GB2232034A - Horizontal and vertical sync pulse derivation - Google Patents

Description

-I- A METHOD AND CIRCUIT FOR DERIVING HORIZONTAL AND VERTICAL FREQUENCY

SYNC PULSES This invention relates to a method and circuit for deriving horizontal and vertical frequency sync pulses from a three-level sync signal of a high definition television signal.

For synchronising the equipment in a television transmission chain a sync signal is required, which is normally contained in the television signal and must be separated from the latter. This sync signal comprises two components, a horizontal component for synchronising the horizontal deflection and a vertical component for synchronising the picture deflection. The horizontal component is formed by H frequency sync pulses, also called line sync is pulses, whilst the vertical component is formed by V frequency sync pulses, also called frame sync pulses. These two components must be separated from one another in each television receiver. Methods and circuits are known by which it is possible to split up the sync signal hitherto used in standard television systems and which has only two different levels. For high-definition television (HDTV) systems, preference is given to sync signals which have three different levels (cf. for example SEPTE Journal% N6vember 87, pp. 1150 to 1152). The advantage of such sync signals is that they are free from mean values, that is they contain no d.c. voltage component.

1 t The object of the present invention is to provide a method by which a sync signal having three levels can be split up into the H and V frequency components.

According to the present invention there is provided a method for deriving horizontal and vertical frequency sync pulses from a three-level sync signal of a highdefinition television signal, comprising deriving a two-level pulse signal from the three-level sync signal by separating the sync signal components located below half the total amplitude of the sync signal, deriving from the leading edges of the twolevel pulse signal first gate pulses whose duration is greater than the duration of the negative-going part of the H sync component of the three-level sync signal, deriving horizontal frequency synchronising pulses from those trailing edges of the two-level pulse signal, or those differentiated positive-going edges of the three-level sync signal, which occur during the first gate pulses, deriving from the V enable pulses of the two-level pulse signal second gate pulses whose duration is greater than the line duration, and deriving vertical frequency synchronising pulses from those leading edges of the horizontal frequency synchronising pulses which occur during the second gate pulses.

The invention has the advantage that it operates in 1 1 a flexible and reliable manner in the presence of noise.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, wherein:

Figure 1 is a circuit diagram for performing the invention. and Figure 2 shows certain of the pulse signals occurring at the similarly lettered points in Figure 1.

Referring to the drawings, the circuit shown in Figure 1 shows a threelevel sync signal S applied to the terminal 1 (whereof the 1250 and 625 lines are shown in Figures 2a and 2a' respectively) passing via an input stage 2 to a circuit 3 for clamping the black level. The three-level sync signal S is then supplied to a clipper circuit 4, in which the sync signal components located at roughly half the negative peak value (or 25% of the total amplitude) are separated to form a two-level pulse signal (Figure 2b). This circuit 4 can, for example, comprise a comparator circuit. to whose inverting input is applied the three-level sync signal and to whose non- inverting input is applied a reference voltage at 25% of the total amplitude.. so that the two-level pulse signal is provided at its output.

1 The output of the circuit 4 is connected on the one hand to a first branch 6 producing the H sync pulses and on the other hand to a second branch 7 producing the V sync pulses. The branch 6 comprises two monostable multivibrators 8, 9 whose pulse inputs are each connected to the output of the circuit 4. The first monostable multivibrator 8 is controlled by the negative-going leading edges of the two-level pulse signal of Figure 2b and produces the first gate pulses shown in Figure 2c, whose duration (approx. 31js) is roughly three times the duration of the negativegoing part of the H sync component of the threelevel sync signal. The first gate pulses at the output of the first multivibrator 8 are supplied to the switching input of the second multivibrator 9. The leading edges of the negative-going H sync pulses of Figure 2d are produced from those positive-going trailing edges of the two-level sync signal which occur during the period of the first gate pulses of Figure 2c. It would alternatively be possible to use the positive- going edges of the three-level sync signal directly, that is without prior conversion into the two-level pulse signal, by initially differentiating the three-level sync signal S and then rectifying it. The resulting needle-shaped pulse signal can then be supplied to the pulse input of the second monostable multivibrator 9 in place of the two-level pulse signal of Figure 2b, so that from it can be produced the leading edges of the H sync pulses during the period of the first gate pulses of Figure 2c. The trailing edges are given in each of the two cases referred to above by the time constant of the second multivibrator 9 or by the end of the first gate pulses. As the second monostable multivibrator 9 is only gated for about 3jusec, for the H sync components of the three-level sync signal S and the starting pulse I st of the second field in the 625 line, the positive-going edges of the two- level sync signal must follow the negative-going edges within this time in order to produce corresponding H sync pulses. The H sync pulses with a pulse duration of approx. 2.usec provided at the output 11 of the second multivibrator 9 can be used directly for the horizontal synchronisation of an oscillator circuit not shown in the drawings.

