GB2091971A - Apparatus for processing a PAL color video signal - Google Patents

Abstract

Apparatus for processing a PAL color video signal eg in the playback circuit of a VTR includes: a control loop in which the color burst in the video signal and an output signal of a reference oscillator 10 oscillating at the subcarrier frequency (fc) are supplied to a phase comparator 15 to derive a phase comparison error signal, from which is produced a control signal for a variable frequency oscillator 12 providing an output signal for the frequency conversion (fs to fc) (9, 11) of the carrier chrominance signal; signal generator means 24, 25 generating an output signal O whose polarity is inverted in alternate horizontal scan periods for addition to the phase comparison error signal, the resulting sum signal being passed through a low pass filter 19 to give said control signal and also to feed detector means 26, 27, 28, 29, 31 detecting the polarity of said control signal in alternate periods of horizontal scanning; and control means inverting the phase of the output signal of said signal generator means in response to the output signal of said detector means, <IMAGE>

Description

SPECIFICATION Colour Signal Processing Apparatus This invention relates to an apparatus for processing a PAL color video signal recorded and reproduced by a video tape recorder of two rotary head type.

In a video tape recorder of two rotary head type used for the recording and reproduction of a PAL color video signal, a PAL color video signal is recorded generally by subjecting the luminance signal to angular modulation, for example, frequency modulation, converting the frequency of the carrier chrominance signal into that of a lower frequency band, mixing the carrier chrominance signal of the low frequency band with the angular modulated luminance signal of the high frequency band, and recording the signal mixture on a magnetic tape. In the playback mode, the angular modulated luminance signal is demodulated into the original luminance signal by a demodulator, and the carrier chrominance signal of the low frequency band is re-converted into that of the original frequency by a frequency converter.

An AFC circuit and an APC circuit have generally been provided in a prior art playback system to meet the necessity for elimination of a phase variation of the carrier chrominance signal in the playback mode due to jitter. The AFC circuit produces, on the basis of the reproduced horizontal synchronizing signal, a signal having a frequency which is, for example, n times (n: an integer) as high as that of the horizontal synchronization signal.

The APC circuit compares the phase of the reproduced burst signal with that of a reference sub carrier signal, and its output signal representing the result of phase comparison is used to control a variable frequency oscillator in the APC circuit. The output signal of the AFC circuit and the output signal of the variable frequency oscillator in the APC circuit are supplied to a frequency converter to derive a signal having a frequency representing the sum of the input frequencies, and this signal is used as a frequency converting signal for re-converting the frequency of the carrier chrominance signal into the original frequency.

Since the PAL system is featured by the fact that there is the phase difference of 90 between the phase of the color burst in one period of horizontal scanning and that in the next period of horizontal scanning, phase errors corresponding to the phase difference of 45 and having different signs appear in one horizontal scanning period and the next horizontal scanning period in the outputs of a phase comparator constituting a part of the APC circuit. According to a prior art practice for preventing an undesirable hue change in the line, the time constant of the APC circuit has been increased so that the APC circuit may not respond to such a phase error. However, the increased time constant of the APC circuit in the prior art playback system has given rise to a problem as pointed out presently.That is, when the AFC circuit is affected by noise resulting from drop-out or noise and skew produced during high-speed reproduction by the magnetic head traversing a plurality of recording tracks in one field, the AFC circuit acts upon the APC circuit as a source of disturbance, and the hue has tended to change over a long period of time when the APC circuit has the increased time constant.

It is therefore a primary object of the present invention to solve the prior art problem pointed out above and to provide a color signal processing apparatus which inhibits an udesirable hue change in the line even when the low-pass filter constituting a part of the APC circuit has a small time constant.

The color signal processing apparatus according to the present invention is featured by the fact that a signal corresponding to the phase difference of about 45 and having different signs in alternate periods of horizontal scanning is added to the output signal of the phase comparator constituting a part of the APC circuit so as to cancel the phase comparison error component produced due to the alteration of the phase of the color burst line by line.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of a preferred embodiment of the present invention taken in conjunction with the accompanying drawings, in which: Figure 1 is a block diagram of a playback circuit of a video tape recorder to illustrate the structure of a preferred embodiment of the present invention; Figure 2 shows waveforms appearing at various parts of Fig. 1; Figure 3 is a detailed circuit diagram of one form of the current source circuit 25 in Fig. 1; and Figure 4 shows waveforms to illustrate the operation of the playback circuit shown in Fig.

