JPS62232206A - Limiter circuit - Google Patents

Abstract

PURPOSE:To prevent the dynamic range from being lowered even when the level fluctuation is rapid by adding a voltage limiter to a feedback circuit depending on the selection of the signal path. CONSTITUTION:Switches 26, 40 of a limiter circuit 24 are thrown to the position of a contact point R in the recording mode, a full feedback amplifier is constituted by a feedback circuit 34 and an operational amplifier 32, which acts like a simple buffer amplifier to a sound signal to be recorded, a voltage limiter 38 is activated via a resistor 36 and the sound signal applied with amplitude limit is fed to an FM modulation circuit 8. Further, in the reproduction mode, the switches 26, 40 are thrown to the position of a contact point P and since the feedback circuit 34 is constituted while being added with the resistor 36 and the voltage limiter 38, the inverse characteristic is applied to the amplitude limit by the voltage limiter 38 at the time of recording, the amplitude component of the voltage limiter 38 at the time of recording is compensated to reproduce the original signal at the time of recording.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a limiter circuit that limits signal amplitude.
In particular, it relates to prevention of reduction in dynamic range.

[Conventional technology]

Traditionally, VTR (video tape recorder) audio recording and
When attempting to achieve high fidelity (lli-fi) in playback, an FM recording/playback method is adopted.

By the way, in a 1li-Fi recording/playback VTR, the audio FM signal of the second stereo channel is inserted into the narrow frequency band between the luminance signal (low carrier FM signal) and the low frequency conversion color signal, so the luminance Signal, low frequency conversion color signal and audio FM
To prevent interference with the signal, the maximum frequency deviation for the audio FM signal (±150 ktlz in the VH3 standard) is determined, and a limiter circuit is used to limit the amplitude of the audio signal so that it does not exceed this range. .

FIG. 9 shows the audio signal system of a general FM recording/playback type VTR. In the FM recording/playback method, during recording, the audio signal applied to the input terminal 2 is passed through a logarithmic compression/expansion type noise reduction circuit (PNP) 4 and applied to a limiter circuit 6, which reduces the amplitude. After limiting the frequency to a certain value or less, an FM modulation circuit (FMMOD) 8 performs FM modulation on a specific carrier wave, and the modulated output is applied to a magnetic helad 10 to be recorded on a magnetic tape. in this case,
The limiter circuit 6 includes a resistor 12 and a diode 14.1.
Consists of 6.

Furthermore, during playback, the FM signal obtained through the magnetic head 10 is sent to an FM demodulation circuit (FMDEM) 18, which demodulates the audio signal from the FM signal, and outputs the demodulated output via the noise reduction circuit 20. It is output from the terminal 22 to the audio reproduction system.

[Problem that the invention seeks to solve]

Incidentally, in an FM recording/playback system using such a limiter circuit 6, when an audio input signal is encoded by the logarithmic compression/expansion type noise reduction circuit 4, an audio input signal whose level fluctuation is relatively gradual (a static For example, the logarithmic compression effect of the noise reduction circuit 4 reduces the dynamic range significantly (for example, 100
A signal component with a dynamic range of dB is 50 dB
Since the signal is compressed to a level such that the amplitude is not limited by the limiter circuit 6, it never reaches a level that would cause the amplitude to be limited by the limiter circuit 6. For example, as shown in A in FIG. 10, when an audio input signal with gradual level fluctuations is applied to the noise reduction circuit 4,
The encoded output (recorded output signal) of the noise reduction circuit 4 is logarithmically compressed within the range of the limit voltage levels VCI and VCZ set in the limiter circuit 6, as shown in B in FIG. When the audio input signal is passed through the limiter circuit 6 and magnetically recorded via the FM modulation circuit 8, the reproduced output signal can faithfully restore the original signal as shown in C in FIG.

However, when the input signal contains many high frequencies and is a pulsed signal (dynamic signal) with large level fluctuations, the dynamic range is compressed due to the attack time constant of the noise reduction circuit 4 and the effect of pre-emphasis. Even at a normal input level, the limiter circuit 6 may often limit the amplitude. In other words, when the signal level suddenly increases, due to the delay of the time constant circuit,
This is because logarithmic compression cannot be achieved instantaneously, and the level is further enhanced due to the high frequency enhancement effect of pre-emphasis, reducing the dynamic range compression effect.

