JPH0317277B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a luminance signal recording circuit of a VTR,
The present invention relates to a luminance signal processing circuit equipped with a comb filter circuit for separating video luminance signals and an AGC circuit for stabilizing video processing.

[Background of the invention]

As is well known, VTRs for household use use completely different signal processing methods for luminance signals and chromaticity signals among video signals to be recorded and reproduced. Accordingly,
In the luminance signal recording circuit, from the input video signal,
A filter is required to remove the chromaticity signal. However, since this filter also attenuates the high-frequency components of the luminance signal, there was a problem in that resolution and sharpness decreased, especially in the case of the NTSC system. These matters will be explained using FIGS. 1 and 2.

FIG. 1 is a block diagram of a conventional luminance signal recording circuit. The video signal input from input terminal 1 is
An AGC circuit consisting of a variable gain amplifier circuit 2 and an AGC detection circuit 4 controls the level to a certain specified level. The chromaticity component of the output of the AGC circuit is removed by a low-pass filter (hereinafter referred to as LPF) 3. Note that this output is fed back via the AGC detection circuit 4 in order to control the level of the AGC circuit. The output of the LPF 3 is recorded on a magnetic tape 9 via an emphasis circuit 5, an FM modulator 6, a recording amplifier 7, and a video head 8.

FIG. 2 shows the recording signal frequency distribution of 8 mm video. Since the luminance signal recording band is approximately 0.6MHz wider than that of conventional VTRs, the band of the LPF shown in Figure 1 can be expanded. However, if the width is made as wide as it is, the removal of the chromaticity signal will be insufficient.
This appears as dot interference in the luminance signal and as moiré in the chromaticity signal. Therefore, a luminance recording comb filter (hereinafter referred to as a Y-shaped comb filter) is required to widen the band of the luminance signal and remove the chromaticity signal to eliminate the above-mentioned interference. FIG. 3 shows the frequency characteristics when a Y-shaped comb filter is used.

The following conditions are necessary for the Y-shaped comb filter. These are: (1) sufficient removal of chromaticity signals, and (2) chromaticity reproduction comb filter (hereinafter referred to as C).
(3) There is no waveform deterioration of the luminance signal, (4) It is easy to integrate into an IC, and (5) PAL has a high carrier wave frequency for the chromaticity signal. This should be easily removed when using the method.

There has never been a Y-shaped comb filter that satisfies the above requirements.

[Purpose of the invention]

An object of the present invention is to provide a luminance signal processing circuit that satisfies the above-mentioned conditions in order to secure a luminance signal recording band and achieve the best image quality.

[Summary of the invention]

The present invention uses an LPF and a Y-shaped comb filter in an overlapping manner, amplifies the input level of the Y-shaped comb filter, and attenuates the output level, thereby suppressing the crosstalk of the chromaticity signal. ensure the degree of attenuation.

[Embodiments of the invention]

Embodiments of the luminance signal processing circuit of the present invention will be described below.

FIG. 4 is a block diagram of the video signal system of the present invention. The recording switches 12 and 24 are arranged as shown in the figure. First, the luminance signal recording system will be explained. In the same figure, the video signal input to input terminal 1 is
The level is controlled by the AGC circuit 2 and input to the LPF 10. The signal from which the chromaticity signal component has been removed by the LPF 10 and the comb filter 13 is fed back to the variable gain amplifier circuit 2 from the terminal 15 via the AGC detection circuit 4, where level control is performed. Therefore, due to the variation in the gain of the comb filter 13, the terminal 1
Even if the level of output terminal 5 fluctuates, the AGC circuit operates to keep this signal at a constant level.
The video signal processing in the frequency modulator 6 and the like can be made constant. The output of the frequency modulator 6 is sent to a magnetic tape 9 via a recording amplifier 7 and a video head 8.
recorded in

Next, the chromaticity signal recording system will be explained. The video signal input to the input terminal 1 is input to a band pass filter (BPF) 25 via a switch 24.
The chromaticity signal having a band of chromaticity subcarrier frequency f _sc ±500KHz is extracted by BPF25 and ACC circuit 26
is controlled to a certain level. The chromaticity signal whose level has been controlled to a constant value is passed through the recording chromaticity signal processing circuit 27, converted to a low frequency chromaticity subcarrier frequency f _Lsc ±500KHz by the converter 28 during recording, and then outputted to the LPF 2.
9. The output of the LPF 29 is added to the luminance signal in an adder 16 and recorded on a magnetic tape 9 via a recording amplifier 7 and a video head 8.