1.

The present invention will now be described in detail with reference to an application of the APC circuit of the present invention to a video tepe recorder, by way of example.

In a video tape recorder of two rotary head type used for the recording and reproduction of a PAL color video video signal, the general procedure in the record mode includes subjecting the luminance signal to frequency modulation, converting the frequency of the carrier chrominance signal into that of a lower frequency band, mixing the carrier chrominance signal of the low frequency band with the frequency-modulated luminance signal of the high frequency band, and recording the signal mixture on a magnetic tape.

The frequency of the carrier of the recorded carrier chrominance signal is generally selected to be a value which is n times (n: an integer) the horizontal scanning frequency fH plus or minus 1/8 H' for example, (40 + 1/8) H' so that components of cross modulation with the FM luminance signal may not be substantially interfere with viewing by the viewer of the reproduced picture.

Fig. 1 is a block diagram of a preferred embodiment of the reproduced signal processing circuit of the present invention which processes a color vide signal reproduced from a magnetic tape on which the signal is recorded by a video tape recorder in a manner as described above.

Referring to Fig. 1, a mixture signal of a frequency modulated luminance signal and a carrier chrominance signal of a low frequency band is reproduced from a magnetic tape (not shown) by a magnetic head 1 and is then supplied through a playback amplifier 2 to a high-pass filter 3. The frequency modulated luminance signal extracted by the high-pass filter 3 is supplied through a limiter 4 to an FM demodulator 5 to be demodulated, and the demodulated luminance signal is supplied to a mixer 6.

The signal reproduced from the magnetic tape and passed through the playback amplifier 2 is also supplied to a low-pass filter 7 which extracts the chrominance signal of low frequency band. The extracted chrominance signal is supplied through an ACC (automatic chrominance control) circuit 8 to a frequency converter circuit 9 which provides an output signal having a frequency representative of the frequency difference of two input signals, so that the carrier frequency of the carrier chrominance signal of the low frequency band is re-converted into the frequency of the chrominance sub-carrier which is about 4.43 M Hz. The frequency-converted carrier chrominance signal is then supplied to the mixer 6.

Consequently, the reproduced composite color video signal of PAL system appears at an output terminal 14.

The reproduced carrier chrominance signal appearing from the frequency converter circuit 9 is also supplied to a phase comparator 1 5 in which the phase of the reproduced carrier chrominance signal is compared with that of an oscillation output signal (a reference signal) of a reference oscillator 10 which oscillates at the frequency (fc = 4.43 MHz) of the subcarrier.The phase comparison between the reproduced carrier chrominance signal and the reference output signal of the reference oscillator 10 is carried out in the phase comparator 1 5 during the burst period only, and the operation of the phase comparator 1 5 is controlled by a burst gate pulse signal obtained by delaying the synchronizing signal output of a synchronizing signal separator circuit 1 6 by a delay circuit 1 7. The output signal of the phase comparator 1 5 is supplied through an adder 1 8 and a low-pass filter 1 9 to a variable frequency oscillator 1 2 to control the oscillation frequency of the oscillator 1 2.

The output signal of the variable frequency oscillator 1 2 is supplied to a 1/8 frequency divider 13, and the output signal of the frequency divider 1 3 is applied to a frequency converter circuit 11. The oscillation output signal of the reference oscillator 10 is also applied to the frequency converter circuit 11.

An output signal having a frequency representative of the sum of the frequency of the output signal of the reference oscillator 10 and the frequency of the output signal of the frequency divider 1 3 is generated from the frequency converter circuit 11 to be supplied to the frequency converter circuit 11 to be supplied to the frequency converter circuit 9 as a signal for converting the frequency of the carrier chrominance signal.

Therefore, an APC circuit is provided which attains synchronization of the phase of the color burst in the output signal of the frequency converter circuit 9 and that of the output signal of the reference oscillator 10.

Let fs be the carrier frequency of the carrier chrominance signal of low frequency band applied from the ACC circuit 8 to the frequency converter circuit 9. Then, the APC circuit is locked when the oscillation frequency of the variable frequency oscillator 1 2 is in the vicinity of 8 fs, and the frequency of the signal supplied from the frequency converter circuit 11 to the frequency converter circuit 9 for converting the frequency of the carrier chrominance signal applied from the ACC circuit 8 to the frequency converter circuit 9 is given by (fc + fs).