When these signals are recorded and reproduced with their amplitude limited by the limiter circuit 6, the waveform of the reproduced output signal rises slowly, making it impossible to restore the original signal. To the auditory sense, the pulsative high-range sounds are removed, giving the impression that the high-range frequency response has deteriorated. For example, as shown in A in FIG. 11, when an audio input signal with large level fluctuations is applied to the noise reduction circuit 4, the encoded output (recorded output signal) of the noise reduction circuit 4 is
As shown in B in the figure, since it has a steep signal component that exceeds the range of the limiting level V CI s V C1 set in the limiter circuit 6, when such a human input signal is passed through the limiter circuit 6, When the signal portion exceeding the limiting level VCZ is removed by the limiter circuit 6, and the audio signal from which such signal components have been removed is magnetically recorded via the FM modulation circuit 8, the reproduced output signal is as shown in FIG. As shown in C, the original signal cannot be faithfully restored, resulting in a reduction in the dynamic range.

In such an FM recording/playback system, the limiter circuit 6 is essential for preventing interference between FM frequencies, but due to the nature of the noise reduction circuit 4, the function of the limiter circuit 6 to prevent overmodulation prevents static noise. The dynamic range of recording and playback changes greatly between standard input signals and dynamic input signals, and the dynamic range becomes narrower for dynamic signals. The operational characteristics of the circuit 6 become a problem.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a limiter circuit that prevents the dynamic range from decreasing even when level fluctuations are rapid.

[Means for solving problems]

As shown in FIG. 1, the limiter circuit of the present invention includes an amplifier (operational amplifier 32) that amplifies an input signal selected from a plurality of input signals, and a limiter circuit that is installed between the input and output of this amplifier and It is composed of a return circuit 34 whose signal paths 34a and 34b can be switched according to the signal, and a voltage limiter 38 which is added to this feedback circuit by switching the signal paths.

In other words, when the limiter circuit of the present invention is implemented in a recording/reproduction method, for example, in response to the amplitude limitation during recording, the limiter circuit expands the amplitude during reproduction, expands or contracts the signal amplitude as necessary, and performs recording/reproduction. The objective is to substantially improve the dynamic range regardless of mode.

[For production]

The limiter circuit of the present invention selects a signal path of a feedback circuit for an amplifier according to an input signal selected from a plurality of input signals, and adds a voltage limiter to the feedback circuit by selecting the signal path. . For example, for a certain input signal, an amplifier and a feedback circuit constitute a full feedback amplifier to form a buffer amplifier, and the amplitude of the signal passing through the buffer amplifier is limited by a voltage limiter.
For other input signals, the amplitude is expanded through a feedback circuit with a voltage limiter added to reproduce the original signal.

Therefore, even if a signal requiring amplitude limitation contains a sharp attack component, the attack component is not lost, and the dynamic range of the signal is prevented from decreasing.
For example, even when the signal is subjected to FM modulation and magnetically recorded, the amplitude of the signal reproduced from the magnetic recording can be compensated for by using characteristics that are opposite to those during recording, and the original signal can be faithfully reproduced.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the limiter circuit of the present invention.

In the embodiment shown in FIG. 1, a limiter circuit 24 according to the present invention is installed in the FM recording/reproducing system shown in FIG. In this limiter circuit 24, a switch 26 is installed to select an input signal to be applied to the non-inverting input terminal (+) of the operational amplifier 32 from a plurality of input signals applied to the input terminals 28 and 30, and is provided at a contact R. An audio input signal to be recorded is applied to the side from the noise reduction circuit 4, and an audio signal demodulated by the FM demodulation circuit 18 from the FM signal detected by the magnetic head 10 is applied to the contact P side. .

