GB2099658A

GB2099658A - Video signal processing circuit for a PAL VTR system

Info

Publication number: GB2099658A
Application number: GB8213950A
Authority: GB
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1981-05-18
Filing date: 1982-05-13
Publication date: 1982-12-08
Also published as: JPS57190490A; DE3218538A1; GB2099658B; DE3218538C2

Abstract

Known magnetic tape systems for recording and reproducing PAL television signals of the type shown in Figure 1 in which during reproduction the luminance and chrominance signals are processed separately and then combined 19 to produce a signal for application to a domestic television receiver, suffer from the disadvantage that the chrominance signal in the final signal applied to the television set, lags the luminance signal by one line. This defect becomes important if tapes are repeatedly played and re-recorded since the lag becomes cumulative. According to the invention the defect is overcome by delaying the luminance component by one line either before recording (as in Figure 3) or during playback. Several methods for accomplishing this are discussed using glass acoustic delays and semi-conductor delays, such as CCD devices. <IMAGE>

Description

SPECIFICATION Video signal processing circuit for a PAL VTR system The present invention relates to a signal processing circuit for correcting undesired displacement between the luminance (luma) and chrominance (chroma) signals of a picture reproduced by a magnetic video tape recorder (hereinafter referred to as a STIR), and more particularly, to a signal processing circuit for preventing occurrence of time difference between the luminance signal and the chroma signal when a phase-alternation line (PAL) system video signal is reproduced in a home VTR.

In a conventional helical-scan home VTR, a reproduction signal processing circuit essentially comprises a chroma signal comb filter as a means of suppressing chroma crosstalk components from the adjacent video tracks which are reproduced when a head traces across the adjacent video tracks.

Fig. 1 of the accompanying drawings shows by way of example, a prior art PAL signal reproducing circuit for a home VTR (e.g., the video home system (VHS) type). In the Figure, reproduced signals extracted from magnetic heads 1 a and 1 b are respectively supplied to preamplifiers 3a and 3b via rotary transformers 2a and 2b, respectively. The head 1 a and the rotary transformer 2a and the head 1 b and the rotary transformer 2b are respectively combined such that the peaking frequency corresponds to near the white carrier (4.4 MHz in the VHS system) of the FM luminance signal. The outputs from the preamplifiers 3a and 3b are supplied to a switching circuit 4 which is switched by head switching pulses (25 Hz pulses in the PAL system) so that the signals picked up by the two heads may be combined into a continuous signal.A part of this continuous signal forms an FM luminance signal a through a high-pass filter 5 (hereinafter simply referred to as a HPF) and the other part is converted to a down-converted chrominance signal 6 (627kHz in the VHS system) through a low-pass filter (LPF) 14. The FM luminance signal a is passed through an FM dropout compensation circuit (referred to as DOC circuit). The DOC circuit comprises a switch 6 responsive to dropout detection for permiting DOC to the subsequent circuits and a 1 H delay line 8 where 1 H is on horizontal period of the picture. The FM luminance signal is supplied through a phase equalizer 9, a limiter 10, a demodulator 1 1, a lowpass filter 12 to a de-emphasis circuit 13 to develop a demodulated video signal or luminance signal f. The demodulated signal fis introduced into a first mixer 1.On the other hand, the downconverted chrominance signal which was passed through the LPF 14 is converted to a highfrequency signal by an up-converter (highband converter) 1 5 thus producing a chroma signal C.

Then, the chroma signal C is passed through a comb filter 16 comprising a 2H delay line 17 to suppress cross-talk components and a second mixture 1 8 which functions as a subtractor thereby generating a chrominance signal e having a reduced effect of the crosstalk from the adjacent tracks on the video tape. The chrominance signal e is combined with the luminance signal fby the first mixer 1 9 and the resulting video signal v is generated at an output terminal 20.

