24 and a capacitor 63 connected between the ground and the junction point of the resistors 61 and 62. It should be noted that the output of the operational amplifier 35 supplied to the

  Low-pass filter 6 is inverted and is added to the signal of

  input formation at the junction point of the resistors 21, 22.

  The control circuit 7 coupled to the input terminal 1 and which consists of the weighting circuit 71 and the rectifying and smoothing circuit 72 gives the control voltage described above which serves to adjust the value of the variable resistor. 38 and thus the gain of the variable gain amplifier 3. The weighting circuit 71 consists of a high-pass filter formed of a first series circuit comprising a capacitor 73 and a resistor 74 as well as a second circuit serial

  connected in parallel to the first circuit in series and includes

  capacitor 75 and resistor 76. The output of the

  series circuits connected in parallel is connected to the ampli-

  A controller 77 which is preferably an operational amplifier having a negative gain and having a feedback resistor as shown. It should be noted that the weighting circuit 71 has a high-pass filter characteristic substantially equal to that of the high-pass filter connected between the input and the output of the operational amplifier 35 and forming the low-pass filter 4. The output of the amplifier 77 is coupled to the circuit 72, the latter consisting for example of a diode connected to a capacitive filter. The circuit 72 gives a continuous control signal which is a function of the level of the high frequency components that pass through the circuit.

weighting 71.

  Thus, as indicated, the noise reduction circuit of FIG. 7 gives a variable deemphasis which modifies the degrees of de-emphasis as a function of the level of the

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  input signal. In this way, the noise reduction circuit 10 of FIG. 7 provides a significant deemphasis for the high frequencies of the low level input signals, to effectively suppress the noise modulation and to allow the expansion of the dynamic range. all in

  avoiding the passing of high level input signals.

  The circuit of FIG. 7 thus gives the characteristics

  level expansion shown in Figure 5.

  In the embodiments of the invention described above, the noise reduction circuit has been used as a level-boosting circuit in a signal decoder.

  information recorded in the form of magnetic signals.

  It is also necessary to provide a level compression circuit in an encoder, whose compression characteristics

  level are complementary to those shown in Figure 5.

  In this way, the noise reduction circuit of FIG. 4 can also serve as an encoder (FIG. 8). More particularly, the noise reduction circuit 10 is connected to the negative feedback path (feedback) of an operational amplifier 210 whose non-inverted input is

  connected to the input 201 to receive the input signal to be

  gistrer; the inverted input is connected to the output terminal 5 of the noise reduction circuit 10 of FIG. 4. The output of the amplifier 210 is connected to the input terminal 1 of the

noise reduction circuit 10.

  Interestingly, the noise reduction circuit 10 is selectively mounted to operate either as an encoder or as a decoder in the circuit 200. For this the amplifier 210 includes a switch 211 schematically represented by a mechanical two-state switch. switching. When the switch 211 touches the contact e, the noise reduction circuit 10 is connected as a feedback circuit between the output and the inverted input of the amplifier 210. When the switch 211 is connected to the contact, the resistance of reaction 212 is connected between the output and the inverted input of the amplifier 210 defining the gain of the amplifier; the output of the amplifier 210 is further connected to receive the amplified information signals for

  the noise reduction circuit 10. Thus when the switching

  211 is connected to the contact e, the circuit 200 shown

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  works as an encoder and gives information signals

  level on the output terminal 202. On the other hand,

  that the switch 211 is connected to the contact d, the circuit 200

  operates as a decoder and gives high level signals

  killed on the output terminal 5. As shown, the terminal of

  output 5 is connected to another output terminal 203 which itself

  can be connected to a magnetic recording transducer

  tick. It should be noted that by using the noise reduction circuit 10 in two modes that can switch, the same circuit can be used both as an encoder and as a decoder, thereby splitting the components. In a typical recording and playback apparatus such as an audio tape recorder, the information signals are not recorded and reproduced simultaneously. Thus, rather than having a separate coding and decoding circuit, it is advantageous to use the same noise reduction circuit 10 to separately perform the coding and decoding. Moreover, by using the same noise reduction circuit for both modes of operation, it is not difficult to adapt the

  encoder and decoder characteristics.

  The characteristics of the reduction circuit of

  noise 10 have been described in detail above and this description

  tion will not be repeated for the purpose of simplification. It should be noted that when the switch 211 touches the fixed contact d, the circuit 200 operates essentially of the same

  as explained above for the modes of

  FIGS. 4 and 7, that is to say that the information signal

  input is amplified by the amplifier 210 and its level is appropriately emphasized with a de-emphasis

variable by circuit 10.

  When the switch 211 touches the contact e, the transfer characteristics of the circuit 10 are used as negative feedback gains B of the circuit 200. If the open-loop gain of the amplifier 210 is equal to A, the total gain or characteristic of transfer of the circuit 200 is equal to A * This is the gain of an amplifier having a

1 + AB

  feedback circuit. If the product AB is sufficiently large, that is, if AB> 1, the gain or the characteristic

  of the circuit 200 put into coding mode is essential.

