JPH0529198B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an equalization circuit for low-frequency conversion carrier color signals, and is particularly used in color under recording VTRs to reduce low frequencies (particularly below 300 kHz) due to tape head system characteristics. The present invention relates to an equalization circuit that increases the level of the low frequency of a low frequency converted carrier color signal during recording in advance by the amount of the gain decrease in order to correct the band limitation of the reproduced carrier color signal caused by the gain decrease.

Conventional technology A color under recording type VTR uses a low-frequency conversion carrier obtained by converting a carrier color signal to an empty low frequency region of a frequency-modulated luminance signal obtained by frequency-modulating a carrier wave with a luminance signal. It is well known that a color signal is frequency division multiplexed, the frequency division multiplexed signal is recorded on a magnetic tape by a rotating head, and this is reproduced, and is currently widely used.

FIG. 7 shows a block system diagram of the color signal recording system of such a color under recording system VTR. In the figure, a color video signal input to an input terminal 1 is separated from a carrier color signal by a bandpass filter 2, and then supplied to a frequency converter 3, where the frequency is converted to a low frequency and the color subcarrier frequency is fs low-frequency conversion carrier color signal. This low-pass converted carrier color signal is supplied to an equalization circuit 5 after unnecessary high frequency components are removed by a low-pass filter 4.

The present invention relates to this equalization circuit 5, and the conventional equalization circuit 5 is simply composed of a CR circuit as shown in FIG. 8, and is configured to enhance the level of the low frequency of the input low frequency conversion carrier color signal as shown in FIG. A
Provides equalization characteristics as shown in . this is,
The frequency components in the band above the color subcarrier frequency fsc (3.58MHz for NTSC system) of the reproduced carrier color signal are
Color subcarrier frequency of low-pass conversion carrier color signal during recording
This is a low frequency component below fs, but due to tape head system characteristics, this low frequency component is attenuated by a gain reduction in low frequencies (especially below 300 kHz), so this reduction is added as a correction amount in advance during recording. By doing so, it is possible to widen the band of the reproduced carrier color signal.

In other words, due to the characteristics of the tape head system, the reproduced carrier color signal is subject to band limitation due to the gain reduction in the low frequency range mentioned above, and since the carrier color signal is biaxially orthogonally modulated, in a band where two types of color difference signals exist, , if the above equalization circuit 5 is not provided, the upper sideband cannot be widened due to the above band limitation, so
Even if only the lower sideband is widened, the transient characteristics will worsen at the time of demodulation. For this reason, the reproduced carrier color signal should have a band that is symmetrical to the color subcarrier frequency fsc. Frequency bands below fsc, which are easy to widen, are also subject to similar band restrictions. Then,
Broadband reproduction carrier color signals are achieved by increasing the color subcarrier frequency.
This can only be achieved if the band becomes wider than fsc.

The low-frequency conversion carrier color signal whose low frequency component has been level-enhanced by the equalization circuit 5 is frequency-division multiplexed with a frequency-modulated luminance signal from a luminance signal recording system (not shown), and then supplied to the rotating head 6. recorded on tape (not shown).

Problems to be Solved by the Invention However, since the group delay time characteristic of the above-mentioned conventional equalization circuit is not flat as shown in FIG. 9B, it deteriorates the transient characteristics of the reproduced carrier color signal.
In some cases, the color changed to a different hue in a transient state. Therefore, a problem arises in that an additional correction circuit is required to flatten the group delay time.

In addition, the conventional equalization circuit described above is for commercial use.
It was only installed in VTRs, and was not installed in general home VTRs, which require a particularly low price. Therefore, in general home VTRs, the bandwidth of the reproduced carrier color signal depends on the characteristics of the tape head system. However, the problem was that the band was narrow.

SUMMARY OF THE INVENTION In view of the above-mentioned points, it is an object of the present invention to provide a low-pass conversion carrier color signal equalization circuit that can realize a wide band of reproduced carrier color signals with a simple circuit configuration.

