GB2200011A

GB2200011A - Television sync extraction circuit

Info

Publication number: GB2200011A
Application number: GB08729904A
Authority: GB
Inventors: Termano Testi; Mario Pallotini; Massimo Ferreri
Original assignee: GTE Telecommunicazioni SpA
Current assignee: Siemens Telecomunicazioni SpA
Priority date: 1986-12-22
Filing date: 1987-12-22
Publication date: 1988-07-20
Also published as: FR2608880A1; IT1201539B; IT8622801A0; DE3742488A1; GB8729904D0

Description

9 t 1 2200011 TELEVISION SYNC EXTRACTION CIRCUIT The present invention

relates to a television sync extraction and amplitude measurement apparatus for instance for u se in television transceivers such as for satellite transmissions.

In television signal receiving and transmitting systems, especially satellite systems, it is necessary_ to perform extraction of synchronisation (sync) signals and measurement of the amplitude thereof.

Extraction of syncls necessary for synchronisation of the various circuits which process the television signal (timers. spreading signal generators,-clamp circuits, and circuits for insertion and extraction of the sound-in-sync).

Measurement of the amplitude of sync signals is required to obtain information on the level of the television signal and thus supply any alarm signals needed.

As known. a television signal has a peak to peak value which varies with its information content: the 2 maximum level corresponds to white, the minimum level corresponds to black. However, the value of the sync signals does not vary with the information content but only if the general level of the television signal actually varies. The result is that correct information on the television signal level can be obtained by measuring the sync amplitude.

To achieve this, it is necessary to employ a sampling system in which the sampling intervals are locked to the extracted sync. Extraction of sync is normally performed by the use of a diode clamp circuit followed by a comparator and a timer. comparison is performed with a fixed reference value. This implies considerable reduction of immunity to noise in the comparison when a television signal is received with a very low signal-tonoise ration (S/N).

This is the case with satellite reception where with INTELSAT STANDARD B stations it is possible for the signal received at radio frequency to have a very low carrier-to-noise ratio (C/N) so as to arrive near the operating threshold of the system. Under these conditions the S/N ratio is below the standard fixed values of international standards and the sync extraction system, operating with comparison to a fixed reference, can therefore commit intolerable errors. In 4 1 6d 1 3 other words, it is not possible to employ such systems in situations near the reception threshold.

Measurement of the amplitude of sync signals is effected in general separately from extraction by simple sampling of the synchronizing level of the sync pulse having adjusted the reference, given by the blanking level (or maximum level at end of pulse) to the value of 0 volts. This reference may however display thermal and supply voltage drifts which could invalidate the correctness of the measurement.

Furthermore. the abovementioned measurement is not applicable in the presence of SIS, i.e. sound-in-sync. In this case the level measured would be affected by amplitude modulation due to the effect of audio channel modulation.