Then, even when the carrier chrominance signal of the low frequency band to be subject to frequency conversion in the frequency converter circuit 9 may include a frequency variation due to jitter, the carrier frequency of the carrier chrominance signal appearing from the frequency converter circuit 9 is maintained constant at the subcarrier frequency fc by the action of the APC circuit.

When the APC circuit alone functions to eliminate a large frequency variation due to jitter as described above, the variable frequency oscillator 1 2 is required to have a wide frequency variable range, and an oscillator, for example, an astable multivibrator is commonly used for this purpose.However, the requirement for a wide frequency variable range will sometimes result in such a situation that the frequency of the signal supplied from the frequency converter circuit 11 to the frequency converter circuit 9 for the purpose of converting the frequency of the carrier chrominance signal deviates for (fc + fs) by a value which is n times (n: an integer) the horizontal scanning frequency fH. When such a situation occurs, the carrier frequency of the frequencyconverted chrominance signal will also deviate from the subcarrier frequency fc by the value which is n times the horizontal scanning frequency fH. In such a case too, the APC circuit is locked because of the utilization of the color burst signal which is an intermittent signal, and the so-called side lock occurs.Reference numeral 20 in Fig. 1 designates a side lock detector provided for preventing this side lock as disclosed in, for example, Japanese Patent Application Laid-open No. 54-57829. This side lock detector 20 controls the oscillation frequency of the variable frequency oscillator 12 to maintain it in the vicinity of 8 fs.

The phase of the color burst signal applied to the phase comparator 1 5 in one period of horizontal scanning is altered by about 90 from that in the next period of horizontal scanning, and the APC circuit operates so that the average phase of the color burst is synchronized with the phase of the oscillation output signal of the reference oscillator 1 0.

Therefore, phase errors corresponding to the phase difference of about 45 and having different polarities in alternate periods of horizontal scanning appear at the output of the phase comparator 1 5. Fig. 2, at B, shows such an output current of the phase comparator 1 5. Fig. 2, at A, shows the carrier chrominance signal, and symbols P and P' in A of Fig. 2 represent the burst signals different in phase by 90 from each other.When such an output current of the phase comparator 1 5 is directly supplied through the low-pass filter 1 9 to the variable frequency oscillator 12, the oscillation frequency of the variable frequency oscillator 1 2 will vary in the line resulting in a variation of the phase of the carrier chrominance signal in the line. In the present invention, an arrangement which will be described now is provided to prevent such an undesirable hue change in the line.

Again in Fig. 1, a horizontal synchronizing signal separator circuit 21 separates the horizontal synchronizing signal from the output signal of the synchronizing signal separator circuit 16. The horizontal synchronizing signal separated by the horizontal synchronizing signal separator circuit 21 is applied to a buffer oscillator 22 which generates stably a pulse signal at the horizontal scanning frequency fH even if a portion of the horizontal synchronizing signal may be absent or missing due to, for example, drop-out.A 1/2 frequency divider 23 divides the frequency of the output signal of the buffer oscillator 22 to generate a continuous pulse signal having a frequency of 1/2 fH. The output signal of the frequency divider 23 is applied to a phase inverter 24 which can selectively invert the phase of its input signal, and the signal of the same phase or inverted phase is supplied from the phase inverter 24 to a current source circuit 25.

Fig. 3 shows the detailed structure of the current source circuit 25. Referring to Fig. 3, the output signal S, of the 1/2 frequency divider 23 at the frequency of 1/2 fH is applied across a pair of input terminals 251 and 252 to cause switching operation of a pair of transistors 253 and 254 of differential connection. Another pair of transistors 255 and 256 constitute a current mirror circuit.

Another transistor 257 acting as a current source is connected at its collector to the common-connected emitters of the transistors 253 and 254 of differential connection. In response to the application of a burst gate pulse from the delay circuit 1 7 to a terminal 258, the transistor 257 is turned on and maintained in the on-state during the burst period. Consequently, the current lo flowing through the collector of the transistor 257 to appear at an output terminal 259 has a polarity variable depending on the state of the input signal S,, as shown by the solid and dotted arrows in Fig. 3. The output current of the current source circuit 25 is supplied to the adder 1 8 to be added to the output current of the phase comparator 1 5.