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  actually equal to 1 / B. Thus when the circuit 10 is connected in the feedback loop on the amplifier 210, the overall characteristics of the circuit 200 are inverse or

  complementary to the transfer characteristic B of the decoder.

  It should be noted in these conditions that if circuit 10 is

  used as an encoder, we obtain a pre-accentuated signal of which-

  the level is compressed, which is complementary to the

  decoder device for recording on the recording medium.

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Claims (3)

A noise reduction circuit characterized in that it consists of a first signal path having a variable gain amplifier and receiving a subtraction signal to amplify it with variable gain and means for significantly deemphasizing the high frequency components of the subtraction signal after amplification, provided by the variable gain amplifier, with respect to the low frequency components to provide an output signal, a second signal path for performing at most one de-emphasis relatively low frequency components of the output signal relative to the low frequency components, subtraction means for subtracting the output signal from the second signal path with respect to an input information signal and to provide a subtraction signal in response thereto as well as gain control means of the variable gain amplifier for superior gain r when the level of the input information signal is relatively high and a lower gain when the signal level is relatively low.   20) Circuit according to claim 1, characterized   in that the second signal path comprises a low-pass filter.   Circuit according to Claim 2, characterized in that the low-pass filter comprises a first and a   second resistors in series between the subtractor and the amplifier   variable gain controller and a capacitance between the junction point of the first and second resistors and a reference potential.   Circuit according to Claim 1, characterized in that the means giving a significant deemphasis comprises a low-pass filter.   53) Circuit according to claim 1, characterized in that the variable gain amplifier consists of a voltage controlled amplifier which receives the subtraction signal, an impedance path receiving the subtraction signal and a adder for adding the output signals of the voltage-controlled amplifier and the impedance path to provide the amplified subtraction signal. 6 ') Circuit according to claim 1, characterized in that the variable gain amplifier comprises an amplificateu: a feedback impedance to ensure the feedback of a portion of the amplified signal to the input and an input impedance   to apply the subtraction signal to the amplifier.   Circuit arrangement according to Claim 6, characterized in that the input impedance path comprises a first impedance element having a fixed impedance and a second variable impedance element in parallel with the first impedance element between the input of the amplifier and the subtractor, the impedance of the second variable impedance element being controlled by the control means.   Circuit according to Claim 7, characterized in that the input impedance path consists of a series connection of an anti-light device and a third fixed value impedance, the series circuit being itself connected in parallel to the first impedance and the second variable impedance.   Circuit according to Claim 8, characterized in that the anti-liming means is composed of a first diode circuit connected in parallel with a second circuit   with diodes and opposite polarization.   Circuit according to Claim 6, characterized in that the means giving a significant deemphasis consists of a low-pass filter.   Circuit according to claim 10, characterized in that the low-pass filter comprises a resistor and a capacitor connected in series between the input and the output of the amplifier. 12) Circuit according to claim 1, characterized   in that the first signal path comprises anti-fault means   limiter connected between the subtractor and the variable gain amplifier to ensure the expansion of high levels of subtraction signal.   Circuit according to claim 1, characterized in that the gain control means of the variable gain amplifier comprises a weighting circuit which receives the information signal for deriving a conair signal from   gain from the high frequency components of the information signal.   and means for supplying the gain control signal to the variable gain amplifier whereby the gain thereof is directly related to the control signal level of the amplifier. 232474736   gain. 142) Circuit according to Claim 13, characterized   in that the variable gain amplifier comprises an amplifier   voltage controlled controller and the means for applying the gain control signal comprises a rectifier providing a gain control voltage according to the control signal of gain.   c) Circuit according to claim 13, characterized   in that the weighting circuit comprises a high-pass filter.   16c) Circuit according to Claim 15, characterized in that the high-pass filter has a filtering characteristic   essentially opposed to that of the means giving a de-emphasis noticeable   17> Circuit according to claim 15, characterized in that the high-pass filter comprises a first series circuit of a first resistance element and a first capacitance and a second series circuit connected in parallel with the first circuit. series and that includes a second   resistance element and a second capacitance.   18) A level expansion circuit having a transfer characteristic which is more sensitive to the frequencies of the low level input signals than for the high level input signals so that the higher frequency components undergo a stronger adjustment. of low level input signals, circuit characterized by a first signal path comprising a variable gain amplifier for amplifying a subtraction signal provided to it and a low pass filter for emphasizing the low frequency components relative to to the high frequency components of the amplified subtraction signal provided by the variable gain amplifier to provide an output signal, a second feedback signal path giving at most a relatively low emphasis to the low frequency components of the output signal relative to the components high frequency of this signal, a subtractor to subtract the output signal from the second path of s ignal with respect to the input signal and provide a response subtraction signal and control means for controlling the gain of the variable gain amplifier so that the gain increases as the level of the input signal increases. 19. Circuit according to claim 18, characterized in that 24747364   that the first signal path includes an anti-limiter connected between the subtractor and the variable gain amplifier   to ensure the expansion of the subtraction signal in high levels.   220 ') Circuit according to claim 11, characterized in that the control means comprises a high-pass filter for filtering the input signal and a smoothing means for smoothing the filtered input signal and giving a control signal of gain that is provided to the gain amplifier variable to control his gain.