With the above signal processing, the chrominance signal is generated by mixing the chrominance signal which is not delayed by the chrominance signal comb filter 1 6 and the chrominance signal which is delayed by 2H via the 2H delay line 1 7. Consequently, the information center of the chrominance signal introduced into the first mixer 19 is delayed by 1 H (where 1 H=1 horizontal period) relative to the luminance signal. Thus, the vertical displacement or chrominance displacement occurs and the picture is therefore deteriorated. The reason will now be described with reference to Fig. 2.

In Fig. 2 showing the time relation between a luminance signal 31 and a chrominance signal 32, a waveform Fig. 2(A) shows the time relation between the luminance signal fand the chrominance signal b shown in Fig. 1, a waveform Fig. 2(D) shows the time relation between the luminance signal fand the chrominance signal d delayed by 2H, and a waveform Fig. 2(E) shows the time relation between the luminance signal f and the chrominance signal e which is the output of the comb filter 1 6. In Fig. 2(A), the center of energy distribution (the point of symmetry) of the chrominance signal is at 33 between C2 and C3 and the corresponding centre position of the luminance signal exists between Y3 and V4.

However, due to subtraction by a subtractor 1 8 the energy distribution of the chrominance signal e changes to be centered between V4 and V8 of the luminance signal fas indicated in Fig. 2(E).

Thus, the chrominance signal e is delayed in time by 1 H as compared with the case of Fig. 2(A). In terms of the picture on the TV screen, this is equivalent to that the chrominance signal is shifted downward by 1 H with respect to the luminance signal for every reproduction of the record in the VTR. In particular, if dubbing is performed repeatedly in the VTR, chrominance displacement takes place by 1 H for every dubbing and thus the resulting increase in such displacement becomes so large on the screen that it is no longer tolerable.

It is therefore the primary object of the present invention to provide a signal processing circuit for PAL system magnetic video tape recorders in which vertical displacement or chrominance displacement on the screen due to a relative delay of the information center of the chrominance signal is corrected, thereby improving the picture quality and the dubbing characteristic.

A feature of the present invention resides in that a recording or reproduciton signal processing circuit is provided with a delay circuit for delaying a luminance signal by one horizontal period of a video signal to be processed (1 H), thereby compensatin'g a relative delay of the chroma signal.

The present invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a circuit diagram of a prior art reproduction signal processing circuit; Figs. 2 and 4 are waveforms diagrams showing the relative time relations between the luminance signal and the chrominance signal during recording and reproduction; Fig. 13 is a circuit diagram showing an embodiment of a signal processing circuit in which the invention is applied to a recording signal processing circuit; Fig. 5 is a circuit diagram showing an example of a wideband delay circuit using a glass delay line; Fig. 6 is a circuit diagram showing an embodiment of the signal processing circuit of this invention, which concurrently utilizes an FM signal dropout compensation circuit; and Fig. 7 is a circuit diagram showing another embodiment of the signal processing circuit of this invention, which concurrently utilizes a video dropout compensation circuit.

Referring to Fig. 3, there is illustrated an embodiment of a signal processing circuit according to the invention, in which a recording signal processing circuit is provided with a 1 H delay circuit for delaying a luminance signal by 1H.

In this embodiment circuit, the luminance signal in a video signal to be recorded is supplied to an input terminal 23 and delivered to the output of an LPF 24 (cutoff frequency =3.0 MHz) and the chrominance signal is delivered to the output of a band-pass filter (BPF) 28. The luminance signal is through a 1 h delay circuit 25, frequency moduiated by an FM modulator 26 and then delivered to a recording amplifier 27, and the chrominance component of the video signal to be recorded is converted to a down-converted chrominance signal by a down-converter 29 and then delivered to the recording amplifier 27, thereby delivering them as recording signals to a recording head 30.

The operation of compensating chrominance displacement according to the embodiment of this invention shown in Fig. 3 will now be described with reference to Fig. 4 showing the relative time relation between the luminance signal and the chrominance signal during recording and reproducing operations.

Here, time slots n, n+1, ....... indicate the line numbers on the screen and each line length corresponds to 1 H.

