GB2025186A - Circuit for the reproduction of tape-recorded colour television signals - Google Patents

Abstract

A circuit for cross talk reduction during the reproduction of colour television signals recorded in closely packed oblique tracks on a tape with a line-staggering of 1.5 lines with the carrier frequency colour signal having its polarity reversed every 4th line during recording. The output of an oscillator 5 driving a mixer 4 is reversed every 4th line to reverse back said polarity, after which the signals are passed through a comb filter 7 having a two-line delay, the reversal of the oscillator output being effected by the line pulses supplied at 15 and divided by 8 in a divider 14. An additional pulse is supplied to divider 14. An additional pulse is supplied to divider 14 through OR gate 19 at predetermined intervals whenever colour bursts filtered out at the comb filter output do not comprise a half-line frequency component (detected at 13). <IMAGE>

Description

SPECIFICATION Circuit for the reproduction of tape-recorded colour television signals This invention relates to a circuit for the reproduction of colour television signals which are recorded in oblique tracks on a magnetic tape.

Oblique track equipment usually records one field per oblique track, the colour television signal being split up into the luminance signal and into the carrier-frequency colour signal; a carrier oscillation is frequency-modulated with the luminance signal, the carrierfrequency colour signal is converted into a frequency range which lies below the frequency band adopted by frequency-modulated luminance signal and is then added to the luminance signal. If the oblique tracks are recorded immediately beside one another without a guard band or gap in order to increase the packing density or to save tape material, the cross-talk from track-to-track can be eliminated or greatly reduced with regard to the higher frequencies of the luminance signal by means of an azimuth staggering of the heads between adjacent tracks.On the other hand, this aid is not sufficient for the low frequencies of the converted colour carrier and it is therefore known already to effect polarity reversal of the colour carrier line by line, so that the cross-talk components can be made harmless by means of comb filters during the reproductions.

For a PAL colour television signal, for example, it has been proposed to record adjacent tracks as shown in Fig. 1 with a line staggering of 1.5 lines and to reverse the polarity of the chrominance carrier after every four lines.

The resulting vector diagram for the recorded chrominance carrier is shown for two adjacent tracks in Fig. 2: the lines 1 to 4, 9 to 1 2 etc, surrounded by a thin line show the normal PAL burst alternating by 90 from line to line, which appears polarity reversed by 180 in the lines 5 to 8, 13, to 1 6 etc. surrounded by thick lines. If the phase has its polarity reversed back again during the lines with thick lines round them during the reproduction and the colour signal thus obtained, as shown in Fig. 3 is supplied through a comb filter with the two-line delay 1 to an adding network 2, then the cross-talk components always cancel each other out.

In order to reverse back the polarity of the chrominance carrier phase during the lines with a thick line around them, a control voltage is obviously necessary, the frequency of which must amount to an eighth of the line frequency and which can therefore be obtained in a simple manner by appropriate division of the line frequency. What is difficult on the other hand, is to determine and produce the correct phase position of this control voltage in relation to the scanned colour signals. For this purpose it has already been proposed to effect a phase comparison between the input and the output of the two-line delay 1 in Fig. 3 and to use the resulting error signal, after appropriate integration, to set back the line frequency divider. This method is complicated and expensive, however.

In accordance with this invention, there is provided a circuit for the reproduction of colour television signals which are recorded in closely packed oblique tracks on a tape with a line staggering of 1.5 lines and the carrierfrequency colour signal of which has its polarity reversed after every 4th line during the recording comprising means for reversing back the polarity of the carrier-frequency colour signal after every 4th line during the reproduction together with a comb filter containing a two-line delay line, a frequency divider for receiving the line synchronising pulses to derive a control voltage for the polarity reversal means, and means for adding an additional pulse to the line pulses supplied to the divider at predetermined intervals of time whenever colour bursts filtered out at the output of the comb filter do not comprise a half-line frequency component.

In an embodiment to be described herein, which is a combined recording and playback apparatus the phase position of the reverse polarity control voltage is detected without an additional phase comparator is altered automatically until it agrees with the phase position of the polarity reversing control voltage employed during the recording.

In accordance with this invention, it is recognised that the normal output of a comb filter as shown in Fig. 3 supplies information regarding the phase position of the reversing back polarity control voltage. This can be seen from Fig. 4 where, in Fig. 4a, part of the recorded track is illustrated once again; in Figs. 4bto 4ethe reversing back polarity control voltages each displaced by one line are shown and under them the resulting vectors of the colour bursts. As can be seen, a properly alternating burst only results at the output of the comb filter with a correct position of the control voltage as shown in Fig.

4b; in the other phase positions, there is either no burst at all (Fig. 4d) or a burst with always the same phase in every other line.