Claims

According to first and second aspects of the invention. there are provided apparatuses as defined in the appended Claims 1 and 12, respectively. Preferred embodiments of the invention are defined in the other appended claims. It is thus possible to provide an apparatus for sync extraction which is substantially free from the 4 above-described disadvantages and provides effective comparison in the extraction phase in the presence of reduction of the television signal level and/or in the presence of noise increase. The amplitude measurement circuit may comprise first and second samplers driven by the extraction circuit so as to sample the synchronising level and the blanking level, respectively, of the frame sync signals, means being provided for determining the amplitude of the sync signals as the difference between the two sampled levels. Locking of the reference level of the extraction circuit comparator to measurement of the amplitude of the sync signals, accomplished by double sampling of the frame sync, provides a comparison under optimal conditions whether in the presence of reduction of the television signal level or in the presence of noise increase. This implies better immunity to noise introduced by a satellite-receiver path, i.e. allows operation under very low CIN ratio conditions in the proximity of the receiver's operating threshold. This means that even under such critical conditions it is possible to have correct television signal alarm W 1 data and correct drive of the extraction circuit so as to remove from the- television signal the dispersion signal introduced in transmission. 1 Such a system is also suitable for operation in the presence of SIS signals. Thisis obtained by appropriately delaying the sampling of the synchronising level of the frame sync so as to perform measurement of the synchronising level in a zone free from SIS signals. The invention will further described. by way of example, with reference to the accompanying drawings, which: Figure 1 is a block diagram of a system constituting a preferred embodiment of the invention; Figure 2 shows a series of representative graphs of the behaviour of signals at various points of the system of Figure 1; and - Figures 3-5 show respective application extensions of the system of Figure 1. An extraction and amplitude measurement system 1 for synchronisation (sync) signals illustrated in Figure 1 comprises an amplitude measurement circuit 2 and an - 6 extraction circuit 3 interconnected and both supplied with the TV signal from a receiving terminal 4 through a stop-band amplifier 5 which brings the signal to the optimal level for subsequent processing and eliminates therefrom the colour subcarrier (at a frequency of 4.43 MHz for the 625 line standard of 3.58 MHz for the 525 line standard). The amplitude measurement circuit 2 comprises two samplers 6 and 7 formed by electronic switches 8 and 9 equipped with holding capacitors 10 and 11. The sampling is thus of the 'sample and hold' type. The outputs of the samples 6 and 7 are connected to the negative or-inverting input and to the positive or noninverting input, respectively, of a differential amplifier 12 with an output integrator 13 made up of a resistor 14 and a capacitor 15 in parallel with each other and connected to ground. The output signal A provides a measurement of sync signal amplitude and is made available at an output terminal 16. The extraction circuit 3 comprises a clamp circuit 17 formed by a capacitor 18 and a diode 19 connected in serieg. Across th-e diode 19, which is forward-biased. there is a voltage Vd, which fixes the synchronising level of the sync pulses at the output 20 of the clamp circuit 17 at the voltage value -Vd. The latter is 1 4 ik 1 1 7 selected in such a manner as to be approximately equal to half the overall level of the television signal. In this manner, the output voltage 5 of the clamp circuit 17 swings between the values +W and -Vd. The output 20 of the clamp circuit 17 is connected to the inverting input of a comparator 21 whose noninverting input is connected to the output of the differential amplifier 12 through an attenuator 22. The signal thus taken from the amplitude measurement circuit 2 constitutes the reference threshold R for the comparator 21. Considering the selection made for the voltage Vd, the threshold will under nominal conditions be approximately at the centre of the signal interval i.e. approximately equal to Ov, but will rise and fall with increases and decreases in the maximum level of the sync signal. More precisely, if the signal decreases, its maximum, or blanking, level will decrease while the minimum, or synchronising, level will also remain fixed at -Vd. In this case the reference level R will also fall but will remain at the centre of the signal amplitude interval. The output of the comparator 21 is connected to a terminal 23 at which there are thus available the sync signals B app ropriately squared. a 1 8 The output of the comparator 21 is also connected to a timer 24.which drives two monostable multivibrators 25 and 26 assigned to control of the samples 7 and 6, respectively. of the amplitude measurement circuit. The monostable multivibrator 25 is triggered when the rising edge of the output signal of the timer 24 occurs. The monostable multivibrator 26 is triggered when the falling edge of the output signal of the timer 24 occurs. The latter is selected such that the falling edge of the output signal coincides with the synchronising level of the-frame sync pulses with a predetermined delay in relation to the beginning of the synchronising level and such that the rising edge coincides with the blanking level of the sync pulses. The television signal TV is amplified and the color subcarrier removed in the amplifier 5 and is supplied to the extraction circuit 3 and the amplitude measurement circuit 2. At each line or frame sync pulse, the diode clamp circuit 17 supplies at the output the signal 5 shown in the graph a) of Figure 2. The comparator 21 compares this signal with the reference signal R giving rise to the square signal of the graph b) of Figure 2.. As mentioned above, the reference signal varies with the 1 P h 11 4 9 sync signal amplitude measured by the measurement circuit 2 so that the comparator operation threshold is held always at the centre of the sync signal amplitude interval. This permits optimal comparison even with a very small SIN ratio. The square signal B at the output represents the extracted pulse.

1 The timer 24 delays activation for a predetermined time T in relation to the falling edge of each square pulse at the output of the comparator 21 so as to isolate the frame sync, of longer duration, from the line sync, of shorter-duration, and so as to operate in a zone of the frame pulses free from the so-called SIS ($ound-in-synt) signals located in the initial part of the synchronising level of the sync pulses. The series of a line sync pulses SR and of a frame sync pulses SQ is illustrated in graph c) of Figure 2 and the corresponding output signal from the timer 24 is illustrated in the graph d) of Figure 2.

The timer output signal drives the two monostable multivibrators 25 and 26 each of which generates a pulse having a duration of 2.5 microseconds at the rising edge and the falling edge, respectively., of the signal as shown in graphs e) and f) of Figure 2.

The monostable multivibrator 26 drives the sampler 6 which samples the synchronising level Lm of the frame sync. The monostable multivibrator 25 drives the sampler 7 which samples the blanking level LM of the frame sync.

In the amplitude measurement circuit 2, the differential amplifier 12 calculates the difference between the two sampled values, giving at the output the signal A representative of the sync signal amplitude and correcting as required the reference level R of the comparator 21 of the extraction circuit 3.