A feedback circuit 34 is added between the output side of the operational amplifier 32 and the inverting input terminal (-) for feeding back the output of the operational amplifier 32 to the input side. This feedback circuit 34 includes a signal path 34a directly connecting its output side and an inverting input terminal (-), a resistor 36 between its output side and the inverting input terminal (-), and a voltage limiter 38 added thereto. signal path 3
4b, and each signal path 34a, 34b is provided with a switch 4
0 allows selective switching.

Two output terminals 42 and 44 are provided on the output side of the operational amplifier 32, and an audio signal as a playback output is selectively extracted from the output terminal 42, and an audio signal to be recorded is selectively extracted from the output terminal 44. . Therefore, the audio signal taken out from the output terminal 42 is transmitted to the noise reduction circuit 20.
The audio signal taken out from the output terminal 44 is also applied to the FM modulation circuit 8. The audio signal applied to the FM modulation circuit 8 has a carrier wave of I? due to magnetic recording.
M modulation is performed, and the modulated output is applied to the magnetic head 10 and magnetically recorded on the magnetic tape.

Therefore, in the recording mode, the limit/Z-curve circuit 24:
After the switch 26.40 is switched to the contact R side and the audio signal passes through the noise reduction circuit 4 from the input terminal 2 side by switching the switch 26 to the contact R side,
It is applied to the non-inverting input terminal (+) of the operational amplifier 32.

In this case, by switching the switch 40 to the contact R side, the feedback circuit 34 is configured to directly connect the output side of the operational amplifier 32 and the inverting input terminal (-), and the operational amplifier 32 is switched to the contact R side, as shown in FIG. As shown in FIG. 3, a full feedback amplifier 32' is constructed, a resistor 36 is inserted in series between the full feedback amplifier 32' and the FM modulation circuit 8, and a voltage limiter 38 is connected to a No. 13 circuit in which the resistor 36 is inserted. route 34
b is added to the FM modulation circuit 8 side. In this case, the full feedback amplifier 32' functions as a mere buffer amplifier for the audio signal to be recorded, and the voltage limiter 38 works via the resistor 36, so that the amplitude-limited audio signal is sent to the FM modulation circuit 8. can be added to Therefore, the amplitude-limited audio signal is recorded on the magnetic tape from the magnetic disk 10 through FM modulation. In this case, since it is the recording mode, the noise reduction circuit 20 and the FMli [circuit 18 of the reproduction system have stopped operating.

In the playback mode, the switch 26.40 is switched to the contact P side, and by switching the switch 26 to the contact P side, an FM signal is applied from the magnetic head 10 side to the FMfim circuit 18, and the demodulated output is the audio signal. A signal is applied to the non-inverting input terminal (+) of operational amplifier 32. In this case, by switching the switch 40 to the contact P side, the feedback circuit 34 is configured such that a resistor 36 is inserted between the output side of the operational amplifier 32 and the inverting input terminal (-), as shown in FIG. At the same time, a voltage limiter 38 is added to the signal path 34b on the inverting input terminal (-) side of the operational amplifier 32.

In this case, since the feedback circuit 34 is configured by adding a resistor 36 and a voltage limiter 38, it has an inverse characteristic to the amplitude limitation by the voltage limiter 38 during recording, and compensates for the amplitude limitation of the voltage limiter 38 during recording. to reproduce the original signal at the time of recording. Therefore, the audio signal whose original signal amplitude has been restored passes through the noise reduction circuit 20 to the output terminal 2.
taken out from 2.

In this limiter circuit 24, it is preferable to use a voltage limiter 38 that does not have temperature characteristics, can provide stable operation, and can gently limit the amplitude when a specific limiting voltage is exceeded. It is ideal in that it does not cause signal distortion and in restoring the original signal.

Naturally, when such a limiter circuit 24 is used in a recording/reproduction system, the signal at the input section of the FM modulation circuit 8 is used for FM modulation, recording on a magnetic tape, detection thereof, and FM demodulation during recording. It is necessary that the signal be generated with exactly the same amplitude as the signal after the signal is applied. And record
In the reproduction system, in order to make the signal amplitude the same during recording and reproduction, for example, FM modulation is performed using a voltage controlled oscillator in the FM modulation circuit 8 during recording, and voltage control of the FM modulation circuit 8 is performed during reproduction. By configuring a PLL (phase locked loop) type FM demodulation circuit 18 using an oscillator, recording,
During playback, if the frequency control sensitivities of the voltage controlled oscillators are set exactly the same, the signal obtained when the signal reproduced from the magnetic recording is FM demodulated through FM modulation and magnetic recording will be the same as the original signal. can be made into amplitude.