Since the prior art recording signal processing circuit does not include the 1 H delay circuit 25 of Fig. 3, the recording video signal is the same with the conventional video signal from the input terminal 23 and this video signal is assumed to be the signal in Fig. 4(A). When the video signal of Fig. 4(A) is passed through the reproduction signal processing circuit of Fig. 1 the resulting reproduced video signal from the output terminal 20 becomes as shown in Fig. 4(B). While the centre (the center position) of the chroma information of the chroma signal C in the conventional video signal Fig. 4(A) thus lies between the line (n+3) and the line (n+4), in the case of the prior art circuit the center position lies between the line (n+4) and the line (n+5) showing that the center of the chroma information of the reproduced video signal is delayed by 1 H.

In accordance with the embodiment of the invention shown in Fig. 3, the 1 H delay circuit 25 delays the luminance signal by 1 H so that the recording video signal becomes as shown in Fig.

4(F) and the centre of the chroma information leads the conventional video signal Fig. 4(A) by 1 H. If the video signal Fig. 4(F) is passed through the reproduction signal processing circuit of Fig.

1, the resulting reproduced video signal from the output terminal 20 becomes as shown in Fig. 4(1).

The centre of the chroma signal in the reproduced video signal Fig. 4(1) lies between the line (n+3) and the line (n+4) and this corresponds to the center of the chroma information in the conventional video signal Fig. 4(A). As a result, the shift of the chroma information in the vertical direction is suppressed.

While, in the above embodiment of the invention, the 1 H delay circuit 25 is connected before the FM modulator 26 in the recording signal processing circuit, the 1 H delay circuit 25 may be connected after the FM modulator 26. As an alternative it is also possible to constantly connect the 1 H delay circuit 25 in the luminance signal processing circuit of Fig. 1 in series to constantly delay the luminance signal by 1 H.

The requirements accompanying the realization of the invention are the need to use a wideband delay circuit for the 1 H delay circuit 25 and the increased cost due to the additional provisional of the 1 H delay circuit 25.

Fig. 5 shows an embodiment of the 1 H delay line 25 comprising an acoustic delay line such as a glass delay line for realizing a wideband delay line which is one of the requirements.

The circuit of this embodiment comprises an up-converter 37, a 1 H glass delay line 38 having a band center frequency of about 12 MHz and a down-converter 39 and the circuit utilizes a characteristic such that the bandwidth of the glass delay line 38 increases with the band center frequency. For example, by selecting the band centre frequency about 12MHz, it is possible to ensure a band of about 12+4 MHz and also practically ensure a band of the luminance signal (about O to 4 MHz) and its frequency modulated signal (about 4.4+3 MHz).Where it is necessary to ensure a luminance signal band width of about O to 4 MHz as in the case of Fig. 3, the signal from an input terminal 36 is up-converted in the upconverter 37 by using a carrier signal of about 9MHz, down-converted to the original signal by the down-converter 39 and then delivered to an output terminal 40, thereby maintaining the desired bandwidth.

Also, by using a semiconductor delay device, such as charge-coupled device (CCD) or MOS memory for the 1 H delay circuit as a means of ensuring the bandwidth, it is possible to attain the desired video band.

Next, approaches of reducing the increase in the cost will be described. Fig. 6 shows an embodiment of the invention for this purpose in which a conventional dropout compensation (DOC) 1 H delay line 45 using an FM signal is commonly used as a 1 H delay circuit 45 for compensating a shift in the vertical contour of the chroma information, resulting in cost reduction and simplified circuit. In this embodiment, the output of the 1 H delay circuit 45 is always delivered to the input terminal of an FM demodulator 46 and a dropout compensation switch 44 applies to the input terminal of the 1 H delay circuit 45 the output of a HPF 43 when there is no dropout and the output of the 1 H delay circuit 45 when there is any dropout.Thus, a dropout-compensated signal is always delivered to each of the input and output terminals of the 1 H delay circuit 45 thereby effecting the dropout compensation. The envelope detection of pickup input waveform may be used as the method of detecting dropouts and its explanation will be omitted since it is well known in the art. Further, since the signal delayed by 1 H is always applied to the input of the FM demodulator 46 as mentioned previously, the demodulated luminance signal is delayed by 1 H as compared with that in the prior art circuit and thus the shift in the vertical direction of the chroma information it suppressed.

