GB2056801A

GB2056801A - Synchronizing an oscillator from a television signal

Info

Publication number: GB2056801A
Application number: GB8025257A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-08-04
Filing date: 1980-08-01
Publication date: 1981-03-18
Also published as: ES493928A0; JPS5624882A; ES8106643A1; IT8068241A0; DE2931758A1; FR2463558A1; IT1130825B

Abstract

In a television signal receiver the synchronizing signal is separated at 2, and applied to a pulse shaper 3 to provide discrete pulses coincident with the synchronizing pulses. These discrete pulses are applied to phase discriminator 4 along with pulses derived from an oscillator 7, controlled by the output of the phase discriminator 4. The oscillator 7 produces a pulse train which is frequency divided in a divide-by-20 stage 8 and a divide- by-2 stage 9 to produce pulses at line frequency which are applied through the normal closed contacts of a switch 11 to the input 6. During the period in the field blanking interval when the pre- and post- equalizing pulses of twice line frequency are present the switch 11 provides a path between the output of divider 8 and input 6 to apply twice line frequency pulses to that input. The action of the switch 11 is controlled by a gate 12 controlled from the television signal. The arrangement accelerates pull-in synchronization of the oscillator 7 during the field blanking period. <IMAGE>

Description

SPECIFICATION Synchronizing an oscillator from a television signal The invention relates to a circuit arrangement, particularly in a television receiver or a television signal tape recording device, for synchronizing the frequency of an oscillator with respect to synchronizing pulses of line frequency comprised in a television signal, in which an oscillation derived from the synchronizing pulses and a reference oscillation derived from the oscillator oscillations are applied to a phase comparator stage which applies a control quantity to the oscillator for frequency control.

In known circuits, the synchronization is carried out between the synchronizing pulses which are separated from the television signal, and the pulses of line frequency produced by the oscillator; the oscillator itself may oscillate at a higher frequency, for example at double the line frequency; in certain types of video tape recording devices an oscillator is provided which oscillates at 625 kHz, which is forty times the line frequency of 1 5,625 Hz and from which the pulses of line frequency are derived by means of divider stages.

In certain circumstances, for example during scanning of a video signal recorded on a magnetic tape, phase jumps occur at certain instants, for example owing to the switch from one magnetic head to another shortly prior to the vertical flyback interval, in the position of the synchronizing pulses which may amount up to 16 ,us, as to 25% of the duration of the line period. This phase jump must be rapidly removed so as to ensure that the lines occupy their correct position again at the beginning of the next picture display.When the abovementioned 625 kHz oscillation is used as the carrier in the recording of colour signals, restoration of the synchronization is also required, so that the carrier oscillations which are preferably reconverted to the normal frequency of 4.43 MHz (PAL system), have the correct phase position in the demodulation of the chrominance signals, and colour defects, for example desaturation, are avoided.

These possible defects are smaller according as the time constants in the phase control circuit are smaller. In view of their sensitivity to noise, these time constants can, however, not be reduced to any desired extent. This applies in particular to the control circuits incorporated in a television receiver which is connected to a recording device and whose time constants in the display of recorded signals can often not be reduced.

The invention provides a circuit arrangement suitable for use in a television receiver or a television signal tape recording device, for synchronizing the frequency of an oscillator contained therein with respect to synchronizing pulses of line frequency present in a television signal when applied to said arrangement, in which an oscillation derived from the synchronizing pulses and reference oscillation derived from the oscillations of the oscillator are applied to a phase comparator stage for applying a control quantity to the oscillator for frequency control, characterized in that said phase comparator comprises a phase discriminator having a first input for receiving the oscillation derived from the synchronizing signal, an output for supplying the control quantity to the oscillator for the frequency and/or phase control thereof, and a second input for receiving a reference oscillation having a fixed relationship with the oscillations of the oscillator and whose repetition frequency, at least during part of the field blanking interval, is equal to a multiple of the line frequency, the television signal during the said part of the field blanking interval containing pulses whose repetition frequency corresponds to that of the said multiple of the line frequency.

The said pulses may have a repetition frequency of, for example, three or four times that of the line frequency are inserted into the television signal. When such an arrangement is used in a television signal tape recording device the said pulses may be inserted into said television signal during recording.

The oscillation received at the first input of the phase comparator may include pulses corresponding to those during and between the pre-equalizing and post-equalizing periods of the field blanking interval, and the said multiple of the line frequency may then be twice the line frequency.

