Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:An observation I have made, and I'm sure you too, is that sync pulse widths are not consistent, not only from different sources but within the same piece of video.
Klaas Robers wrote:I used always the down going edge of the sync pulse. However in my generators (e.g. EPROM-) I inserted one black (blanked) pixel at the end of each line, just before the sync starts. That is no more visible in the 10 kHz low passed video signal, but it gave much less sync jitter due to the video content. Anyway a flywheel (Nipkow disc or in software) in the line generator is eliminating the remaining line jitter.
Klaas Robers wrote:I prefer active clamping over DC restoration.
Steve Anderson wrote:Klaas Robers wrote:I prefer active clamping over DC restoration.
So do I. But what I'm driving at here is that detecting the sync pulses and thereby generating the clamp pulse as per 26/1 and 26/3 suffer from the same shortcomings as above. If you look at the second op-amp in both references you'll note that the time-constant is about 220ms, (330n + 680k), very similar to that illustrated above (190ms). If you can't detect the sync pulse you can't generate the clamp pulse. At least with this arrangement.
Steve Anderson wrote:The test signal is a 'Club Standard' NBTV signal comprised of four frames of white followed by four frames of black complete with missing syncs continuously repeated. This DC-coupled signal was then passed through a single CR hi-pass filter with a -3db cut-off of 10Hz to simulate the output of an audio source. Now -3db at 10Hz is not a shabby performance of any audio gear, and here only one hi-pass stage is employed. Two waveforms of the test signal are below, (Blk2Whi 1.gif and Whi2Blk 1.gif) the yellow trace being the TSG output, the cyan being after the hi-pass filter. One shows the black-to-white transition, the other white-to black.
Now I'll be the first to admit this is an extreme waveform, but it is legit, and although unlikely to happen on real progam material, it could and the system should cope with it.
In most cases there's another CR hi-pass filter interposed, some form of buffer amplifier, here is no exception, but I did set it at a very long time-constant of one second, -3db at 0.16Hz.
The results shown in (Wht2Blk 2.gif) are woefully inadequate...which is not surprising. You can twiddle the sync-slice pot nut you end up slicing genuine video and missing the otherwise valid sync pulses.
Klaas Robers wrote:Steve, the first thing I see is that your input signal has a high pass RC-time of 15 msec. That is a -3 dB point of 11 Hz. Such signals are difficult to handle anyway. The -3 dB frequency should be much lower.
Klaas Robers wrote:Then I see that the sag-speed of the DC-restorer has a time constant which is much longer. That is asking for problems. That will never work. You see that happening. It should be the other way around.
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