JPS5942365B2 - Compression/expansion method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention particularly relates to a compression/expansion method for improving the recording and playback characteristics of a tape recorder.

In general, tape recorders have extremely poor waveform transmission characteristics, with waveform deterioration particularly severe due to delay distortion in the low frequency range. 1st
In the figure, a and 38c7 are the cassette tape recorder and 38c7.
An example of actual measurement of the group delay time characteristics (group delay) of an N12 track tape recorder is shown.

As is clear from the figure, there is almost no region where the group delay time is flat over the entire frequency range, and especially in the low frequency region, as the frequency decreases, the group delay time increases approximately in inverse proportion to the square of the frequency f. I can see it happening. Due to the group delay characteristics mentioned above, the response output waveform of a cassette tape recorder to a square wave input of, for example, 200H2 becomes as shown in Figure 2. As a result of this waveform distortion, the peak-to-peak of the waveform The value is 60% of the original waveform (square wave) value.
It can be seen that this number is also increasing. Therefore, if a peak value control type compression/expansion system is used for such a cassette tape recorder, the level control signal of the decoder side expansion device will be much larger than that of the encoder side, resulting in a large error. . In order to solve the problems of the prior art, the present invention provides a first-order phase shift circuit that approximates the group delay time characteristic of a tape recorder at least in the low frequency region at the input stage of the level detection device in the encoder-side compression device. of,
Furthermore, the present invention is characterized in that a second-order phase shift circuit is provided at the input stage of the level detection device in the decoder-side expansion device for correcting a decrease in group delay characteristics in a relatively narrow specific frequency band.

The present invention will be described below with reference to embodiments shown in the drawings.
In the embodiment shown in FIG. 3, numeral 1 is a cassette tape recorder, and a peak value controlled compression/expansion system is used for recording and reproduction.

That is, the encoder-side compression device 2 consists of a voltage-controlled amplifier 4 and a level detector 5, and the decoder-side expansion device 3 consists of a voltage-controlled amplifier 6 and a level detector T. Therefore, the transmission distortion in the range where the frequency characteristics of the tape recorder 1 are flat is:
This is nothing but delay distortion based on the group delay time of the curve a shown in FIG. 1, especially in the low frequency region. Therefore, in the present invention, a first-order phase shift circuit 8 that approximates such group delay time characteristics is provided at the input stage of the level detector 5 to compensate for the delay distortion. FIG. 4 shows an example of the primary transfer circuit.

In the figure, R is a resistor of the same value, C is a capacitor, and 9 is a differential amplifier. Group delay time TL (
0) is q as shown in the formula below. Therefore, in the frequency range ωCR〉1, TL is inversely proportional to the square of the frequency f, and the first
It will look like curve C in the figure.

Therefore, after the group delay time characteristics of the tape recorder shown by curves a and b in FIG.
It can be seen that approximation can be achieved using the first-order phase shift circuit shown in the figure. In the first-order phase shift circuit of FIG. 4, the group delay time characteristic (calculated value) when CR=3.times.10@-3 seconds is selected, for example, as curve C in FIG. 1.

In this case, the response waveform of the circuit of FIG. 4 to a 200 Hz square wave input is as shown in FIG. 5, and as is clear from FIG. 2, it closely approximates the response of the force set recorder. Next, the group delay time characteristic shown by curve a in Fig. 1 is 2
It has a concave portion in a relatively narrow frequency band of ~3 KHz.

If this concave portion is corrected to be flat, matching in the compression/expansion system will be improved. However, as shown in FIG. 7, the group delay time of the second-order phase shift circuit as shown in FIG. 6 becomes large near a specific frequency. It is possible to correct concavities in the group delay time characteristics fairly well. However, in Fig. 6, R1 to R4 are resistances, Cl,
C2 is a capacitor, and 10 is a differential amplifier. Therefore, for example, the secondary phase shift circuit 11 of FIG. 6 is replaced with the one shown in FIG.
If inserted into the input stage of the level detector 5 in the decoder-side decompression device 3, the delay distortion of the level detection system in both the encoder and decoder will be approximately equal, realizing a compression/decompression system for tape recorders with good input/output matching. can.

As explained above, according to the present invention, the gain control signal of the compression/expansion device is correctly complementary to the transient input signal to the tape recorder, so the matching of the input/output signals of the compression/expansion system is greatly improved. do.

[Brief explanation of drawings]

Fig. 1 is an actual measurement curve diagram of the group delay time characteristic of a tape recorder, Fig. 2 is a response output waveform diagram of the tape recorder to a 200 Hz square wave input, Fig. 3 is a block diagram of an embodiment of the present invention, and Fig. 4 The figure shows an example of an l-order phase shift circuit.
The figure shows the response waveform of the circuit in Figure 4 to a 200Hz square wave input, Figure 6 shows an example of a second-order phase shift circuit, and Figure 7 shows the response waveform of the circuit in Figure 4.
The figure is a curve diagram showing the group delay time characteristics of the circuit in Figure 6.
The figure is a block diagram of another embodiment of the present invention. 1...Power set tape recorder, 2...
・Compression device, 3... Decompression device, 4, 6...
... Voltage control amplifier, 5, 7... Level detector, 8... Secondary phase shift circuit, 11... Secondary phase shift circuit.

Claims (1)

[Scope of Claims] 1. In a compression/expansion system used during recording and playback of a tape recorder, the group delay time of the tape recorder at least in the low frequency range is input to the input stage of the level detector in the encoder side compression device. A compression/expansion method characterized by providing a first-order phase shift circuit that approximates characteristics. 2 At the input stage of the level detector in the decoder side expansion device,
2. The compression/expansion system according to claim 1, further comprising a second-order phase shift circuit for correcting a decrease in group delay characteristics in a relatively narrow middle frequency band of said tape recorder.