Thanks for posting your timing diagram, Steve.
For decades, the only SSTV video standards I ever saw in U.S. publications came directly from an article written by Copthorne MacDonald for the January 1961 issue of QST magazine:
- sstv_standards.gif (131.9 KiB) Viewed 8054 times
"Item D" from your illustration was completely new to me until very recently when I saw it mentioned in an on-line SSTV document. I only recently made that addition to my SSTV generation software to keep the phase of the horizontal sync consistant from frame-to-frame while performing my current sync detection experiments.
My TriplePIC follows a "triggered sweep" philosophy, and so it doesn't care about the phase of the horizontal sync between video frames. My original SSTV
Oscilloscope Adapter had a horizontal sweep that was triggered by each individual sync pulse as well.
But a year or so later when I built a Robot 70 (with a PLL-triggered horizontal sweep), I often saw on-air signals where the first few lines of the image were out-of-alignment as the PLL was forced to re-acquire lock at the start of every new frame of video. I thought (at the time) that the problem was due to the PLL being thrown out of lock by the vertical sync pulse, but I now have a better understanding of what was going on. I felt that the poor sync handling of the Robot 70 was its biggest downfall.
In the early 1970s, Copthorne had an SSTV column in "CQ" magazine (published in the U.S.). In the August 1974 issue, he wrote a product review of the Robot 70 where he stated:
"This reviewer's only real complaint about sync performance is the line-to-line jitter which starts to appear when the signal drops to the point where "snow" is visible in the picture".
In the September issue, he offered his own solutions to the jitter problem that was prevalent in probably the majority of SSTV monitors and scan converters at the time. In fact, the cover photo of that very same issue shows the output of W0LMD's SSTV keyboard after being scan converted and displayed on an NTSC (fast-scan) television set, complete with very noticeable line-to-line jitter!
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In Cop's opinion, the biggest cause for jitter was what Robot (and others) had employed in their P7 monitors at the time: A sync separator that stripped sync from the demodulated video, followed by a PLL-triggered horizontal sweep. He even cited his original P7 monitor as having instability problems as it employed a 1200 Hz bandpass detector followed only by half-wave rectification and no post detection filtering at all.
I adopted his suggested techniques in my TriplePIC scan converter, using modern-day components, and the results have been very good. But now I realize it may be possible to do even better.
73 de John, KD2BD