The oscillator may produce oscillations having a repetition frequency equal to the or a further multiple of the line frequency with divider means for deriving from the oscillations of said oscillator line frequency pulses and a switching arrangement for normally applying said line frequency pulses to the second input of said phase comparator, said switching arrangement, at least during the said part of the field blanking interval, applying said reference oscillations to said second input which reference oscillations are derived from said oscillator. The change in operation of said switching arrangement may be controlled from a gating pulse derived from said television signal.

The invention also has for its object to accelerate in a circuit arrangement of the type defined in the opening paragraph, the phase and/or frequency control in the relevant time range, in the vicinity of the vertical synchronizing pulses, without changes occurring in the circuit, for example, in the time constants of a frequency and/or phase control circuit, determined by a low-pass filter.To this end the circuit arrangement according to the invention is characterized in that a phase discriminator which receives the synchronizing signal at its first input and whose output supplies a -frequency and/or phase control quantity for the oscillator, receives at its second input, preferably via a gate circuit, a reference oscillation which is fixedly coupled to the oscillations of the oscillator and whose repetition frequency is equal to the above-mentioned multiple of the line frequency and this during the occurrence of pulse edges comprised in the television signal during the field flyback interval, the repetition frequency of these pulse edges being equal to a multiple of the line frequency.

Since that operation is carried out with a multiple (a harmonic) of the line frequency, a higher number of reference instants are available, so that an error signal can be built-up more rapidly and also decreases more rapidly when a re-adjustment is carried out.

To that end the circuit arrangement according to the invention may be characterized in that pulse edges whose repetition frequency is equal to a multiple of the line frequency, for example three times or four times this line frequency, are inserted into the television signal. In a television signal tape recording device such an insertion may be effected during recording.

It is alternatively possible to use a normal television signal and the circuit arrangement according to the invention may be characterized in that the oscillator is synchronized by at least some of the pre-equalizing pulses, and/ of the post-equalizing pulses and/of the serration pulses in the field synchronizing signal.

The invention will now be further explained, by way of example, with reference to the block schematic circuit diagram shown in the accompanying drawing.

A television signal comprising picture, blanking and synchronizing signals is applied from a terminal 1 to a synchronizing pulse separating stage 2. The synchronizing pulses thus separated are applied to a pulse shaping stage 3 and converted so that narrow pulses 21 of the same duration and amplitude are produced at the ouput of the stage 3 at approximately the leading edge instants of the applied synchronizing pulses. The duration of these pulses must be shorter than that of the pre-qualizing and post-equalizing pulses contained in the field blanking period, the preequalizing and post-equalizing pulses having half the duration of the pulses of the line frequency; in a CCIR television signal the pulses at the output of the stage 3 must therefore have a duration of less than 2.3 ,us.

These pulses 21 are- applied to a first input 5 of a phase discriminator 4, whose second input 6 receives pulses 22 of the line frequency, which are applied to the input 6. of the discriminator 4 from an oscillator 7 via a first frequency divider 8 and a subsequent second frequency divider 9 and via the first make contact and the centre contact of a change-over switch 11. The control voltage produced at the output terminal 1 3 of the discriminator 4 is applied to the control input of the oscillator 7 via a low-pass filter 1 4j which comprises at least one time constant element. In the circuit shown in the drawing, the phase position of the oscillator 7 is checked with respect to the applied synchronizing signals 21 at intervals of one line period, and a control voltage is formed therefrom.

During the occurrence of the pulse edges of the higher repetition frequency in the vicinity of the field pulses, a reference oscillation of double the frequency is applied to the discriminator 4 and in this manner the phase position and the generation of a control voltage takes place at intervals of half a line period. To this end the change-over switch 11 is switched from the rest position to the operating position in the time interval in which pulse edges 21 of double the line frequency occur in the applied television signal. By means of a gate circuit 12, the above-mentioned time interval is determined in the television signal and a corresponding signal is applied to the changeover switch 11.The oscillations of the oscillator 7 of, for example, 625 kHz, are divided by the first frequency divider 8 and the second frequency divider 9 by 40 to the line frequency of 1 5,625 Hz. The second frequency divider 9 is a divide-by-two divider which halves the frequency of the oscillation supplied by the first divider 8, which divides by 20. A, for example, pulse-shaped oscillation, whose frequency is equal to double the line frequency can therefore be taken from the output of the first frequency divider 8. In the operating position of switch 11 correspondingly high frequency reference oscillations 23 are consequently applied to the phase discriminator 4, so that each edge of double the line frequency, comprised in the picture signal, is used to control the frequency of the oscillator 7. This considerably accelerates the re-adjustment of this oscillator.

When the invention is used for line synchronization in a television receiver, the oscillator 7 oscillates at double the line frequency and the divider 8 can be dispensed with. The change-over switch 1 1 then switches from the a input to the output of the divide-by-two divider.

