I've put together the 'side-chain' SSTV sync detection/extraction circuit. It uses the filters, (IC122a/b and IC121b) with permission, from John Magliacane's, KD2BD, Triple-PIC SSTV up-convertor. Website here with all the information, circuits and software for construction...
https://www.qsl.net/kd2bd/TriplePIC.htmlI've also attached the complete article below...
I've made some minor changes to John's original, mainly to ease sourcing components, e.g. changing 68n caps to 47n, with appropriate changes to associated resistor values. Also to suit the +/-12v supplies I'm using.
I've also asked John if the Triple-PIC is 525/60 output only, though most modern TVs/monitors will sync to both 525/60 and 625/50. Update upon John's reply.
14/06/2021, John's reply...
my scan converter generates a 262 line non-interlaced at 60 Hz vertical, 15.734 kHz horizontal that's close enough to the NTSC standard to display properly on every U.S. TV set and monitor I've tested. The zero-crossing demodulation will follow in due course...so far this has only been simulated, not built/tested.
Steve A.
As with most diagrams posted left-click the in-line image for a better resolution version, browser 'back' button to return...at least in Chrome it works...