JPS58157210A - Variable gain amplifier level detection device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a level detection device for a variable gain amplifier suitable for a compander type noise reduction system.

[Technical background of the invention and its problems] Recently, in the field of tape recorders, for example, audio signals are recorded with logarithmic compression, and when played back, they are expanded to the original size and the hiss noise of the magnetic tape is Noise reduction systems that reduce noise are becoming widely used.

Such a noise reduction system, for example,
It is constructed as shown in the figure and includes an encoder 12 connected to the recording input terminal side of the tape recorder 11 and a decoder 13 connected to the playback output terminal side.

These encoder 12 and decoder 13 include variable gain amplifiers 12a and 13a and level detection device 1, respectively.
2b and 13bf, and are adapted to perform predetermined operations such as compression and decompression of audio signals.

In other words, these encoders 12 and decoders 13 have input-to-output static characteristics as shown at 12a and 13c in FIG. For example, if a signal of -6 Odb is input to the encoder 12, -30 db
When the -50 db signal is input to the decoder 13, it is attenuated to a -6 O db signal.

However, such characteristics are shown only for static characteristics, and the actual dynamic characteristics are, for example, in the case of the encoder 12, when the human signal level is instantaneously changed from -60db to Odb, the final result is II/ The output changes from -30db to Odb, but the broken line J in Fig. 2
? It changes as shown in d. This state can be observed as an overshoot waveform in which the output level suddenly exceeds Odb at 150 db and then gradually settles down to Odb, as shown in FIG.

This oversheet TD, the time constant circuit of a general K level detection device, and the variable emfax circuit that amplifies the high frequency range by changing the gain depending on the overall level of the audio signal It is known that this phenomenon occurs easily and its level increases, and it has a large negative effect on sound quality. Moreover, depending on the magnitude of the overshoot, the magnetic tape exceeds the limit and enters the magnetic saturation region, making it impossible to perform accurate decoding.

This type of overshoot can be improved by speeding up the response (dynamic characteristics) of the level detection device that controls the gain of the variable gain 4-amplifier, but Sensitivity to natural noise also increases, making the sound quality less desirable.

[Purpose of the invention]

This invention was made in view of the above points, and example tld
'An object of the present invention is to provide a level detection device for a variable gain amplifier that is capable of reducing as much as possible the oversheet of a variable gain amplifier mounted in an encoder of a noise reduction system.

[Summary of the invention]

This invention is used in a compander-type noise reduction/ion system having a variable gain amplifier, in which a current level signal obtained by double-wave rectification of an audio analog signal is averaged through a time constant circuit as a load. In a level detection device for a variable gain amplifier that can derive a gain control signal for a variable gain amplifier by converting it to a voltage level, when a DC level signal obtained by double-wave rectification of an audio analog signal is extracted, it is also converted to a Kvt pressure level. death,
When this voltage level is larger than the averaged voltage level obtained by the time constant circuit, the extracted current level is converted into a voltage level and is passed through the time constant circuit as a gain control signal for the variable gain amplifier. It is characterized by being derived.

[Embodiments of the invention]

An embodiment of the present invention will be described in detail below with reference to the drawings.

That is, FIG. 4 schematically shows the configuration of the level detection device according to the present invention, in which the input terminal IN is connected to the double-wave rectifier circuit 1.
1 input terminal. This double wave rectifier circuit 11
The output terminal of is connected to the input terminal of a current detection circuit 12 formed of, for example, a current mirror circuit. One output terminal of the current detection circuit 12 is connected to the input terminal of the current-voltage conversion circuit 13, and the other output terminal is connected to the input terminal of the averaging circuit 14, which is, for example, a time constant circuit.

The output terminal of the current-voltage conversion circuit 13 is connected to the input terminal of the averaging circuit 140 in common with the other output terminal of the upper ml current detection path 12 via an amplifier 15 and a switching element S consisting of, for example, a diode. . The output terminal of this averaging circuit 14 is connected to an output terminal OUT, and is configured to supply a gain control signal to a gain control terminal of a variable gain amplifier (not shown).

In other words, the level detection device configured as described above is used, for example, in an encoder of a noise reduction system, in which a signal supplied from a variable gain amplifier (not shown) via the input terminal IN is converted into a current level by the double-wave rectifier circuit 11. It is designed to be rectified. The rectified signals are supplied to the current-voltage conversion circuit 13 and the averaging circuit 14 separately.