Therefore, by the provision of the circuits 21 to 25, the current, whose polarity changes in alternate periods of horizontal scanning and which is as shown in C or D in Fig. 2 depending on the switched position of the phase inverter 24, is added to the output current of the phase comparator 1 5 in the successive burst periods.It will be seen in E of Fig. 2 that the outout current of the phase comparator 1 5 is cancelled in the adder 1 8 by the output current lo of the current source circuit 25 when the level of the output current lo of the current lo of the current source circuit 25 is equal to that of the output current of the phase comparator 1 5 corresponding to the phase difference of 45 and when the polarity of the former current is opposite to that of the latter current as shown in C in Fig. 2.On the other hand, when the polarity of the output current lo of the current source circuit 25 is the same as that of the output current of the phase comparator 1 5 as shown in D in Fig. 2, the output current of the adder 1 8 supplied to the low-pass filter 1 9 has a polarity changing in alternate periods of horizontal scanning, and its level is about two times as high as that of the output current of the phase comparator 1 5 as shown in F in Fig. 2.

The frequency components of the output current lo of the current source circuit 25 include 1/2 fH and its odd harmonics and have constant levels. Therefore, the output current of the current source circuit 25 affects only the 1/2 fH and higher frequency components of the output current of the phase comparator 1 5 and would not act to unlock the APC circuit. Thus, even when, for example, the current shown in F in Fig. 2 is supplied to the low-pass filter 1 9 and its output voltage varies as shown in G in Fig. 2, an increased hue change in the line will merely occur.

Therefore, the APC circuit will be maintained in the locked condition, and the output voltage of the low-pass filter 1 9 will be either in the state in which is maintained substantially constant without giving rise to a hue change in the line or in the state in which it varies considerably every other period of horizontal scanning as seen in G in Fig. 2.

An arrangement is therefore provided in the present invention to detect the state of the output voltage of the low-pass filter 1 9 and to control the polarity of the output current of the current source circuit 25 so that the undesirable hue change in the line can be prevented. This arrangement includes a highpass filter 26, a half-wave rectifier circuit 27, a pulse shaper circuit 28, a pulse counter 29 whose output signal turns from its "0" level to its "1" level when it has counted four pulses, a 1/8 frequency divider 30 and a flipflop 31, as shown in Fig. 1. The voltage-state detecting and current-polarity control operation of the above arrangement will be described with reference to Fig. 4.When the polarity of the output current of the phase comparator 1 5 is the same as that of the output current of the current source circuit 25, and the output voltage of the low-pass filter 1 9 varies every other horizontal scanning period as shown in G in Fig. 2 and in J in Fig. 4, the output voltage including such an AC component passes through the high-pass filter 26 and is then half-wave rectified by the half-wave rectifier circuit 27. The output signal of the half-wave rectifier circuit 27 is supplied to the pulse shaper circuit 28, which is, for example, a Schmidt circuit, so that the waveform portion higher than a predetermined level is converted into a pulse signal as shown in K in Fig. 4.This predetermined level is generally selected to be higher than the maximum level of the phase error output signal appearing from the phase comparator 1 5 due to the presence of a jitter component commonly included in a reproduced chrominance signal. Preferably, this predetermined level is generally selected to be the phase error outout level appearing when the phase difference between the two input signals compared in the phase comparator 1 5 is more than 20". The pulse counter 29 is reset as soon as the output signal of the 1/8 frequency divider 30 turns into its "1" level as shown in I in Fig. 4. The pulse counter 29 starts to count the output signal of the pulse shaper circuit 28 as soon as the output signal of the frequency divider 30 turns into its "0" level.The output signal of the pulse counter 29 turns into its "1" level from its "(9" level when the counter 29 has counted four pulses in the output signal of the pulse shaper circuit 28, as shown in L in Fig. 4. Consequently, the output signal of the flip-flop 31, to which the output signal of the pulse counter 29 is supplied, turns into its "1" level from its "0" level, as shown in M in Fig. 4, to control the phase inverter 24. Consequently, the phase of the signal applied from the phase inverter 24 to the current source circuit 25 is inverted as shown in N in Fig. 4, and the polarity of the output current of the current source circuit 25 is turned from the state shown in D in Fig. 2 to the state shown in C in Fig. 2.Consequently, the output current of the polarity shown in C in Fig. 2 is now supplied to the adder 1 8 to cancel the output current of the phase comparator 1 5 thereby preventing an undesirable hue change in the line. Also, when the output current of the current source circuit 25 has the polarity which cancels the output current of the phase comparator 1 5 as shown in C in Fig. 2, the output voltage of the lowpass filter 1 9 does not appreciably vary unlike the waveform shown in G in Fig.

