JPS58119288A - Signal reproducing circuit - Google Patents

Abstract

PURPOSE:To prevent a luminance signal and a chroma signal from being timewise shifted, by converting a reproducing signal for high frequency at the reproductionof PAL signals in a VTR, passing the signal through a broad band delay line, and using the signal for a main signal not only for dropout compensation. CONSTITUTION:Signals are switched at a head changeover circuit 11 via heads 1, 2 and preamplifiers 9, 10 with a head switching pulse and become continuous signals. A part of the signals passes through a low pass filter 13 to be a chroma signal (b), which is frequency-converted at a high band conversion circuit 22, and applied to a mixing circuit 26 via comb type filters 23, 24. A luminance signal is obtained from the other output via an automatic gain control circuit 35, a high frequency conversion circuit 36, an oscillator 37 and an amplifier 100, the dropout is detected at a dropout detection circuit 56 at the next stage, and the output controls a switch 39. Normally, a 1H delay luminance signal B through a vertical corelation circuit 101 is dropped out, then a luminance signal A not through a delay line is outputted and inputted to a mixing circuit 26 via a switch 51, and a video signal (v) is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for preventing a time lag between a luminance signal and a chroma signal when reproducing a PAL signal in a home VTR.

Figure 1 shows the PAL of a home-use (for example, VH8 system) VTR.
This shows a conventional example of a signal reproducing circuit, in which the reproduced signal from the head 1.2 is transmitted through rotary transformers 3, 4, . Resonant capacitors 5, 6. The head 1. rotary transformer 3 and the resonant capacitor 5. and the head 2. For VH8, the peaking frequency is selected to be around 4.4 M fJz.Damping resistor 7
．． 8 is for adjusting the Q of peaking. The output of preamplifier 9.10 is a head switching pulse (NTSC5C
A continuous signal is generated by a switch circuit 11 that is switched by HIz; P A L25Hz), and part of it passes through a bypass filter (hereinafter abbreviated as HPF) 12 to become a luminance signal a, and a part passes through a low-pass filter (hereinafter abbreviated as LPF).
15 through the chroma signal (627 KHz for VH8
) becomes b.

The FM brightness signal a is sent to switch 14. IH delay line (IH is 1
horizontal period) consisting of 15 FM dropout compensation circuits (
(hereinafter abbreviated as FMDOC circuit) 162 phase equalizer 17. Limiter 18. Demodulator 19. It passes through LPF'20+de-emphasis circuit 21 and becomes a luminance signal f. On the other hand, LPF1
The chroma signal passing through 3 is converted to 4 by the high frequency conversion circuit 22.
．． After becoming a 43 MHz chroma signal C, it passes through a comb filter 25 consisting of a 2H delay line 23 and a subtraction circuit 24, thereby becoming a chroma signal e with reduced influence from adjacent tracks on the tape. The chroma signal e is mixed with the luminance signal f in a mixing circuit 26 to form a video signal V.

Figure 2 shows the time relationship between the luminance signal 27 and the chroma signal 28, where (1) is the time relationship between the luminance signal f and the chroma signal in Figure 1, and (2) is the luminance signal f and the 2H delay. The time relationship (6) with the chroma signal d is the luminance signal f,
The time relationship with the chroma signal e which is the output of the comb filter 25 is shown. In (1), the center of gravity (left-right symmetry point) of the chroma signal is at the intersection 29 between C2 and C3, and the position of the corresponding luminance signal is between Y3 and Y4. However, in (3), the center of gravity of the chroma signal is located at 6o between Y4 and Y5 of the luminance signal, and is shifted later in time by 1H compared to (1). On a TV screen, this corresponds to the fact that the chroma signal is shifted 1H below the luminance signal by performing recording and reproduction once on the VTR. In particular, when dubbing is repeatedly performed on a VTR, the chroma center of gravity shifts downward by 1H each time, resulting in extremely large deterioration in image quality.

SUMMARY OF THE INVENTION An object of the present invention is to provide a signal reproducing circuit in which the chroma center of gravity does not shift when reproducing a PAL signal.

In order to achieve the above object, the present invention delays the luminance signal by 1H during reproduction to eliminate the shift of the center of gravity of the chroma signal. In particular, in order to use a glass delay line which is lower in cost than COD etc., the reproduced signal is converted to a high frequency band, and the glass delay line is wideband so that the signal does not deteriorate. Furthermore, a surface acoustic wave filter (hereinafter abbreviated as SAW filter) is used for reproduction equalization. With the above, a high-performance video DOC can be realized at the same time. Another major feature is that since a broadband delay line is used, it can be easily expanded to include functions such as vertical correlation.

The present invention will be explained in detail below based on specific examples.

FIG. 5 shows an embodiment of the present invention, which differs from the conventional example shown in FIG. 1 in the following points: (1) resonance trimmer condenser 5.
(2) an automatic gain control circuit (hereinafter abbreviated as AGC circuit) 35 is used after the head switching circuit 11; ．． High frequency conversion circuit 36
9 oscillator 37. (6) Switch 69.The reproduction equalization SAW filter 3B and the amplifier 100 are used. Wideband 1
There is an FMDOC consisting of an H delay line 4° amplifier 41, the output of the switch 39 is divided into two, and one side is connected to a limiter 42. Demodulator 46LPF44. De-emphasis circuit 2001 and the other 1H delay line 40. After passing through the amplifier 41, the limiter 45. Demodulator 46. LPF47. By using the de-emphasis circuit 201 and two demodulation systems, (4) the luminance signal A that is the output of the de-emphasis circuit 200 and the 1H delayed luminance signal B that is the output of the de-emphasis circuit 201, Differential amplifier 48 limiter 49. The vertical correlation circuit 101 is configured by the differential amplifier 50, and the video DOC is also configured by the switch 51.