FIG. 4 shows a specific example of the circuit configuration of the limiter circuit 24 shown in FIG. 1 as an IC.

As shown in FIG. 4, an audio signal to be reproduced is applied to the input terminal 28 during reproduction, and an audio signal to be recorded is applied to the input terminal 30 during recording. In this case, each input terminal 28.30
is not intended to be an external terminal (pin) of an IC.

The operational amplifier 32 has a switch 26.40 added thereto, and has two differential amplifiers 46A and 46A as pre-stage amplifiers.
46B and one output stage amplifier 48, the differential amplifier 46A is set for reproduction, and the differential amplifier 46B is set for recording, so that the operation can be switched depending on the recording or reproduction mode. That is, the differential amplifier 46A is
It is composed of a transistor 50.52.54.56.58 and a resistor 60.62.64.66, and during playback, a bias human power V as a control voltage is applied to the base of the transistor 54.
p is added and it becomes operational.

Further, the differential amplifier 46B includes the transistor 56, 58 and the resistor 60, 62 of the differential amplifier 46A in common, and is composed of the transistor 68, 70, 72 and the resistor 74, and is controlled by the base of the transistor 72 during recording. A bias human power Vn as a voltage is applied to bring it into operation. A constant bias voltage 8 applied to the bias input terminal 76 is applied to the base of the transistor 50.68 that constitutes the input section of these differential amplifiers 46A and 46B. side, applied to the transistor 68 side through the resistor 80,
This bias voltage (2) is set, for example, to a voltage value that is 1/2 of the power supply voltage VCC.

The output stage amplifier 48 also includes transistors 82, 84 .
86.88 and resistors 90.92.94.96, transistor 82 is installed as a load of differential amplifiers 46A, 46B, and transistor 56 is installed as a load of differential amplifiers 46A, 46B. The transistor 58 constitutes a current mirror circuit.

In this case, during reproduction, the reproduction signal PB is transmitted to the transistor 84.
．． It is taken out from the output terminal 42 provided on the collector side of 88. The resistor 36 connects the common collector of the transistors 84 and 88, which are the output terminals of the output stage amplifier 48, and the transistor 52 on the inverting input terminal (-) side of the differential amplifier 46A.
installed between the base of the During recording, the output signal REC to be recorded is taken out from the output terminal 44 formed on the base side of the transistor 52.

In addition, the voltage limiter 38 has a function of suppressing the signal amplitude by increasing the impedance by a little over 1/4 when the audio signal amplitude exceeds a specific level, and has the function of suppressing the signal amplitude at a specific midpoint level of 8
, the allowable amplitude VCI is set symmetrically on the positive and negative sides, and the positive limiter 100A and the negative limiter 100B, in which the allowable amplitude is set with VC2, uniformly limit the positive and negative amplitudes. It is configured. In this case, the positive limiter 100A is the transistor 102.104.10
6.108 and 1 anti-110.112.114.11
6, and the negative limiter 100B is composed of transistors 118, 120, 122, 124 and resistors 12
It consists of 6.128.130.132. The series resistance circuit consisting of resistors 134, 136, 138, and 140 constitutes a voltage divider circuit, and the limiting voltage VCI and VC2 is set. In this case, the bias voltage 8 set at the midpoint of the resistors 136 and 138 is shared with the bias voltage of the operational amplifier 32, and is the midpoint level of the audio signal during recording and the midpoint of the audio signal during playback. This ensures that the levels match.

Also, transistors 142.144.146.148.
150 and resistance 152.154.156.158.1
Reference numeral 60 constitutes a constant current circuit that provides operating current to the positive and negative limiters 100A and 100B. , 100B operates.

