Television – Synchronization – Sync separation
Patent
1996-04-02
1998-09-29
Grant, Chris
Television
Synchronization
Sync separation
348525, H04N 508
Patent
active
058152130
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a device for extracting synchronization from a composite video signal. In a wider sense, the device in accordance with the invention is used in the video field. The invention is particularly suitable for integration on an integrated circuit.
An example of a synchronization extraction device according to the prior art is represented diagrammatically in FIG. 1. This device receives the composite video baseband signal (called CVBS signal) on a connection 1, through an external clamping capacitor 2. According to the present example, the CVBS signal is of positive polarity, which means that the synchronization pulses are formed by square-waves which are, negative with respect to the black level. The device further comprises a black-level recovery unit 3, a low-pass filter 4, an amplifier 5, a peak detector 6, a voltage divide-by-2 circuit (8), these elements being mounted in series. An external capacitor 7 stores the voltage value supplied by the peak detector 6. The outputs of the amplifier 5 and of the divider 8 are connected to the inputs of a comparator 9.
The CVBS input signal is aligned ("clamped") to a black reference voltage internal to the circuit 3 by means of this circuit 3 and or the capacitor 2. This clamping is carried out by means known to the person skilled in the art.
The low-pass filter 4 reduces the synchronization burst, the chrominance subcarrier and the noise component. The peak detector 6 detects the value of the peak voltage of the synchronization pulses previously amplified, and, in the present example, inverted, by the amplifier 5. The resultant value is stored by the external capacitor 7. The threshold for detecting a synchronization pulse is situated at 50% of the sum of the black level and the value held in the capacitor 7. The comparator 9 supplies a logic signal at the point 10 of the device, this signal corresponding to the threshold being crossed or not crossed. This signal is in the high state when a pulse is detected (signal above the threshold) and in the low state in the opposite case.
This circuit exhibits disadvantages under certain working conditions. This is because, in practice, a video signal only rarely has the profile of a theoretical CVBS signal. An example of such a signal is given diagrammatically in FIG. 2. According to this example, in accordance with the CCIR standards, the amplitude of the synchronization pulses 11 is ideally 300 mV. However, in real signals, this value can vary between 20 and 800 mV. The change in this value can occur within the time for a given transmission or more abruptly upon a change from one transmission to another.
If a large-amplitude pulse is followed by pulses the amplitude of which falls off rapidly with respect to this first pulse, these lower-amplitude pulses risk not being taken into account by the extraction device, the capacitor of the peak detector not having time to discharge sufficiently rapidly to follow the weakening of the amplitudes; with the peaks of these weaker pulses being situated below the detection threshold mentioned above, they will be forgotten. The variation in amplitude from one pulse to another generally does not exceed 10%, but this variation can be continued in the same sense over a certain number of pulses. A similar problem exists during the frame return when certain types of clamping circuits (type known as "IXI", where X represents the ratio between the charging and discharging currents of the clamping capacitor) are employed; the peak detector (or bottom detector if the signal is not inverted by the amplifier) cannot always follow the rapidity with which the clamping is performed. In both cases, a problem is encountered in sizing the storage capacitor of the peak detector; a compromise has to be found between the rapidity of the device and its sensitivity to low currents,
The amplitude variation can be introduced by an automatic gain control (AGC) device. This problem is particularly acute when the signal is of the SECAM L type, in which the video signal is positive-a
REFERENCES:
patent: 3706847 (1972-12-01), Smeulers
patent: 4550342 (1985-10-01), Buchanan et al.
Soviet Patent Abstracts, Section EI, Week 8841, 23 Nov.1988, Derwent Publn. Ltd., London & SU Patent 1385326.
Patent Abstracts of Japan, vol. 10, No. 137, 21 May 1986 & Japanese Patent No. 60-264167.
Grant Chris
Thomson Multimedia S.A.
Tripoli Joseph S.
Wein Frederick A.
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