Fig. 7 shows another embodiment of the invention in which a dropout compensating 1 H delay circuit using a video signal is used commonly as the 1 H delay circuit 45 for compensating a shift in the vertical direction of the chroma information, thereby improving the dropout compensation performance and reducing the increase in the cost. In this embodiment, the dropout compensation circuit comprising the dropout compensation switch 44 and the 1 H delayNcircuit 45 is connected between the FM demodulator 46 and the mixer 47, thereby effecting the dropout compensation by means of the video signal and improving the performance.

Also, the luminance signal is always delayed 1 H by the 1 H delay circuit 45, thereby compensating the vertical displacement or the chroma displacement on the picture screen.

In accordance with the embodiment of the present invention shown in Fig. 7, it is possible to use a semiconductor delay device such as CCD having a pass band in the video band, making it suitable from a system point of view to construct the ciruitry with integrated circuits and reducing the circuitry in size much than previously. In Figs.

6 and 7, only the principal component parts of the circuitry in Fig. 1 are shown for purpose of simplifying the description. Circuit blocks 48 to 51 are identical with circuit blocks 14, 1 5, 1 7 and 1 8 in Figure 1, respectively.

By using the present invention it is possible to compensate a shift in the vertical chroma contour on the screen and also reduce the increase in the contour shifting due to a dubbing.

It should be understood that in accordance with the embodiment of Fig. 5, the luminance signal is up-converted (e.g., the reproduction FM signal of 4.5+3 MHz is raised by 1 OMHz to 14.5+3 MHz in the VHS system) and this makes possible the use of a wideband delay line, thereby allowing to use a principal signal the 1 H delayed signal which has heretofore been used only for dropout compensation purposes due to the narrow band with the low center frequency. Thus, if the 1 H-delayed luminance signal g is used as the luminance signal in the PAL signal reproduction, the time relation between the chroma signal and the luminance signal becomes as shown in Fig. 2(G). In other words, the center position 35 of energy distribution of the chroma signal lies between V3 and V4 and thins is equivalent to the center position of the conventional signal shown in Fig. 2(A). However, the chroma signal distribution is expanded.

Claims

1. A signal processing circuit for a magnetic video tape recorder for recording and reproducing PAL system television signals in which a reproduction signal processing circuit includes a chroma signal comb filter for suppressing crosstalk components in the chrominance signal, the improvement comprising delay mean for constantly delaying a luminance signal of a video signal to be recorded or reproduced by one horizontal period (1 H) of said video signal, said delay means being connected in series to or in a luminance signal processing circuit of the recording or reproduction signal circuit.

2. A video signal recording and reproduction processing circuit for a PAL system video tape recorder wherein an FM luminance signal processing circuit comprises a dropout compensation circuit including one-horizontalperiod (1 H) delay means for producing a 1 H delayed luminance signal and a switch circuit responsive to dropout detection for permitting the output of said 1 H delay means to be supplied to said luminance signal processing circuit, the improvement wherein when said switch detects no dropout said 1 H delay means is constantly coupled in series to a luminance signal path of the luminance signal processing circuit.

3. A signal recording and reproducing processing circuit according to claim 1 or 2, wherein an FM demodulator for demodulating said luminance signal is connected before or after said 1 H delay means.

4. A signal processing circuit according to claim 1, 2 or 3, wherein said 1 H delay means comprises an acoustic delay line or a chargecoupled device (CCD).

5. A signal processing circuit according to claim 1, 2, 3 or 4, wherein said luminance signal processing circuit comprises up-converter means for up-converting a reproduction signal, and down-converter means for down-converting said reproduction signal, said 1 H delay means being connected between said up-converter means and said down-converter means.

6. A signal processing circuit according to claim 5, wherein said luminance signal processing circuit comprises a playback equalizer for equalizing the up-converted signal from said upconverter means.

7. A signal processing circuit constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.