The branch 7 contains a known integrator circuit 12 for the V signal, to which is applied the two-level pulse signal of Figure 2b. Both the H pulse components (pulse duration approx. 0.89,usec) and the so-called V enable pulses I W (pulse duration approx. 8,usec) between the H pulses at the end of the fields, undergo integration, so that there is a signal configuration according to Figure 2e at the output of the circuit 12. This signal is now supplied to a circuit 13 for obtaining second gate pulses. The circuit 13 can for example comprise a comparator circuit, whose non-inverting input is connected to the output of the integrator circuit 12 and at-whose inverting input is applied a reference voltage producing a clipping potential A. The second gate pulse, having a duration of more than one line, are shown in Figure 2f and are provided at the output of the circuit 13. These second gate pulses are supplied to the switching input of a third monostable multivibrator 14 for gating purposes and the output signals of the second monostable multivibrator 9 are applied to its pulse input. The negative-going leading edges of the H sync pulses of Figure 2d produce, during the gating periods, the leading edges of the V output pulses, whose trailing edges are given by the time constant of the third multivibrator 14 or the end of the second gate pulses. Thus at the output 15 of the third multivibrator 15 of the third multivibrator 14 one obtains the V sync signals according to Figure 2g for the start of the second field (line 625) of a given frame or V sync signals according to Figure 2h for the start of the first field (line 1250) of the next frame.

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Claims (10)

1. A method for deriving horizontal and vertical frequency sync pulses from a three-level sync signal of a high-definition television signal, comprising deriving a two-level pulse signal from the threelevel sync signal by separating the sync signal components located below half the total amplitude of the sync signal, deriving from the leading edges of the two-level pulse signal first gate pulses whose duration is greater than the duration of the negativegoing part of the H sync component of the threelevel sync signal, deriving horizontal frequency synchronising pulses from those trailing edges of the two-level pulse signal, or those differentiated positive-going edges of the three-level sync signal, which occur during the first gate pulses, deriving from the V enable pulses of the two-level pulse signal second gate pulses whose duration is greater than the line duration, and deriving vertical frequency synchronising pulses from those leading edges of the horizontal frequency synchronising pulses which occur during the second gate pulses.

2. A method according to claim 1, wherein the twolevel pulse signal is derived by comparison of the three-level sync signal with a reference voltage at about 25% of the total amplitude of the three-level aync signal.

3. A method according to claim 1, wherein the first gate pulses are produced by controlling a first monostable multivibrator with the leading edges of the twolevel pulse signal.

4. A method according to claim 1, wherein the horizontal frequency synchronising pulses are derived by controlling a second monostable multivibrator with the trailing edges of the two-level pulse signal during gating by the first gate pulses.

5. A method according to claim 1, wherein the horizontal frequency synchronising pulses are derived by controlling a second monostable multivibrator with the differentiated positive edges of the threelevel pulse signal during the gating by the first gate pulses.

6. A method according to claim 1, wherein the second gate pulses are produced by integration of the V enable pulses with a subsequent comparison of the integrated pulses with a clipping potential.

7. A method according to claim 1, wherein the vertical frequency synchronising pulses are derived by con trolling a third monostable multivibrator with the leading edges of the horizontal frequency synchronising pulses during gating by the second gate pulses.

8. A circuit for performing the method according to one of the claims 1 to 7, comprising a first comparator circuit to whose inverting input is applied the three-level sync signal and to whose non-inverting input is applied a reference voltage and at whose output is provided the two-level pulse signal, a first circuit branch with first and second monostable multivibrators whose pulse inputs are connected to the output of the comparator circuit, the output of the first multivibrator being connected to the switching input of the second multivibrator at whose output are provided the horizontal frequency synchronising pulses, and a second circuit branch with a V integrator circuit connected to the comparator circuit output and a following second comparator circuit whose noninverting input is connected to the output of the V integrator circuit, whose inverting input is supplied with a reference voltage and whose output is connected to the switching input of a third monostable multivibrator, the pulse input of the latter being connected to the output of the second multivibrator and at whose output are provided the vertical frequency synchronising pulses.

9. A circuit according to claim 8, wherein upstream of the first comparator circuit there is connected a circuit for clamping the black level.

10. A method as claimed in claim 1, substantially as described with reference to the accompanying drawing;:

P,iblishel-1990 at ThePatent Office. State House. 66 71 High Holborn. LondonWCIR4TP. Further copies maybe obtainedfroin The Patent Office Sales Branch, St, Mary Cray. Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1187