Television – Synchronization – Sync generation
Reexamination Certificate
1998-08-19
2001-08-07
Eisenzopf, Reinhard J. (Department: 2614)
Television
Synchronization
Sync generation
C348S500000, C348S525000, C348S531000, C348S533000
Reexamination Certificate
active
06271888
ABSTRACT:
BACKGROUND OF INVENTION
The invention relates to a method for obtaining line synchronization information items from a video signal, and also to an apparatus for carrying out the method.
The invention is based on a method for obtaining line synchronization information items from a video signal of the generic type of the independent claim 1. Even though digital system solutions will become increasingly predominant in future television technology, analogue source signals will still exist for many years in the future. Examples include the terrestrial reception of video signals, which is still widespread to date, and the analogue recording methods, e.g. according to the VHS standard in the case of video recorders. Such, analogue signal sources represent critical signal sources for digital systems, and their signal processing requires special measures.
To date, television receivers with digital signal processing (e.g. in the case of the 100 Hz technology) have been operating, as a rule, with clock systems which are synchronized with the respective input signal. Since the input signal is the analogue CVBS signal, either the horizontal sync pulse (line-locked clock) or, alternatively, the colour subcarriers or colour synchronizing pulses (burst) (colour subcarrier-locked clock) are frequently used as reference point for the synchronization. The sync separation in the video lines has usually been carried out to date by means of analogue methods using so-called sync separator stages and a PLL filter stage connected downstream. In television receivers with digital signal processing, a PLL filter stage which is a digital realization of the known analogue sync signal processing is usually used. The filter stage is then a digital PLL (Phase-Locked Loop). Examples of such digital PLL circuits are the circuits SAA 7111 from Philips, HMP 8112 from Harris and Digit 3000 from Intermetall. The principal problem with such digital PLL circuits is that the known instabilities in the picture occur when the input signal present is an analogue video signal picked off from an analogue video recorder which is currently operating in the search mode (fast forward or reverse run) . Many users of analogue video recorders are sufficiently acquainted with such instabilities. Specifically, disturbing horizontal stripes appear in the picture when the video recorder is operating in the search mode. These disturbing stripes originate from the fact that in the search mode, the video heads no longer run on a single slanted track but rather sweep across two or more slanted tracks, depending on the search speed. During the transition from one slanted track to the next, abrupt sudden phase changes arise with regard to the occurrence of the sync pulses of the video lines. These sudden phase changes are actually governed by the geometry in magnetic tape recording in accordance with the slanted track method. The sudden phase changes are therefore determined by the system and, in addition, virtually unavoidable.
Irregular occurrence of line sync pulses also arises, however, in the case of video signals generated by camcorders. In this case, the instabilities that occur are, as a rule, more severe than in the case of a normal video recorder, because the regulation of the head-drum speed is subject to greater fluctuations on account of the larger component tolerances.
EP-A 0 266 147 discloses a digital PLL circuit for a television receiver. In the case of this digital PLL circuit, in order to avoid the abovementioned problem in the search operating mode in video recorders, a switching unit is provided which drastically shortens the time constant of the phase-locked loop in the event of identification of a sudden phase change caused by the head changeover at the end of a slanted track, with the result that the region of instability in the picture is reduced in size. The disadvantage of this solution is that the reduction of the time constant of the phase-locked loop provided by this solution means that noise components in the video signal are able to be suppressed less well and disturbing lines still remain visible, even though to a lesser extent than when the time constant is larger.
SUMMARY OF THE INVENTION
Taking the abovementioned prior art as a departure point, the object of the invention is to specify a method for obtaining line synchronization information items which enables a further improvement in the picture quality even in the case of noise and unstable analogue source signals, and at the same time can be realized in a simple manner in terms of circuitry, in particular with digital technology.
The object is achieved separately in each case by a plurality of steps but combining the various measures affords an optimized solution. The first improvement measure consists in accurately determining the position of a line sync pulse that occurs. This is done differently from the previously known methods, in which frequently it is simply a case of evaluation only of the falling or rising edge of the line sync pulse, in that the respective video line or only part of the video line is convolved with a pattern function and the position determination for the line sync pulse is carried out by analysis of the result function of the convolution operation. An ideal line synchronization pulse is preferably used as the pattern function. An improvement in the position determination is advantageous particularly in the case of terrestrially transmitting video signals with multipath propagation which are subjected to a great deal of interference.
The other, very essential improvement measure according to claim 1 consists in replacing the digital PLL circuit that is usually used otherwise by a filter unit in which filtering of the positions of the line sync pulses in the temporal direction takes place, in which a linear or non-linear estimation is carried out for the respective position to be corrected of a line sync pulse. Thus, a feedback control loop is no longer used and the known settling problems of these control loops are deliberately avoided. As a result, it is possible to achieve a significant improvement in the picture quality for critical source signals. Therefore, particularly in the case of signals from video recorders, in the case of noisy antenna signals or else antenna signals in surroundings where interference due to multipath propagation occurs, it is possible to improve the picture quality of the video signals displayed. Since the new approach additionally implies a free-running, crystal-generated system clock, the solution is excellently suited to digital video signal processing in computer systems.
Further improvements to the method are possible by virtue of the measures evinced in the dependent claims. As already mentioned, an ideal line sync pulse is preferably selected as the pattern function for the convolution operation. This pulse has a slightly trapezoidal shape.
The analysis of the result function of the convolution operation preferably comprises determination of the maximum or minimum of the result function. These operations can easily be carried out mathematically by various methods. The position of the maximum or minimum then specifies the position of the line sync pulse (the centre of the line sync pulse). It is thus possible to perform an unambiguous position determination in a simple manner. The measure is based on the insight that the convolution result of two square-wave pulses is a triangle function having an absolute maximum or minimum.
It is advantageous if, in order to determine the maximum or minimum of the result function in the convolution operation, the first derivative in the result function is formed and then the zero of the first derivative is determined. A triangle function exhibits only one zero in the first derivative, which can easily be determined by customary computation methods.
Since the signals involved are signals which are limited in terms of their bandwidth and are processed digitally, the peak of the result function of the convolution operation may be rounded. For this case, th
Lares Roland
Rothermel Albrecht
Desir Jean W.
Deutschethomson-Brandt GmbH
Eisenzopf Reinhard J.
Sragow Daniel E.
Tripoli Joseph S.
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