Regarding the limiting operation, if we focus on the positive limiter 100A shown in FIG.
R1 is the resistance value of the resistor 36, RNF is the resistance value of the resistor 36, and I is the constant current supplied from the transistor 142 constituting the constant current source. When a signal voltage exceeding the limited amplitude level is supplied, the current flowing through the transistor 104 is 11, and the current flowing through the transistor 102 is I2.The voltage level Δ■ exceeding the limited amplitude level is ΔV=RE (II I2) +Vyln( 1+/Iz) ・−・fD,
VT In (1+/It) <<RE (I
When I - [2), the voltage level ΔV is ΔV#RE (It -1z) ... (
2). Also, constant voltage KI. and current 1. ,I.

is, 10=1. +12...The relationship is as shown in (3). Here, let the resistance value of the resistors 114 and 116 be R, and R
1.11 >>vt tn (1+/ΔI), the current Δ1 flowing through the transistor 106 in response to the voltage level Δ■ is Δl#[, -1°Δ■/RE (4) Become.

Therefore, the input impedance R of the positive limiter 100A
IM is R18=ΔV/ΔI#RE (5).

Therefore, for an input exceeding the voltage 1, a voltage divided by the input impedance (R18-h) and the resistor 36 (R□) is obtained. Therefore, the limiting is not applied completely, the amplitude is limited with the level attenuated, and the limiting level is determined by voltage division with the power supply.
Since the amount of attenuation is almost determined by the resistance 36, 110, 112, etc., the influence of temperature etc. is very small. Such an operation is the same for the negative limiter 100B.

Therefore, along with the operation of the voltage limiter 38, the recording and playback modes are switched by the control voltages V2 and 8 to the transistors 54.72, and the differential amplifiers 46A and 46B are selectively operated according to each mode, and the feedback The signal paths 34a and 34b of the circuit 34 are switched.

During recording, the differential amplifier 46B and the output stage amplifier 48
Since the feedback circuit 34 constitutes a buffer amplifier, the input signal to be recorded applied to the input terminal 28 is taken out from the output stage amplifier 48 as a buffer output via the differential amplifier 46B. This buffer output signal is applied to a voltage limiter 38 via a resistor 36, and since the voltage limiter 38 has human output characteristics as shown in FIG. Signal amplitude is positive limit voltage ■. 1 and the negative side limit voltage VC2, the positive side limit voltage VCI and the negative side limit voltage VC
Performs a gentle amplitude limiting operation according to the amplitude exceeding t,
As shown in FIG. 7B, the signal is converted into a recording output signal having a certain amplitude or less, and is taken out from the output terminal 44.

Furthermore, during reproduction, the human input signal to be reproduced that is applied to the input terminal 30 is outputted as an output signal PB from the output stage amplifier 48 via the differential amplifier 46A.

In this case, a resistor 36 is inserted into the feedback circuit 34, and a voltage limiter 38 is added to obtain input/output characteristics for expanding the amplitude limiting bone during recording, as shown in FIG. Compensates the amplitude of the reproduced signal using input/output characteristics, faithfully reproducing the original signal during recording. For example, the seventh
As shown in C in the figure, when a reproduced input signal with amplitude limitation is applied, the input/output characteristics shown in Fig. 8 result in:
Amplitude compensation is applied to the portion where the signal component has been removed, and the seventh
As shown in D in the figure, the same output signal as the input signal during recording shown in A in FIG. 7 is reproduced, and the original signal is faithfully reproduced. In other words, when recording, the operational amplifier 32
By simply adding a resistor 36 between the output side and the inverting input terminal (-), a full feedback state is created, so the input and output signals have the same amplitude, but the level is large and the voltage limiter 38 is When the working voltage is reached, amplitude limitations are applied to positive and negative parts in the signal paths 34a and 34b of the feedback circuit 3°4, so the amplitude of the input signal is expanded, and the amplitude limitations during recording are canceled and compensated. The original signal is restored.

In this case, as for the amplitude expansion operation of the voltage limiter 38, a gentle amplitude expansion operation corresponding to the amplitude level can be obtained, similar to the amplitude limiting operation.