Means for Solving the Problems The low-band conversion carrier color signal equalization circuit of the present invention has characteristics such that the group delay time characteristic is flat at least in the band of the low-band conversion carrier color signal, and Provides equalization characteristics that enhance the level of low frequency components of color signals.

Function: In order to compensate for the decrease in gain of the low frequency component of the low-frequency conversion carrier color signal caused by the process of magnetic recording and reproduction, the level of the low-frequency conversion carrier color signal is increased by the amount of gain decrease during recording, and the group delay time is A magnetic recording medium is recorded with flat characteristics.

Embodiment FIG. 1 shows a block diagram of an embodiment of the present invention. In the figure, the low-frequency conversion carrier color signals inputted to the input terminal 10 each have the same delay time τ (for example,
After being given a delay time of 2τ through the two delay circuits 11 and 12 (300 ns), the signal is supplied to the first adder circuit 13;
supplied to

As shown in FIG. 2, the delay circuits 11 and 12 each consist of a delay line 18 of a predetermined length and matching resistors 19 and 20. The frequency characteristics and group delay time characteristics of each of the delay circuits 11 and 12 are
The characteristics shown in Figures A and B are selected. That is, as shown in FIG. 3A, the frequency characteristics of each of the delay circuits 11 and 12 are flat within the band of the low-frequency converted carrier color signal of the color subcarrier frequency fs, and unnecessary high frequency components beyond that are attenuated. This shows the low-pass filter characteristics. Further, as shown in FIG. 3B, the group delay time characteristics of each of the delay circuits 11 and 12 are flat at least within the band of the low-frequency converted carrier color signal.

Returning to FIG. 1 again, the adder circuit 13
After the output signal is gain adjusted by the gain adjuster 14, it is supplied to the second addition circuit 15, where it is added and combined with the low-pass conversion carrier color signal of delay time τ from the delay circuit 11, and then output. It is output to terminal 16.

The frequency characteristics of the entire circuit shown in Fig. 1 are as follows:
As shown by the solid line in FIG. A, a so-called raised cosine filter characteristic is shown in which the level of low frequency components below the color subcarrier frequency fs of the low-frequency conversion carrier color signal is enhanced, and the level of high frequency components is attenuated.
That is, if the delay circuits 11 and 12 have flat frequency characteristics, the frequency characteristics of this circuit will peak at a frequency of 1/τ in the high frequency region above frequency fs, as shown by the broken line in FIG. 4A. Similarly, the frequency characteristic continues indefinitely as a sine curve as shown by the broken line.

However, according to this embodiment, since the delay circuits 11 and 12 have low-pass filter characteristics as shown in FIG. 3A, the frequency characteristics of the entire equalization circuit become as shown by the solid line in FIG. It is possible to reduce noise at higher frequencies.

Further, as shown in FIG. 4B, the group delay time characteristic of this embodiment has a flat group delay time characteristic at least within the low frequency conversion carrier color signal band. The low frequency converted carrier color signal taken out from the output terminal 16 with this flat group delay time characteristic and the frequency characteristic (equalization characteristic) shown by the solid line in FIG. After being frequency division multiplexed with a modulated luminance signal, it is recorded onto magnetic tape by a rotating head.

Note that the delay times τ of the delay circuits 11 and 12 are always equal, and when the delay time τ is set to a small value, the equalization characteristic becomes as shown by the solid line in FIG. 6, and when the delay time τ is set to a large value, the equalization characteristic becomes is as shown by the solid line in the figure. In this way, by varying the delay time τ, the equalization characteristics can be varied.

The slope of the curve of the equalization characteristic and the range of the curve to be used as the equalization characteristic are arbitrarily determined by the delay time τ and the gain adjuster 14. The optimum equalization characteristic is determined from these two variable amounts.