The necessary detection can be achieved by the usual sub-circuits present in an apparatus for the recording function.

An embodiment of this invention will now be described, by way of an example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of recorded tracks on a magnetic tape; Figure 2 is a schematic diagram showing the changes in phase line-by-line in the colour signals of successive lines in a frame accord ing to a known system; Figure 3 is a circuit diagram of a filter used in the known system; Figure 4a is a reproduction of the phasechanges in the successive lines of a portion of a field and Figs. 4b and 4e show the phases of successive lines at the output of the filter of Fig. 3 for reversal of the control voltage at different lines; and Figure 5 is a circuit diagram of part of arecording and playback apparatus in accordance with this invention.The colour signal separated out of the scanned colour television signal is supplied via a terminal 3 to a mixer 4 in which it is converted back into its standard frequency position with an auxiliary frequency supplied by an oscillator 5. A bandpass filter 6 eliminates disturbing components, so that at the output of a comb filter 7, corresponding to that shown in Fig. 3, a chrominance signal appears which is purified of cross-talk components and which, if it meets the required conditions, is passed on by a control unit 8 to a terminal 9 for the usual further processing.By means of keying pulses supplied at a terminal 10, the colour bursts are passed from the output of the comb filter 7 to one input of a phase comparator 11 and compared there with the output voltage of a sub-carrier frequency oscillator 12, the precise frequency of which is adjusted by the error 'signal of the phase comparator 11. A component of half the line frequency appears in the error signal of the phase comparator 11 when and only when the phase of the colour bursts delivered by the comb filter 7 alternate line by line in accordance with the PAL conditions, that is to say when Fig. 4b is satisfied. The half-line frequency component is recognized in a detector 1 3 and acknowledged by the presence of a voltage at the output of detector 13; inter alia, this voltage releases the chrominance signal via the control unit 8 to the terminal 9.

All the components mentioned so far are already present in a video recording and reproduction apparatus for tightly packed tracks.

Circuit 1 3 and its output form a colour killing circuit. Even a divider 14, which divies the line frequency supplied at a terminal 1 5 by 8 and so actuates a changeover switch 16, is already needed for the recording, because the two output voltages of the oscillator 5 are offset in relation to one another by 180 so that the switch 1 6 also ensures the reversal of polarity during the recording.

The line frequency supplied at the terminal 1 5 is divided in a divider 17, the division ratio of which can be selected freely; for example, the divider 1 7 can deliver a pulse to a monostable multivibrator 1 8 after every 3rd or 4th frame, which monostable multivibrator can only be triggered if it is not blocked by the output voltage of the detector 1 3. In other words, only if no half-line frequencycompo- nent is detected in the detector 13, that is to say when a situation corresponding to Fig. 4c, 4dor 4e is present, does an additional pulse pass through the divider 17, the monostable multivibrator 1 8 and the OR-circuit 1 9 to the divider 1 4 which displaces the control voltage for the switch 1 6 in phase by the duration of one line. This cycle is repeated until the state corresponding to Fig. 4b is detected by the detect6r 1 3 and so the reversing back polarity control voltage has automatically assumed the correct phase position.

Claims (5)

1. A circuit for the reproduction of colour television signals which are recorded in closely packed oblique tracks on a tape with a line staggering of 1.5 lines and the carrierfrequency colour signal of which has its polarity reversed after every 4th line during the recording, comprising means for reversing back the polarity of the carrier-frequency colour signal after every 4th line during the reproduction together with a comb filter containing a two-line delay line, a frequency divider for receiving the line synchronizing pulses to derive a control voltage for the polarity reversal means, and means for adding an additional pulse to the line pulses supplied to the divider at predetermined intervals of time whenever colour bursts filtered out at the output of the comb filter do not comprise a half-line frequency component.

2. A circuit as claimed in claim 1, including a colour killing circuit from which the halfline frequency component is obtained, and in which said pulse adding means comprises a monostable multivibrator the triggering of which is prevented inthe presence of said halfline frequency component.

3. A circuit as claimed in claim 2, comprising a second frequency divider, having a dividing ratio which can be preset, supplies trigger pulses for the monostable multivibrator, the line synchronizing pulses being supplied to the input of the second divider.

4. A circuit as claimed in any preceding claim, wherein by mixing with a fixed frequency, the carrier-frequency colour signal is converted, before its recording, out of its standard position into a lower frequency position, said circuit including mixing means for converting the scanned colour back into the standard position, the voltage supplied by an oscillator for the mixing having its polarity reversed after every 4th line.

5. A circuit for the reproduction of colour television signals, substantially as herein described with reference to the accompanying drawings.