Next, the chromaticity signal reproduction system will be explained. During playback, the switches 12 and 24 are connected to the opposite sides shown. The BPF 25 and the ACC circuit 26 are shared during recording and reproduction, and the comb filter 13 is used as a Y-shaped comb filter during recording and as a C-shaped comb filter during reproduction. In this way, the comb filter 13 is constructed.
1H delay line can also be used.

The low frequency chromaticity signal input from the input terminal 21 during playback is converted to a high frequency range by the converter 23 during playback.
BPF25, ACC circuit 26, comb filter 13,
Output terminal 33 via reproduced chromaticity signal processing circuit 32
Output to. Note that 34 is a carrier generator that generates a carrier of f _sc +f _Lsc .

In the configuration described above, the comb filter 13
A comb filter circuit 100 of the present invention including the following will be described.

As is well known, the principle circuit of a comb filter is the fifth one.
This is the configuration shown in the figure. The same parts as in FIG. 4 are given the same reference numerals. In FIG. 5, the switch 12 is in the position shown in the figure during recording, and is reversed during playback.

First, the time of playback will be explained. A reproduced chromaticity signal is input to the input terminal 30, and this signal and the signal delayed by the 1H delay line 35 are subtracted by the subtracter 38.
This has the characteristics of a C-shaped comb filter and is output to the output terminal 31.

Next, the Y-shaped comb filter during recording will be explained. The luminance signal inputted from the input terminal 11 is delayed by the 1H delay line 35 and added to the adder 37.
and output to the output terminal 15. However, the figure shows a case where the frequency characteristics of the 1H delay line 35 are flat, and the delay line actually used in a VTR is a glass delay line, which is a BPF (Band Pass Filter) with a pass band around 2.5 to 6.5 MHz. It has the characteristics of
Therefore, there is no problem when operating as a C-shaped comb filter, but when operating as a Y-shaped comb filter, in the configuration shown in FIG. This is disadvantageous as it becomes relatively large. Therefore, the configuration shown in FIG. 6 is necessary. The same parts as in FIG. 5 are given the same reference numerals, and some explanations are omitted. 6th
In the figure, the signal input from input terminal 11 is
It is added to the LPF 39 and an HPF (High Pass Filter) or BPF 40 having a cut-off frequency approximately equal to the LPF 39. The output of BPF40 is 1H delay line 3
The signal delayed in step 5 is added to the adder 41, and the luminance component of this band is extracted. and adder 42
Then, it is added to the previous output of the LPF 39 and outputted to the output terminal 15 as the desired signal. If this is done, the above-mentioned inconvenience will not occur. The cutoff frequency of LPF39 is approximately 1H.
It should be selected so that it is at the lower limit of the passband of the delay line 35 and outside the chromaticity signal band (f _sc ±500KHz).
Also, since the 1H delay line 35 has band characteristics, the HPF
Or BPF40 seems unnecessary, but LPF39
It is also necessary to have a complementary relationship with the side shunt signals.

In addition, in an ideal Y-shaped comb filter, the chromaticity subcarrier frequency f _sc and then nfh (n;
The degree of attenuation at frequencies separated by an integer (fh: horizontal synchronization frequency) is infinite. But in reality,
Due to coupling between the input and output of the 1H delay line, etc.
Only about 20dB of attenuation can be obtained. For this reason, the fourth
The LPF 10 shown in the figure has a characteristic that the degree of attenuation of f _sc is about 20 dB.