In this case, the signal supplied to the averaging circuit 14 is converted into a voltage level by a time constant circuit constituting the averaging circuit 14 and is averaged (derives an average level).

On the other hand, the current level signal input to the current-voltage conversion circuit 13 is voltage-converted and then supplied to the input end of the averaging circuit 14 via the amplifier 15 and the switching element S.

As a result, when the output terminal level of the amplifier 15 is higher than the voltage level generated at the input terminal of the averaging circuit 14 due to the signal from the current detection circuit 14, the switching element S is turned on, and the output signal of the amplifier 15 is The gain of a variable gain amplifier (not shown) is controlled to be small via the averaging circuit 14 and the output terminal OUT.

As a result, if, for example, the level of the input terminal IN increases rapidly, the switching element S turns on, and the amplifier 1
Since the output signal of 5 immediately lowers the gain of a variable gain amplifier (not shown) via the switching element S and the averaging circuit 14, it is possible to configure an encoder for a noise reduction system in which overshoot is less likely to occur.

Specifically, such a level detection device is constructed as shown in FIG. In addition, the same parts in FIG. 8g5 as in FIG. 4 are given the same reference numerals and their explanations will be omitted.

In other words, the input terminal IN is connected to the input terminal of a double-wave rectifier circuit 11 that includes a resistor R1, an inverting amplifier A, a switching circuit consisting of transistors Qt and Qs, and a current mirror circuit consisting of transistors Qm and Qs. The commonly connected collectors of transistors Q1-Qa, which serve as the output ends of this double-wave rectifier circuit 1, are as follows:
Current detection circuit 12 becomes transistor Qs+Qs+Q,
is connected to the common pace of a current mirror circuit consisting of.

In this current detection circuit 12, the collector of a transistor Q, which serves as one output terminal, is grounded via a resistor R, which serves as a current-voltage conversion circuit 12. One end of this resistor R1, which is not grounded, serves as the input terminal and output terminal of the current-voltage conversion circuit 13, and serves as the input terminal of the amplifier 15 (the pace of the transistor Qs ).

The base of the transistor Q-, which serves as the output end of the amplifier 15, is connected to the output terminal OUT via a time constant circuit, which serves as an averaging circuit 14, and includes a resistor R and a capacitor CI in parallel.

Further, the collector of the transistor Qγ, which is the other output terminal of the current detection circuit 12, is connected to the output terminal OUT through a time constant circuit made up of a resistor Rjl and a capacitor C1 in parallel, similar to the emitter of the transistor Q. ing.

In FIG. 5, ``c'' is a positive power input terminal, and VD is a negative power input terminal. In this case, the switching element S in FIG. 4 is also used as the transistor Q of the amplifier 15.

That is, in the circuit of FIG. 5, the signal supplied to the input terminal IN is converted to a current level by the resistor R1, the transistor Q3 is turned on in the positive half cycle, and a current approximately equal to the collector current of the transistor Qz is passed through the transistor Qs.
The transistor Q1 is turned on in the negative half cycle, and a current equal to this collector current is supplied to the transistor QsK. As a result, transistor Q,
A signal whose current level has been rectified in both waves by the resistor R1 flows through the resistor R1.

And the transistor Qs and the transistor QIQ that constitutes the current mirror circuit. In each case, an equal collector current flows through the transistor Q1. In this case, a transistor. The collector current ■1 of te is resistor R1
Since it is output to the output terminal OUT via the averaging circuit 14 consisting of 0 capacitors, the output voltage Vo of the output terminal OUT is determined by assuming the average value of the current F as ■1 and the value of the resistance am. If it is expressed by its sign, then V, = Iγ・R.

becomes.

On the other hand, the collector ta of the transistor Q is converted into a voltage by the resistor R, and this voltage vs is input to the averaging circuit 14 via the amplifier 15 formed by the transistors Q and Q-. Here, transistor Q
Assume that there is no collector current of 1, and the transistor.

Brutality.

If the voltage between each pace and collector of Q- is Vlm, the output voltage Vo of the output terminal OUT is Vg = VB -2
Va becomes a curse. The output voltage Vo in this case is approximately equal to the envelope level of the signal supplied to the input terminal IN, as described above.

As a result, when Vs 2V1 recommendation < It・R, the output voltage Vo is ! ? −Rs and Vj −2Vmm
> Iy ・Rm, the output voltage Vo is Va −2
Va becomes a curse.