2, and the pulse shaper circuit 28 is unable to generate the pulses unless the output level of the half-wave rectifier circuit 27 is sufficiently high. Thus, no output pulses appear from the pulse shaper circuit 28 as seen in K in Fig. 4, and no change occurs in the polarity of the output current of the current source circuit 25.

It will be seen that the output current of the current source circuit 25 is so controlled as to cancel the phase comparison errors of different polarities appearing in alternate periods of horizontal scanning due to the locking of the phase of the output signal of the reference oscillator 10 with the average phase of the color burst.

It will be understood from the foregoing detailed description of an embodiment of the present invention that a voltage or current having different signs in alternate periods of horizontal scanning and corresponding to the phase difference of about 45t is added to the output of the phase comparator 1 5 constituting part of the APC circuit, and the polarity of this voltage or current is controlled by detecting the AC component in the output signal of, for example, the low-pass filter 19, so as to cancel the phase comparison error component generated due to the synchronization of the phase of the output signal of the reference oscillator 10 with the average phase of the color burst.

Therefore, when the APC circuit according to the present invention is used to constitute a part of a playback circuit of a video tape recorder recording and reproducing a color video signal of PAL system, as shown in Fig.

1, an undesirable phase variation of the carrier chrominance signal due to jitter can be eliminated at a high speed without giving rise to an undesirable hue change in the line, so that pictures of good quality can be reproduced.

The APC loop would not be unlocked even if the polarity of the output current of the current source circuit 25 might coincide, even temporarily, with that of the output current of the phase comparator 1 5 constituting a part of the APC circuit. Therefore, even when, for example, a color video signal edited to include a discontinuity in the order of fields of pictures may be reproduced, the carrier frequency of the carrier chrominance signal output will not appreciably vary, and such an undesirable situation will not occur in which unlocking of the APC circuit results in the operation of the color killer in the television receiver, hence, in the disappearance of color display on the screen of the television receiver.

While the aforementioned embodiment of the present invention has been described with reference to an application of the color signal processing apparatus of the present invention to the playback circuit of a video tape recorder by way of example, it is apparent that the present invention is also equally effectively applicable to a video disk apparatus of magnetic, optical or any other type in which the carrier chrominance signal is frequency-converted in the record mode and re-converted into the original signal in the playback mode.

Claims (4)

1. An apparatus for processing a PAL color video signal including a control loop in which the color burst in the carrier chrominance signal of the PAL color video signal and an output signal of a reference oscillator oscillating at the subcarrier frequency are supplied to a phase comparator to derive a phase comparison error signal, and a control signal produced on the basis of the phase comparison error signal is used to control a variable frequency oscillator which provides an output signal used for the frequency conversion of the carrier chrominance signal, said processing apparatus comprising:: signal generator means for generating an output signal whose polarity is inverted in alternate periods of horizontal scanning; adder means for adding the output signal of said signal generator means to the phase comparison error signal generated from said phase comparator; detector means for detecting the polarity of the phase comparison error component appearing from said phase comparator in different polarities in alternate periods of horizontal scanning; and control means for controlling the polarity of the output signal of said signal generator means in response to the output signal of said detector means, the output signal of said adder means being passed through a low-pass filter to control said variable frequency oscillator.

2 A processing apparatus as claimed in Claim 1, wherein said detector means includes extractor means for extracting an AC component from the output signal of said adder means, and amplitude detector means for detecting a predetermined level of the amplitude of said AC component.

3. A processing apparatus as claimed in Claim 1, wherein said signal generator means is controlled by a burst gate pulse extracting the color burst signal in the carrier chrominance signal so that said signal generator means generates its output signal during the burst period only.

4. Apparatus for processing a PAL colour video signal including a control loop in which the colour burst in the carrier chrominance signal of the PAL colour video signal and an ouput signal of a reference oscillator oscillating at the subcarrier frequency are supplied to a phase comparator to derive a phase comparison error signal, and a control signal produced on the basis of the phase comparison error signal is used to control a variable frequency oscillator which provides an output signal used for the frequency conversion of the carrier chrominance signal, the apparatus being substantially as herein described with reference to Figs. 1 and 3 of the accompanying drawings.