In the configuration shown in FIG. 6, at least the switch circuit 12 is built into the chromaticity signal processing IC, so Y
When operating as a comb filter, it is inevitable that the chromaticity signal being recorded will experience crosstalk from, for example, a switch circuit. Therefore, in order to eliminate the influence of this crosstalk, the sixth
By providing an amplifier at the output of the BPF 40 shown in the figure and providing an attenuator before the adder 42 of the comb filter output, the crosstalk of the chromaticity signal can be attenuated by the amplification degree of the amplifier.

An example of a filter that satisfies the above requirements is shown in FIG. The same parts as in FIG. 6 are given the same reference numerals, and some explanations are omitted. In the figure, R ₁ to R ₃ are resistors, L ₁ is an inductor, C ₁ and C ₂ are capacitors, and E ₁ is a DC voltage source. During recording, the switch 12 is connected as shown.

The luminance signal inputted from the input terminal 11 is applied to the negative phase input terminal and the positive phase input terminal of the subtracting amplifier 50 through both resistors R ₁ and R ₂ having substantially the same value. A trap consisting of an inductor L ₁ and a capacitor C ₁ connected in series is connected to the positive phase input terminal side. The trap frequency is selected to be near the chromaticity subcarrier frequency _fsc , the passing center frequency of the 1H delay line 35, or the passing center frequency of the BPF 40 shown in FIG. Therefore, a signal corresponding to the output of the LPF 39 in FIG. 6 is obtained at the positive phase input terminal of the subtraction amplifier 54. (There are almost no frequency components higher than the trap frequency of L ₁ and C ₁ , especially in the case of the NTSC system.) On the other hand, since the input signal is directly applied to the negative phase input terminal of the subtracting amplifier 50, the input signal is applied to its output terminal. obtains a signal corresponding to the output of BPF 40 in FIG. This signal is amplified by the amplification circuit 51 for suppressing the crosstalk of the chromaticity signal described above, and then a Y-shaped comb characteristic is obtained by the 1H delay line 35 and the summing amplifier 52. By attenuating this signal with an attenuator 53, crosstalk is suppressed. This output is passed through capacitor _C2 to subtraction amplifier 5
It is added to the negative phase input terminal of No.4.

A DC voltage equal to that of the input terminal 11 is applied here via a voltage source E ₁ and a resistor R ₃ . On the other hand, the same signal as the signal at the positive phase input terminal of the subtracting amplifier 50 is applied to the positive phase input terminal. Therefore, a signal corresponding to the output terminal 15 in FIG. 6 is obtained at the output terminal 15, and the intended purpose can be achieved.

By the way, when the circuit connected to the positive phase input terminal of the subtracting amplifier 50 is implemented as an IC, it is desired to have a simple configuration since it is an external element. Therefore, L ₁ , C ₁
A series trap consisting of In principle, a general LPF may be used, but the frequency characteristics of the circuit connected here should be determined in correlation with the bandpass characteristics of the 1H delay line 35. Therefore, it is desirable that this characteristic be easy to adjust. _If a series trap consisting of _an inductor L _{1 and a} capacitor C ₁ is used as shown in FIG. By changing it, you can easily change its characteristics. For this adjustment,
As shown in Figure 8, inductor L ₁ and capacitor
You can also add a resistor _R4 to the series trap of _C1 .

Furthermore, the 1H delay line 35 generally has considerable loss even within the passband. In FIGS. 8 and 9, a configuration is adopted in which this is compensated for by the summing amplifier 52, but
As shown in FIG. 9, at the front stage of the 1H delay line 35,
A preamplifier 55 may also be provided.

Further, as shown in FIG. 10, an amplifier 56 having the same amplification as the amplifier 51 that amplifies the input signal to the Y-shaped comb filter consisting of the 1H delay line 35 and the summing amplifier 52 is connected to the positive phase input side of the subtracting amplifier 54. 7, the attenuator 53 in FIG. 7 may be removed and its attenuation may be supplemented by the subtraction amplifier 54.