FIG. 5 shows a circuit diagram of another embodiment of the circuit of the present invention. In the figure, the same components as in FIG. 1 are denoted by the same reference numerals, and their explanations will be omitted. In FIG. 5, the low frequency conversion carrier color signal from the input terminal 10 is supplied to a delay line 24 having a delay time τ through a buffer amplifier 22 and a matching resistor 23, respectively. The frequency characteristics and group delay time characteristics of this delay line 24 are the same as those shown in FIGS. 3A and 3B. delay line 2
The low frequency conversion carrier color signal extracted after being delayed by a time τ from 4 is supplied to the adder circuit 15 through the buffer amplifier 25.

Here, since the buffer amplifier 25 is selected to have a high input impedance, the delay line 24
A part of the output low frequency conversion carrier color signal is reflected by the buffer amplifier 25 and sent back to the delay line 24 for a time τ.
The signal is delayed, given a total delay time of 2τ, and is supplied to the gain adjuster 14 through the buffer amplifier 26. As a result, a low frequency conversion carrier color signal having a flat group delay time characteristic and having an equalization characteristic shown in FIG. 4A is outputted to the output terminal 16. According to this embodiment, only one delay line is required, and the circuit can be constructed at a lower cost.

Note that the equalization circuit can be equivalently provided in the reproduction system of the carrier color signal, but since it amplifies a signal whose gain is attenuated, the S/N of the reproduced carrier color signal deteriorates. Therefore, as in the present invention, it is desirable to provide the equalization circuit in the recording system for the low frequency conversion carrier color signal.

Effects of the Invention As described above, according to the present invention, it is possible to widen the band of reproduced carrier color signals, and the components other than the delay circuit (delay line) can be configured inexpensively and compactly by integrating circuits. line) is also inexpensive, and since the group delay time characteristic is flat, there is no need for a correction circuit for correcting the group delay time characteristic, and because of the above, it can be constructed at low cost, making it suitable not only for commercial VTRs but also for general home use. It may be installed in a VTR, and has features such as being able to reduce unnecessary noise at high frequencies because the equalization characteristics have low-pass filter characteristics.

[Brief explanation of the drawing]

FIG. 1 is a block system diagram showing one embodiment of the present invention, FIG. 2 is a diagram showing an example of the configuration of a delay circuit in the block system shown in FIG. 1, and FIGS. 3A and 3B are respectively shown in FIG. A diagram showing the frequency characteristics and group delay time characteristics of a delay circuit, FIGS. 4A and 4B are diagrams each showing an example of the frequency characteristics and group delay time characteristics of the device of the present invention, and FIG. 5 is a diagram showing another embodiment of the present invention. 6 is a diagram showing changes in frequency characteristics when the delay time is changed in the apparatus of the present invention, and FIG. 7 is a block system showing an example of a color signal recording system of a color under recording type VTR. 8 is a circuit diagram of an example of a conventional circuit, and FIGS. 9A and 9B are diagrams showing frequency characteristics and group delay time characteristics, respectively, of the conventional circuit. 10...Low frequency conversion carrier color signal input terminal, 11,
12...Delay circuit, 13...First addition circuit, 1
5... Second addition circuit, 16... Low frequency conversion carrier color signal output terminal, 18, 24... Delay line, 25...
Buffer amplifier with high input impedance.

[Claims]

1 Power supply circuit 1 through A line 13 and B line 14
In a power supply current detection circuit that detects a power supply current I supplied from 5 to a subscriber's telephone (TEL) through a detection resistor 11 inserted at least on the A line 13 side, Feed current I _f
, the positive direction detection current flowing through the first motor resistor 45
A first detection section 41 that detects i _f and a second detection section 42 that detects the reverse direction feeding current I _r flowing through the detection resistor 11 as the reverse direction detection current I _r flowing through the second monitor resistor 46 are provided. Here, the first detection unit 41 includes a first PNP transistor 411 whose base is connected to one end of the detection resistor 11 on the A line 13 side and whose collector is connected to a low voltage source, and the first monitor resistor 45. The first terminal in which the positive direction detection current i _f is applied from one end side.
a 1NPN transistor 412 inserted in series between the first NPN transistor 412 and the one end of the first monitor resistor 45;
1NPN transistor 412 and emitters connected