Apparatus for compatibly locking horizontal sync signal to PAL colour subcarrier

Abstract: Circuits are provided for increasing the frequency of the standard PAL line sync signals so that they have the same phase relationship with the color subcarrier every fourth line. The generation of a standard PAL sync signal is achieved by generating a signal as just described and decreasing its horizontal frequency.

Description: In the present PAL colour television system the colour subcarrier frequency is an integral submultiple of the horizontal scanning frequency plus a frame frequency offset of 25 Hertz for the purpose of making dot crawl from the colour subcarrier less objectionable. This requires more complex circuits for generating the horizontal sync and subcarrier. Whereas this can be done with analogue circuitry, the problem becomes more complex when the signals are to be generated by digital means. The invention provides apparatus for generating synchronizing signals for use in a PAL colour television system, said apparatus comprising a sync generator for providing standard synchronizing signals of a PAL system including a signal at the subcarrier frequency (f sc ), a vertical synchronizing signal (V) and a line synchronizing signal (H) comprising a sequence of line sync pulses, said apparatus further comprising: means responsive to signals provided by said sync generator for producing another line synchronizing signal comprising a sequence of other line sync pulses having a period during the active scan period such that an integral number of quarter cycles of the subcarrier occur between consecutive said other line sync pulses, the timing of said other line sync pulses during each vertical blanking interval incorporating an additional delay of half a subcarrier period such that the average period of said another line synchronizing signal is the same as the average period of said standard line synchronizing signal, and means coupled to said sync generator and to said means for producing said other line synchronizing signal for combining at least said vertical synchronizing signal and said another line synchronizing signal. In a particular embodiment, the horizontal scanning frequency during the active scan period (that is, other than during the vertical blanking interval) may be increased by 100/1135 Hertz. In order to keep the horizontal and vertical scanning frequencies locked, one line during each vertical blanking interval is increased by an amount required to compensate for the accumulated error. In a PAL system sampled at four times the subcarrier frequency, the additional time is two pixels. Use of this altered horizontal sync frequency enables the horizontal and subcarrier frequencies to be locked every four lines while maintaining the beneficial effects of the 25 Hertz frame frequency offset. Thus if PAL transmitters used this horizontal sync signal, there would be no difference in the operation of home receivers but the circuits for studio equipment that cooperates with a transmitter would be far simpler. On the other hand if the horizontal sync frequency remains as it is, the horizontal sync frequency just described could be derived from it and used by studio equipment. Because the phase of the horizontal sync with respect to the colour subcarrier repeats every eight fields, it is far more difficult to attain tight phase lock between sync and the subcarrier than it would be if the phase repeated every four lines, as it would do with the horizontal frequency suggested herein. US Patent US-A-4 450 474 in the name of Mizukami discloses a PAL sync generation system in which a sync generator provides a multi-component signal, one component being a line frequency signal having a period which is an integral multiple of one quarter the subcarrier period. The sync generator output is subsequently corrected, by phase modulation, whereby the line frequency signal provides a standard PAL line frequency signal relative to which the subcarrier has the 25 HZ field frequency offset. Thus, although the line component in the sync generator output generally corresponds to the "other" line frequency synchronising signal which is generated in the present invention, it is then corrected to the standard line frequency signal, and is not combined with a standard PAL vertical sync signal as in the present invention; furthermore this "other" line frequency signal is not derived from a standard PAL sync signal, as in the present invention. Indeed, the purpose in Mizukami is to provide a standard PAL signal, whereas the purpose of the present invention is to provide a signal having a non-standard line frequency component. The invention also provides apparatus for generating synchronizing signals for use in a PAL colour television system, said apparatus comprising a sync generator for supplying standard line and field synchronizing signals (H, V) and a signal (F sc ) of standard subcarrier frequency, said line synchronising signal comprising a sequence of standard line sync pulses, said apparatus further comprising: means responsive to signals supplied by said sync generator for producing another line synchronizing signal comprising a sequence of other line sync pulses, said another line synchronising signal having a shorter period during the active scan period than said standard line synchronizing signal such that an integral number of quarter cycles of said subcarrier occur during each line scanning interval, means responsive to the signals supplied by said sync generator for increasing the time between said other line sync pulses occurring during each vertical blanking interval such that the average period of said another line synchronizing signal is the same as the average period of said standard line synchronizing signal, and means for combining the vertical synchronizing signal supplied by said sync generator with said another line synchronizing signal. The invention will be exemplified by reference to the accompanying drawings in which: Figure 1 is a block diagram of a circuit incorporating the invention in which the desired line scanning frequency is derived from the PAL subcarrier or burst; Figure 2 is a block diagram of a circuit incorporating the invention in which the desired line scanning frequency is derived from the PAL line scanning frequency and subcarrier; and Figure 3 shows a way of generating the standard PAL sync signals. Figure 1 has a phase locked loop 2 that multiplies the subcarrier frequency applied to it from a sync generator 3 by four. It may operate in response to a burst of the PAL subcarrier frequency or the subcarrier frequency itself. The output of loop 2 is applied to one input of an AND gate 4. The output of the AND gate 4 is divided by 1135 in a divider 6. Pulses from a vertical sync generator or separator are applied to a one shot circuit 8 so as to generate a pulse that is two pixels wide during each vertical blanking period. In this embodiment, the pulses are negative and are applied to the other input of the AND gate 4 so as to prevent the pulses applied to the input from reaching the output of the AND gate 4. Alternatively, the output of the one shot circuit 8 can be applied to the counter reset. Vertical synchronizing pulses and waves of the color subcarrier frequency as well as the line synchronizing pulses from the divider 6 are combined in any suitable manner in studio equipment 7 that supplies video signals to a transmitter 9. In Figure 2 a PAL composite sync from a sync generator 11 is applied to a FIFO memory 10. Its write clock input is connected to means 12 for multiplying the line scanning frequency by 4 x 1135. The output of the FIFO 10 is the new PAL composite sync signal. A source 14 of a frequency equal to four times the subcarrier frequency is applied to one input of an AND gate 16. A vertical scanning frequency derived from the PAL composite sync is applied to a two pixel one shot circuit 18, and its output is applied to another input of the AND gate 16. The output of the AND gate 16 is applied to the read terminal of the FIFO 10. Vertical synchronizing pulses and waves of the color subcarrier frequency as well as the line synchronizing pulses from the FIFO 10 are combined in any suitable manner in studio equipment 17 that supplies video signals to a transmitter 18. Note that the read is faster than the write but the number of clock pulses in a field is the same because of the inhibiting by the AND gate 16. If a ROM were used to generate the standard PAL sync signals, it would have to store eight fields because this is the time it takes the phase relationship between horizontal sync and the color subcarrier to repeat. In accordance with another aspect of this invention, a far simpler way of generating the standard PAL sync signals is provided as illustrated in Figure 3 in which components corresponding to those of Figure 2 have the same designations primed. A ROM 20 that provides the new composite sync in which the horizontal sync has the same phase relationship with the color subcarrier every fourth line supplies signals to the input of the FIFO 10′. It only needs to be four lines long. The output of the multiplying means 12′ is connected to the read clock input of the FIFO 10′, and the output of the AND gate 16′ is connected to the write clock input of the FIFO 10′. This slows the higher frequency sync signals provided by the ROM 20 to the standard PAL frequency.