FIG. 11 shows an example of a circuit based on the embodiment shown in FIG. 7. This has a configuration in which the amplifier 51 in FIG. 7 is supplemented by a subtracting amplifier 50. 7th
The same parts as in the figures are given the same reference numerals, and some explanations are omitted. _R1 to _R3 , _R5 to _R9 are resistors, _L1 is an inductor, _C1 to _C3 are capacitors, _Q1 to _Q5 are transistors, _I1 to _I5 are current sources, and _E1 is a DC voltage source. It is.
The degree of amplification of the level input to the comb filter is determined by the resistor _R5 and the emitter resistances of the transistors _Q1 and _Q2 , and is approximately 20 dB in this embodiment. Resistance _R6 ,
R ₇ constitutes the attenuator 53 in FIG. DC fluctuations caused by mode switching of the switch 12 are amplified by a subtracting amplifier consisting of transistors Q ₅ , Q ₄ and the like, and output from an output terminal 15 as a control signal for the AGC circuit.
Therefore, in order to ensure the band of the Y-shaped comb filter and not allow direct current fluctuations to pass through, the capacitor _C3 and the resistor _R7 constituting the HPF are made small. In addition, considering the above factors and the Miller effect of transistor Q ₅ , in order to reduce the impedance of the base of transistor Q ₅ , capacitor C ₂
also needs to be made smaller.

Furthermore, when the chromaticity subcarrier frequency is high and the comb filter is unnecessary, as in the case of the PAL system, it is sufficient to remove at least the trap consisting of the inductor L ₁ and the capacitor C ₁ . In this way, the input signal at the input terminal 11 is amplified by the subtracting amplifier made up of transistors Q ₄ , Q ₅ , etc. via the resistor R ₂ and is output to the output terminal 15 . At this time, the output of the subtracting amplifier consisting of transistors Q ₁ , Q _{2 ,} etc. becomes zero, so the input signal of transistor Q ₅ becomes zero. Therefore, the signal input from the input terminal 11 is amplified and output to the output terminal 15, thereby achieving the intended purpose.

Furthermore, FIGS. 7 to 11 show embodiments of the present invention.
In the figure, the signal that has passed through the switch 12 may be used as the signal that is added or subtracted from the signal that has passed through the 1H delay line that constitutes the comb filter.

〔Effect of the invention〕

As described above, the present invention provides a new configuration of a Y-shaped comb filter circuit used in a brightness recording system of a VTR.

By amplifying the signal input to the comb filter and attenuating the output, crosstalk of the chromaticity signal can be suppressed and the chromaticity signal can be sufficiently removed. Can be used for both purposes.

In addition, since the main signal is processed using only an external LC trap and amplifier, waveform deterioration can be minimized.
When using the PAL system, it can be handled by removing at least this trap, which has the effect of making it easier to perform when integrated into an IC.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional VTR luminance recording system. Figure 2 is a frequency characteristic diagram of an 8 mm video recording signal. Figure 3 is a characteristic diagram of a comb filter. Figure 4 is a diagram of the recording signal processing circuit of the present invention. 5 is a principle circuit diagram of a comb filter, FIG. 6 is a block diagram of a comb filter, FIGS. 7 to 10 are block diagrams showing one embodiment of the present invention, and FIG. FIG. 3 is a circuit diagram showing a specific example of the circuit shown in the figure. 2...Variable gain amplifier circuit, 4...AGC detection circuit, 10...LPF, 100...Comb filter circuit, 50, 54...Subtraction amplifier, 51...Amplifier, 35...1H delay line, 52... ...summing amplifier,
53...attenuator.

Claims (1)

[Claims] 1. A variable gain amplifier and an AGC detection circuit that detects the level of a video signal amplified by the variable gain amplifier and controls the gain of the variable gain amplifier so that the amplified video signal is at a constant level. consisting of
It consists of an AGC circuit, an amplifier circuit that amplifies the output signal from the AGC circuit, a 1H delay line, and a summing amplifier, and a comb filter that extracts a luminance signal from the output signal of the amplifier circuit. It consists of an attenuation circuit that attenuates an output signal, an emphasis circuit that nonlinearly emphasizes the output signal of this attenuation circuit, and a frequency modulation circuit that performs frequency modulation with the output signal of this emphasis circuit. A brightness signal processing circuit, comprising: detecting the level of the brightness signal after passing through the shaped filter; and controlling the gain of the variable gain amplifier so as to keep this level constant.