Television – Synchronization – Automatic phase or frequency control
Patent
1994-02-28
1996-02-27
Kostak, Victor R.
Television
Synchronization
Automatic phase or frequency control
348542, 348546, H04N 512
Patent
active
054952948
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the acquisition and regeneration of synchronising signals in a received communications signal having a repetitive synchronisation signal.
BACKGROUND TO THE INVENTION
The invention will now be described and discussed in terms of one type of repetitive signal, a video signal. However, this should not be considered limiting and is for ease of understanding only. It is to be understood that the invention is applicable to any signal which includes a repetitive synchronisation signal.
Synchronisation signal separators (sync. separators) are widely used in many types of video equipment, for example in video synchronisers which introduce a delay into the incoming video signal in order to synchronise it with other video sources. Video synchronisers perform well with good quality video signals, but their performance is less successful with noisy video such as may occur when processing signals from radio-cameras. Examples of such radio-cameras are described in BBC Research Department Report 1991/15 entitled `The Development of Radio Systems for Portable Cameras Used in Television Production` by C. Gandy. Television signals received from these mobile cameras using microwave links are often of variable quality due to noise and signal loss. This causes problems for video synchronisers, in particular their sync. separator circuitry. Existing sync. separators have been found to be inadequate, as they are prone to losing the lock on the signal in conditions of severe noise, rather than `flywheeling`, a condition in which the sync. separator would generate its own sync. signals without reference to the incoming syncs. Moreover, in the known apparatus, the frequency of the flywheel decays as the control voltage held on a capacitor gradually discharges. Thus, the timing of the sync. signals generated during flywheeling becomes progressively more inaccurate.
A further disadvantage of existing sync. separators is that they can only operate with standard level video signals such as are generated under studio conditions. The noisy environments in which radio cameras operate mean that standard level signals are often not produced.
SUMMARY OF THE INVENTION
The present invention aims to overcome the disadvantages mentioned above. The invention also aims to provide a sync. separator which may be used for any type of signal which includes repetitive synchronising signals and which is transmitted in a channel of variable quality
The invention is defined in the independent claims to which reference should be made.
Preferred and advantageous features of the invention are defined in the dependent claims.
A preferred embodiment of the invention, once it has locked on to the signal can operate even in the event of short but total losses of incoming sync. signals, such as can occur in the event of multipath propagation. It also has the advantage of being able to tolerate longer periods of degraded syncs than prior art separators.
In one embodiment, the separator can recognise the difference between odd and even television fields enabling use with switching type radio cameras.
BRIEF DESCRIPTION OF DRAWINGS
An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:
FIG. 1 is a general block diagram of a sync. separator embodying the invention; and
FIG. 2 is a more detailed block diagram of the digital processing and VCO of the sync. separator;
FIGS. 3A-3E are timing diagrams explaining the operation of the phase comparator; and
FIG. 4 illustrates a further embodiment of the invention.
DESCRIPTION OF BEST MODE
The preferred embodiment is used in connection with video signals, and in particular PAL standard signals. However, the invention is applicable to any signal containing a repetitive synchronising component. In the video application, the invention is applicable to any signal format including, for example, NTSC, SECAM and MAC as well as PAL.
The PAL signal comprises 25 frames of video information per se
REFERENCES:
patent: 4228461 (1980-10-01), Weissmueller
patent: 4740999 (1988-04-01), Wagner
patent: 4847678 (1989-07-01), McCauley
patent: 5005079 (1991-04-01), Satomi
patent: 5036293 (1991-07-01), Aschwanden
patent: 5181115 (1993-01-01), Flamm et al.
patent: 5331347 (1994-07-01), Wu
British Broadcasting Corporation Research Department Report "Radio-Cameras: The development of radio systems for portable cameras used in television production," C. Gandy, 1991.
Evans Richard H.
Gandy Christopher
British Broadcasting Corporation
Fournier Kevin J.
Kostak Victor R.
Lee Michael H.
O'Connell Robert F.
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