Therefore, if configured as in this embodiment, the amplitude limit start voltage is stable and the amplitude limit is performed gently.
By using a limiter circuit that is stable against temperature, even if amplitude is limited during recording, the same limiter circuit is used as a feedback system for the operational amplifier during playback, and the amplitude of signals exceeding the limiting voltage is limited to below the limiting voltage level. Even if
The limited portion can be faithfully restored to the original signal, and substantially the same effect as expanding the dynamic range, ie, the maximum frequency deviation, of an FM recording/reproduction system can be obtained. Therefore, it is possible to expand the dynamic range of the recording/playback system, including noise reduction, not only for pulsed signals but also for static signals, making it possible to easily and faithfully reproduce the original signal in the recording/playback system. can be reproduced.

Note that in cases other than self-recording/reproduction where the recording/reproduction system can be shared, for example, when reproducing recordings such as magnetic tape recorded by other recording devices, the limiter control voltage VL is set to H (ON) / By stopping the limiting operation with L (OFF) (where H is a high potential and L is a ground (GND) potential) and canceling the limiting extension during playback, playback is performed in the normal mode. That is, in this control, the recording and reproduction control voltages Vp, V
*It is controlled by a control circuit, and when recording, V, Hashi,
■.

I], ■, H1 extended playback, ■, H1■,
■, is Hl Also, during normal playback, ■, is H1 ■ Ehari, V
, is set to L.

In the embodiment, the amplitude limitation of the audio signal in the recording/reproduction system has been described, but the present invention can also be applied to the case where the amplitude of a plurality of signals other than the recording/reproduction system is adjusted according to the signal. Further, although magnetic tape is taken as an example of the recording means, the present invention can be similarly applied to recording media other than magnetic tape.

[Effects of the Invention] As explained above, according to the present invention, a signal path of a feedback circuit to an amplifier is selected according to an input signal selected from a plurality of input signals, and the signal path is controlled by selecting the signal path. A voltage limiter is added to the feedback circuit. For example, for a certain input signal, the amplifier and the feedback circuit constitute a total feedback amplifier to function as a buffer amplifier, and the amplitude of the signal passing through the buffer amplifier is limited by the voltage limiter. In addition, for other input signals, the amplitude is extended through a feedback circuit with a voltage limiter to reproduce the original signal, so it is possible to limit the amplitude and reproduce the original signal, not only for signals whose level changes slowly, but also for signals whose level changes rapidly. In addition to being able to expand the amplitude, it is possible to prevent a reduction in the dynamic range of the signal due to amplitude limitation, and it is possible to easily and faithfully reproduce the original signal.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the limiter circuit of the present invention, Fig. 2 is a circuit diagram showing an equivalent circuit of the limiter circuit shown in Fig. 1 in recording mode, and Fig. 3 is a circuit diagram showing an example of the limiter circuit shown in Fig. 1. Figure 4 is a circuit diagram showing the equivalent circuit of the limiter circuit in regeneration mode. Figure J1 is a circuit diagram showing a specific circuit configuration example of the limiter circuit shown in Figure 5. Figure 5 is a circuit diagram showing the positive limiter of the voltage limiter shown in Figure 4.
FIG. 7 is a diagram showing the input/output characteristics of the limiter circuit shown in FIG. 4 in the recording mode, FIG. Figure 9 is a block diagram showing the conventional FM recording/playback system, and Figures 10 and 11 are the FM recording/playback system shown in Figure 9. FIG. 3 is a diagram showing operation waveforms of a reproduction system. 24... Limiter circuit, 32... Operational amplifier, 34
...Feedback circuit, 38...Voltage limiter. Patent applicant Matsushita Electric Industrial Co., Ltd. Figure 3 Figure 5 Figure 9 □□□□□□7F- Limiter output voltage/S IC+ lB Figure 8 cz Figure 1O Figure 11

Claims (1)

[Claims] An amplifier that amplifies a selected input signal from among a plurality of input signals, and a feedback circuit that is installed between the input and output of this amplifier and is capable of switching the signal path according to the input signal. and a voltage limiter added to the feedback circuit by switching the signal path.