Moderators: Dave Moll, Steve Anderson
Harry Dalek wrote:...I wonder what the bottom range is .
Steve Anderson wrote:...I suspect not that much lower, plus it expects equalising pulses and broad pulses within the vertical period. I have used this chip many times for standard 525/625 sources, and it works well. But using it for NBTV I would guess it's a non-starter, as we have no pulses at all within the frame/field sync period. Someone say I'm wrong!
I did do an item which appeared in the NBTVA newsletter some 13 years ago, which predicted where the 'missing pulse' would be. It worked quite well (If I may say so), But it seemingly generated zero interest, so I gave up on it. Now I understand why many give up on creating things like a YouTube channel, or any other form of publishing. I've thought about creating a YouTube channel many times, and not just for NBTV, and thought, "Why bother?" Many others that do create a YouTube channel seem to reach the same conclusion fairly quickly.
However, there are others who are quite successful, I guess they get the 'formula' just right.
Steve A.
Harry Dalek wrote:..but why do we have to stick to standard who made that rule up the club i suppose may be we need to change that rule ! no frame pulse is one way to do it but not the best way to do it ...what i am getting at going off club standard can be fun too .
Steve Anderson wrote:I assume the reason for the 'missing pulse' frame sync is partly historical, and partly technical. The historical and technical reasons are somewhat intertwined. The resurgence of the interest in NBTV was maybe in the 60s or 70s. The only practical method of recording/storing NBTV signals then was primarily open-reel tape recorders, or in time, cassettes. The bandwidth of both domestically was maybe 10kHz, often less in those days. The low-frequency performance was similarly 'not so good'. (read awful).
Devoting a whole line to frame sync meant losing around 3% of 'picture time' which was felt to be wasteful. Not having a line sync when one was expected meant this was frame sync.
The way the unit works in my posting above regenerates the missing sync which also acts as a frame sync. BUT, it does require a good clean signal path and to a degree reasonably accurate hole-drilling if using a disc camera. I doubt it would be useable on the noisy HF amateur bands, especially with their limited audio bandwidth.
Once you get to 100 or more lines (e.g. early SSTV with 120/128 lines), the loss of 1 line in 100 or more is 'acceptable'. Also, SSTV being based on an FM subcarrier the low-frequency problem goes away at the expense of reduced high-frequency video components. Nothing is ever free!
[/quote]Here we are more than two decades into the 21st Century and there is still no sane and simple alternative to the 'missing sync' method whilst keeping things in the analogue domain, and at least some semblance to the origins of analogue TV.
Harry Dalek wrote:I must admit i have never come across NBTV on SW for some reason never took off and every one went with SSTV ,got you it would of been nice if some one could try
Steve Anderson wrote:I've been thinking of using GPS/GNSS for sync pulses - it's hard to get a better timing standard, and they're within a few (20-30) nano-seconds of each other wherever you may be on this planet.
Why? They're so accurate syncs wouldn't need transmitting! Video with no sync pulses...not exactly true, just a one-off 'reference point'. Ionospheric propagation may be an issue though depending on frequencies used for the video signal. Cables, fibres and frequencies unaffected by atmospheric conditions would be more stable and useful...possibly? 'Sort of' thinking aloud...
Steve A.
The more I think on this the easier it's becoming!!
Steve Anderson wrote:I put together a system some years ago where there was just a frame sync, no line syncs. It worked very well (closed-circuit). It was a 48-line system at 12.5fps with standard scanning (left-right/top-bottom). It worked well. I calculated the timing error (as none was visible) which turned out to be a small fraction of one pixel in the whole frame. I assumed the crystals used were really cheap with a +/-100ppm 'accuracy'. (i.e. very poor for a crystal).
Using GPS/GNSS it should be far, far better, especially if both ends of the circuit use the same reference. I wonder if the various GNSS systems (GPS, Europa, GLONASS etc.) are locked together? If a GPS receiver can process more than one system simultaneously (most today can and do) which system should it use if they are even slightly different in timing?
Years ago a good local source of accurate timing/frequency was the sync pulses within standard 525/625 analogue TV off-air. The TV station SPG (Sync Pulse Generator) had to have (by law in the UK strangely) an accuracy of +/- 1Hz for the colour subcarrier at 4.43MHz (0.226ppm). All other timings/pulses were derived from that or a multiple. When GPS came along they were locked to GPS and thus became as good, long-term.
Steve A.
In many countries the 50/60Hz power mains supply is also locked to some atomic reference. Once you filter out the rubbish that gets superimposed on it, it's also a good long-term source of accurate frequency.
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