The first important point of the present invention is to perform high frequency conversion (for example, in VH8, the reproduced F'M signal of 4.5±3 MHz is converted to 1
0 MHz to 14.5 ± 5 MHz), it is possible to use a wideband delay line, so the 1H delay signal, which was conventionally used only for dropout compensation due to its low center frequency and narrow band, can be used as the main signal. It is something that can be used. FIG. 4 shows an example of the frequency characteristics of the 1H delay line. Therefore, in the case of P and AL signal reproduction, if the 1H delayed luminance signal B is used as the luminance signal, the time relationship between the chroma signal and the luminance signal will be as shown in FIG. 2 (4). That is, the center of gravity #55 of the chroma signal is between Y5 and Y4, and is equivalent to the center of gravity in the case of the original signal shown in FIG. 2 (1). However, the chroma signal distribution has expanded. As described above, even if dubbing is repeated, the on-screen chroma signal does not shift downward relative to the luminance signal, and image quality deterioration can be minimized. Further, by using the vertical correlation circuit 101 which can be realized with a simple configuration in conjunction with the present invention, noise and crosstalk can be removed and the S/N can be improved by about 2 dB.

The second important point of the present invention is that the reproduced FM signal is subjected to high frequency conversion to perform FM equalization using the SAW filter 38 having a flat group delay characteristic. At the same time, the frequency characteristics of the preamplifiers 9 and 10 are made almost flat, and are configured to absorb variations in inductance between heads.

In conventional LC resonance type FM equalization, the amount of group delay changes greatly near the resonance point, and the S/N deteriorates because AM noise is converted to PM noise. However, in the SAW filter 38, the amplitude characteristics and group Delay characteristics can be set independently and S/N improvement is possible. An example of the amplitude characteristic 521 group delay characteristic 53 of the SAW filter 38 is shown in FIG. SAW filter 68
The configuration also includes a chroma removal filter 54 (HPF or trap filter).

Further, the output of the SAW filter 68 is applied to the dropout detection circuit 56 through the amplifier 10tJ, so that a dropout pulse P is generated when the signal level becomes, for example, 20 dB below the normal level. After the dropout pulse P undergoes appropriate waveform shaping, it is applied to the FMDOC switch 39, and is switched so that the signal g from the SAW filter 68 is output during normal times, and the signal g is output during dropout. On the other hand, the dropout pulse P passes through the approximately 1H delay circuit 57 and is waveform-shaped so that it is temporally wider than the period corresponding to the dropout of the 1H delayed luminance signal B, and then is applied to the switch 51. As a result, under normal conditions, the 1H delayed luminance signal B passing through the vertical correlation circuit 101
1 At the time of dropout, luminance signal A is output. Note that the above-mentioned vertical correlation circuit 101. Although the video DOC switch 51 is unnecessary in terms of only preventing the center of gravity of the chroma signal from moving, it is more effective to use it because the increase in cost is small and the performance is greatly improved.

On the other hand, in the case of NTSC, since a 1H delay line 58 is used for the reproduction chroma comb filter, the chroma center of gravity shifts by 0.
5H, and correction like the above PAL cannot be performed. Therefore, as shown in FIG. 6, the luminance signal A that has passed through the vertical correlation circuit 102 is used during normal times, and is used as the 1H delayed luminance signal B during dropout. In this case, in the pulse delay circuit 57,
There is no need to delay the dropout pulse P by 1H,
From the SAW filter 38 to the video DOC switch 510
The time during which the luminance signal is delayed (usually 500μS to 1.5μS)
8), it is sufficient to make the period wider before and after the period corresponding to the dropout of the luminance signal A. Even in the case of PAL, if the movement of the chroma center of gravity is not a problem, it is easy to think that the configuration shown in FIG. 6 can be used as in NTSC.

As described above, according to the present invention, it is possible to prevent a time lag between a luminance signal and a chroma signal in PAL signal reproduction. This is particularly effective when dubbing is repeated.

Further, the vertical correlation circuit and video DOC that can be easily obtained from the configuration of the present invention are extremely effective in improving image quality.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of a conventional reproducing circuit, and FIG.
3 is a waveform diagram showing the time relationship between the 14 degree signal and the chroma signal, FIG. 3 is a block diagram showing an embodiment of the present invention, and FIG.
FIG. 5 is a frequency characteristic diagram showing an example of the insertion loss characteristics of a 1H delay line, FIG. 5 is a characteristic diagram showing an example of the gain and delay characteristics of a SAW filter, and FIG. 6 is another embodiment of the present invention. FIG. 25...2H delay line 5...SAW filter 6
9... Switch 40... Wideband 1H delay line 51... Switch Figure 4 Circumferential speed @ (MHz) 57 Number of cycles (MHz)

Claims (1)

[Claims] (t) A circuit for converting a playback signal into a high frequency band into an FM luminance signal system in a playback circuit of a video tape recorder, a playback equalization amount for equalizing the high band converted signal, and dropout compensation for the playback signal. It is equipped with a dropout compensation circuit including a 1H (one horizontal period) delay line to perform PAL processing, and two demodulators that separately demodulate the signal before passing through the 1H delay line and the signal after passing through the 1H delay line. A signal reproducing circuit characterized in that when reproducing a signal, a signal after passing through a 1H delay line is used as a main signal. 2. The signal regeneration circuit according to claim 1, wherein a surface acoustic wave filter is used as the regeneration equalizer.