Does anyone still build their own SSTV equipment these days?
I've had an interest in slow scan TV for a while already, although I have next to no interest in playing with SSTV PC/computer programs. I've been searching on and off for a long-persistence P7-type phosphor CRT for a while now to make a traditional analogue monitor for 120/128 line monochrome images, but nothing has turned up yet. I have a heap of short/medium P1 CRTs, so I've decided to knock up a hybrid analogue/digital monitor in the mean time. I've got most of the circuitry figured out on paper for a video digitiser for the image transmitting unit, but that will be a separate standalone system to the receiver/monitor. For now I've just finished drafting the schematic for the latters video display board.
This board contains the video memory and associated circuitry to generate the video display on what will be the units internal CRT or an externally connected CRO. I think it's fairly self-explanatory, but the frame rate is 61Hz, 128 horizontal lines with 256 pixels per line. 8-bit digitization is used for the video luminescence. All signals are derived from the free-running 15-bit synchronous counter to the left. U10+U11 produce the horizontal deflection sweep signal while U12+U13 handle the vertical. The 15-bit counter is positive edge-triggered. On the negative edge of the counters master clock the data output of the video memory SRAM is synchronously shifted to the parallel video DAC via octal DFF U19. During the hoz. and vert. retrace intervals the video DAC is blanked by asserting U19s asynchronous reset input and forcing the analogue video output to a negative level blacker than black.
A uC-based demodulator/receiver and digitiser will be accommodated on a separate PCB. The uC on this board will assert access to the video memory for writing via the 74HC573 tri-state buffers wired as parallel data bus multiplexors/selectors.
The SRAM is large enough to hold four individual received images. To this end the two MSBs of the SRAM are binary-decoded by a small diode matrix for a 4-position rotary switch which will be used for selecting the image/page to be displayed.
Should get around to starting the board layout tomorrow evening, though I'm going to have to start hunting for some recordings of 7/8 second SSTV images so I have something to decode when the monitor is complete.