In the case of a television signal tape rec- * ording device a modification of the television signal can be effected during recording in such a way that the signal comprises pulse edges whose repetition frequency is a multiple of the line-frequency, for example, 3 or 4 times this -line- frequency. Such a signal is thus inserted in the vicinity of the field synchronization signal, somewhere between the last picture line of one field and the first picture- line of the subsequent field.

Claims

1. A circuit arrangement suitable for use in a television receiver or a television signal tape recording device, for synchronizing the frequency of an oscillator contained therein with respect to synchronizing pulses of line frequency present in a television signal when applied to said arrangement, in which an oscillation derived from the synchronizing pulses and a reference oscillation derived from the oscillations of the oscillator are applied to phase comparator stage for applying a control quantity to the oscillator for frequency control, characterized in that said phase comparator comprises a phase discriminator having a first input for receiving the oscillation derived from the synchronizing signal, an output for supplying the control quantity to the oscillator for the frequency and/or phase control thereof, and a second input for receiving a reference oscillation having a fixed relationship with the oscillations of the oscillator and whose repetition frequency, at least during part of the field blanking interval, is equal to a multiple of the line frequency, the television signal during the said part of the field blanking interval containing pulses whose repetition frequency corresponds to that of the said multiple of the line frequency.

2. A circuit arrangement as claimed in Claim 1, in which the said pulses having a repetition frequency of, for example, three or four times that of the line frequency are inserted into the television signal.

3. A circuit arrangement as claimed in Claim 2 when used in a television signal tape recording device, in which the said pulses are inserted into said television signal during recording.

4. A circuit arrangement as claimed in Claim 1, in which the oscillation received at the first input of the phase comparator includes pulses corresponding to those during and between the pre-equalizing and postequalizing periods of the field blanking interval.

5. A circuit arrangement as claimed in Claim 4, in which the said multiple of the line frequency is twice the line frequency.

6. A circuit arrangement as claimed in Claim 1, in which the oscillator produces oscillations having a repetition frequency equal to the or a further multiple of the line frequency, divider means for deriving from the oscillations of said oscillator line frequency pulses and a switching arrangement for normally applying said line frequency pulses to the second input of said phase comparator, said switching arrangement, at least during the said part of the field blanking interval, applying said reference oscillations to said second input which reference oscillations are derived from said oscillator.

7. A circuit arrangement as claimed in Claim 6, in which the change in operation of said switching arrangement is controlled from a gating pulse derived from said television signal.

8. A circuit arrangement substantially as herein described with reference to the accompanying drawing.

9. A television receiver including a circuit arrangement as claimed in any of the preceding Claims 1, 4, 5, 6, 7 or 8.

10. A television signal tape recording device including a circuit arrangement as claimed in any of the preceding Claims 1 to 8.

11. A circuit arrangement, particularly in a television receiver or a television signal tape recording device, for synchronizing the frequency of an oscillator with respect to synchronizing pulses of line frequency comprised in a television signal, in which an oscillation derived from the synchronizing pulses and a reference oscillation derived from the oscillator oscillations are applied to a phase comparitor stage which applies a control quantity to the oscillator for frequency control, characterized in that a phase discriminator which receives the synchronizing signal at its first input and whose output supplies a frequency and/or phase control quantity for the oscillator, receives at its second input, preferably via a gate circuit, a reference oscillation which is fixedly coupled t6 the oscillations of the oscillator and whose repetition frequency is equal to the above-mentioned multiple of the line frequency and this. during the occurrence of pulse edges comprised in the television signal during the field flyback interval, the repetition frequency of these pulse edges being equal to a multiple of the line frequency.

1 2. A circuit arrangement as claimed in Claim 1, characterized in that pulse edges whose repetition frequency is equal to a multiple, for example three or four times, of the line frequency are inserted into the television signal.

1 3. A circuit arrangement as claimed in Claim 2, characterized in that in the case of a television signal tape recording device the pulse edges whose repetition frequency is equal to a multiple of the line frequency are inserted during recording.

1 4. A switching arrangement as claimed in one of the Claims 1 to 4, characterized in that the oscillator is synchronized by at least some of the pre-equalizing pulses and/or the post-equalizing pulses and/or the serration pulses in the field synchronizing signal.

1 5. A circuit arrangement as claimed in Claim 4, characterized in that the line pulses are derived from the oscillator oscillations by means of frequency division and that the oscillations used for the synchronization, and whose repetition frequency is equal to double the line frequency are derived from the last but one divide-by-two divider stage.