For quite some time I've been using a modified (improved?) version of the Robot 70 SSTV demodulator. The final circuit diagram I have for it is dated March 2015. It perfoms remakably well considering its simplicity and the era it was designed in, the mid-late 60s maybe?
It's biggest flaw to me is the output filter, this has to separate the wanted modulation of up to 1kHz or more (the video signal) from the full-wave rectified sub-carrier, where the sync frequencies get doubled to 2.4kHz due to the full-wave rectification. That is just over one octave, quite a task for an analogue filter. To achieve such a steep cut-off it has to be either a Butterworth or Chebychev version in the analogue world. Both of these introduce overshoots on fast (a relative term) rising or falling signals, e.g. sync or blanking edges.
Initially I considered replacing said filter with a digital FIR version. But if we're introducing digits, why not make it all (or mostly) digital?
Follows is my concept. It uses the zero-crossing method at the input, a look-up table to convert the half-cycle duration into a linear voltage representation with or without Gamma correction and finaly a modulated PWM signal is generated at approximately 48kHz.
Now the output filter can be a sinple Bessel analogue version with far less overshoot, maybe none, as the wanted video frequencies (around 1kHz) and the PWM (48kHz) are widely separated. There is also the possibility of replacing the analogue output filter with a digital version, though still a Bessel, e.g. MAX292/296. That needs investigation though.
Although the circuit diagram below looks quite busy, it's largely due to all my notes and other stuff that should vanish when finalised. This has not been built yet and I still have to write the software for it. I hope to make a start in the next few days.
This in terms of hardware is simpler than the Robot 70 version, two chips (one 20-pin, one 8-pin), instead of three chips (two 14-pin, one 8-pin).
Steve A.
Another item I should look at is the modulator I use which also could do with a re-visit as it also uses similar analogue filters. That thankfully is somewhat simpler...maybe?
A question raised previously..."Will this work with other modes such as Martin, Scottie or PD?" Yes, including those that use VIS codes. It doesn't decode the VIS as shown, but might be persuaded to do so, though that is not a priority for me...a use for the data/ASCII output maybe? (IC401 pin 10/TP6). It all depends on timing, code size and remaining memory to handle this additional requirement. It may require a change of processor to a 28-pin device simply to get enough RAM and program memory, but probably not. At this stage I'm not going to actively pursue these additions, maybe sometime later...