The basic system 1 of Figure 1 is susceptible to various extensions as shown in Figures 3-5.

In particular there can be associated a circuit 31 for suppression of the dispersion signal introduced in transmission. Such an added circuit comprises a clamp switch 32 controlled by a timer 33 which receives the square signal B at the output of the comparator 21 of the extractor circuit 3 of the basic system 1. The time is selected so as to perform the sampling coinciding with the back porch level which follows the rising edge of the line sync pulses. An amplifier 34 is also provided in cascade with the stop-band amplifier 5.

A 51 I a 1 11 To further increase immunity to noise and to any jitter phenomena, even in the presence of reduced television signal levels. it is alto possible to associate a circuit which accomplishes a second comparison, still with the moving reference. i.e. automatically locked to the level of the sync. The additional circuit is illustrated and indicated by reference number 41 in Figure 4 and comprises a comparator 42, a timer 43 and an attentuator 44. the comparator 42 receiving the amplitude measurement signal A produced by the measurement circuit 2 of the basic system 1. The circuit 41 is free from problems connected with drifts of the diode 19 and can therefore provide in certain cases extraction of syncs (signal C) even better than those provided by the basic system 1.

Finally, as shown in Figure 5 there can be provided the combination of the circuits 1, 31 and 41, the last of which drives a clamp switch 45 shunted from a transmission line of the television signal which branqhes out from the receiving terminal 4 via a branching circuit 46 and comprises two amplifiers 47 and 48. The circuit 1 is the basic sync extraction and measurement Circuit, the circuit 31 actuates the clamp coincidentally with the back porch level to obtain a first suppression of the-spreading signal, and the circuit'41 suppresses the spreading signal by means of 12 another clamp circuit and a second comparison with the moving reference.

J p 4 z 13 a 1 k.

CLAIMS 1. An extraction and amplitude measurement apparatus for television synchronisation signals, comprising an extraction circuit and an amplitude measurement circuit, the extraction circuit including a clamp circuit. a comparator. and a timer, the amplitude measurement circuit including at least one sampler frequency-locked to the synchronisation signal. the comparator having a reference level which varies with the amplitude of the synchronisation signal supplied by the amplitude measurement circuit.

2. An apparatus as claimed in Claim 1. in which the amplitude measurement circuit comprises first and second samplers driven by the extraction circuit so as to sample the syncrhOnising level. and the blanking level. respectively, of the frame synchronisation pulses. and means for determining the amplitude of the synchronisation pulses as the difference between the sampled levels.

3. An apparatus as claimed in Claim 2, in which the amplitude determining means comprises a differential amplifier with inputs connected to the outputs of the samplers.

14

4. An apparatus as claimed in Claim 2 or 3. in which-the samplers are of the sample and hold type.

5. An apparatus as claimed in Claim 3 or in Claim 4 when dependent on Claim 3. in which the reference level of the comparator of the extraction circuit is supplied by the output of the differential amplifier.

6. An apparatus as claimed in any one of Claims 2 to 5, in which the timer of the extraction circuit is arranged to drive in temporal succession first and second Tonostable multivibrators which are arranged to control the first and second samplers, respectively, of the amplitude measurement circuit. the timer being arranged to activate the first multivibrator coincidentally with the synchronizing level of the frame syncrhonisation pulses and the second multivibrator coincidentally with the blanking level of the frame synchronisation pulses.

7. An apparatus as claimed in claim 6. in which the timer is arranged to activate the first multivibrator with a predetermined time delay relative to the falling edge of each frame synchronisation pulse.

8. An apparatus as claimed in any one of the preceding claims, in which the output of the comparator A lo of the extraction circuit controls a circuit for suppression of a dispersion signal introduced during transmission, the suppression circuit including a clamp switch controlled by the output of the comparator via a timer.

9. An apparatus as claimed in any one of the preceding claims. including a further extraction circuit having a comparator arranged to receive the output of the amplitude measurement circuit as a variable reference level.

1

10. An apparatus as claimed in Claim 9, in which the output of the further extraction circuit controls a further clamp.switch for shunting a transmission line of the television.signal.

11. An extraction and amplitude measurement circuit substantially as hereinbefore described with refer-ence to and as illustrated in the accompanying drawings.

12. A television sync extractor, comprising means for supplying a reference signal dependent on the level of an input sync signal and a comparator for comparing the input sync signal.with the reference signal to provide an extracted sync signal.

Published 1988 at The Patent OMce, State House. 65'71 High Holborn, London WClR 4TP. Plarther copies may be obtained from The Patent OMce, Sales Branch, St Ma--y Cray, Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray, Kent. Con. 1/87.