Claims: Apparatus for generating synchronizing signals for use in a PAL colour television system, said apparatus comprising a sync generator (3; 11) for providing standard synchronizing signals of a PAL system including a signal at the subcarrier frequency (f sc ), a vertical synchronizing signal (V) and a line synchronizing signal (H) comprising a sequence of line sync pulses, characterised by further comprising: means (2, 4, 6, 8; 12) responsive to signals provided by said sync generator for producing another line synchronizing signal comprising a sequence of other line sync pulses having a period during the active scan period such that an integral number of quarter cycles of the subcarrier occur between consecutive said other line sync pulses, the timing of said other line sync pulses during each vertical blanking interval incorporating an additional delay of half a subcarrier period such that the average period of said another line synchronizing signal is the same as the average period of said standard line synchronizing signal, and means (7; 10,17) coupled to said sync generator and to said means for producing said other line synchronizing signal for combining at least said vertical synchronizing signal and said another line synchronizing signal. Apparatus as set forth in Claim 1 wherein the period of said another line synchronizing signal during the active scan period corresponds to a frequency 100/1135 Hz higher than the frequency of the standard line synchronizing signal. Apparatus as set forth in Claim 1 or 2 wherein said means for producing said another line synchronizing signal is comprised of: an ANDING means (4; 16) having two inputs and an output, means (6; 14) for coupling a signal of four times the subcarrier frequency provided by said sync generator to one input of said ANDING means, and a one shot circuit (8; 18) responsive to said vertical synchronizing signal for applying a signal to the other input of said ANDING means to disable said output of said ANDING means for half a cycle of the subcarrier signal. Apparatus as set forth in Claim 1 or 2 wherein said means for producing said another line synchronizing signal is comprised of: a FIFO memory (10) having a write input, a read output, a write terminal and a read terminal, means for coupling a composite sync signal including said standard line and vertical synchronizing signals provided by said sync generator to said write input, a multiplier (12) coupled to said sync generator for applying to said write terminal a signal of frequency 1135 times the frequency of said standard line synchronizing signals, an ANDING means (16) having an output connected to said read terminal and two inputs, means (14) coupled to said sync generator for applying to one of said inputs a signal having a frequency four times the frequency of the subcarrier signal, and a one shot circuit (18) responsive to the vertical synchronizing signal provided by said sync generator for applying a signal to the other input of said ANDING means effectively to decouple said one input of said ANDing means from said output for a period occurring during each vertical blanking interval that has a duration of half a cycle of the subcarrier signal. Apparatus for generating synchronizing signals for use in a PAL colour television system, said apparatus comprising a sync generator (3; 11) for supplying standard line and field synchronizing signals (H, V) and a signal (F sc ) of standard subcarrier frequency, said line synchronising signal comprising a sequence of standard line sync pulses, characterised by further comprising: means (2, 6; 12) responsive to signals supplied by said sync generator for producing another line synchronizing signal comprising a sequence of other line sync pulses, said another line synchronising signal having a shorter period during the active scan period than said standard line synchronizing signal such that an integral number of quarter cycles of said subcarrier occur during each line scanning interval, means (4, 8; 16, 18) responsive to the signals supplied by said sync generator for increasing the time between said other line sync pulses occurring during each vertical blanking interval such that the average period of said another line synchronizing signal is the same as the average period of said standard line synchronizing signal, and means (7; 10, 17) for combining the vertical synchronizing signal supplied by said sync generator with said another line synchronizing signal.

Inventors: DISCHERT LEE ROBERT, TOPPER ROBERT JOSEPH, DISCHERT, LEE ROBERT, TOPPER, ROBERT JOSEPH
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H04N9
IPC codes: H04N9, H04N5