In other words, if the signal level supplied to the input terminal IN is sufficiently small, V.
Since the emitter current of is approximately zero, the output voltage Vo outputs the average level of the input signal (that is, I, .Rs).

On the other hand, when the input signal level is large, v,
If the condition -2Vim>h·Rlm is satisfied, the output voltage Vo is the envelope level of the input signal. Therefore, when the signal level at the input terminal IN increases rapidly, the envelope level is output from the output terminal OUT, and the gain of a variable gain amplifier (not shown) constituting the encoder is controlled to be reduced, for example. , which operates to reduce the occurrence of overflow.

FIG. 6 shows the response of the circuit of FIG. 5 to tone burst signal input; (a) is the input tone burst signal, and (b) is the signal rectified by the double wave rectifier circuit 1. be. When the amplitude of the tone burst signal shown in (,) is small, VB (2Vi+g), so the capacitor CI is charged only by the collector current of the transistor Qy, and the output terminal OUT is It gradually increases and becomes a constant value at I and R.

If the amplitude of the tone burst signal (&) is sufficiently large, the capacitor C1 is charged by the transistor Qy, as shown by the curve X in FIG. 6(d), and the capacitor C1 is charged by the emitter current of the transistor Q9. Since the battery is rapidly charged, the output voltage of the output terminal OUT rises more rapidly as shown by the curve y than in the case of FIG. 7(c). Then, in period t1, the voltage of the output terminal OUT is VB
-2vlm, and the capacitor C during periods t1 and tl.
! However, the bell detection device has a faster response to large amplitude input signals than a conventional level detection device, and when applied to an encoder mite in a noise reduction system, for example, it constitutes an encoder with less overshoot. It is even better to configure it as shown in FIG.

However, parts in FIG. 7 that are the same as those in FIG. 5 are designated by the same reference numerals and their explanations will be omitted.

That is, in Fig. 8, the averaging circuit has been improved in order to speed up the rise characteristic, and the emitter of transistor Q- is connected to the output terminal OUT, and the capacitor C1l is directly charged by the transistor QsK. Provided for. Further, a resistor R11 is inserted and connected between the connection point between the emitter of the transistor Q9 and the capacitor C1l, and the connection point between the collector of the transistor Q1 and the capacitor c1.

If the capacitor C1l is set to have a capacitance sufficiently smaller than cl, the load on the transistor Q can be lightened, and if the emitter current of the transistor Q9 does not change, the circuit shown in Figure 5 The output voltage of the output terminal OUT can be made to rise faster than the above. As a result, it is possible to obtain a level detection device for a variable gain amplifier that can constitute an encoder of a noise reduction system with substantially less overshoot.

The level detection device for a variable gain amplifier according to the present invention has improved dynamic characteristics as described above, so it works well even when applied to an encoder of a noise reduction system, and can be modified in various ways. It goes without saying that the invention can be applied without departing from the gist of the invention.

〔Effect of the invention〕

As detailed above, according to the present invention, there is provided a level detection device for a variable gain amplifier which is capable of reducing as much as possible the over/neat of a variable gain amplifier mounted in an encoder of a noise reduction system, for example. can be provided.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the outline of a conventional noise reduction system, FIG. 2 is a diagram showing the static characteristics of the noise reduction system in FIG. 1, and FIG. Figure 5 shows a specific example of the device shown in Figure 14. FIG. 7 is a circuit diagram showing another embodiment. 11...Double wave rectifier circuit, 12...Current detection circuit, 1
3... Current voltage conversion circuit, 14... Averaging circuit, 1
5...Amplifier, A...Inverting amplifier, Ql to Q...
...Transistor, R1 to R, ...Resistance, CI...
・Capacitor. Applicant's Representative Patent Attorney Takehiko Suzue Figure 1 Figure '2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] Used in a converter noise reduction system that has a variable gain amplifier, converts an audio analog signal into a current level that has been rectified in both waves, and averages this current level signal using a time constant circuit that serves as a load. In the level detection device for a variable gain amplifier, which is capable of deriving a gain control signal of the variable gain amplifier by converting it to a voltage level of , a second means converting the current level extracted from the first means VC into a voltage.
means, a third means for detecting a state in which the voltage level obtained by the second means is larger than a voltage level obtained by averaging with the time constant circuit, and a detection state of the third means; The voltage level obtained by the second method above!
What about the time constant circuit described in IJ? A level detection device for a variable gain amplifier, comprising: fourth means for deriving a gain control signal for the variable gain amplifier.