Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:I've had a fiddle around with MMSSTV with you comments foremost, but I cannot find the correct calibration 'knob'. This is a live analogue input generated electronically fed straight into the PCs line input, it's not recorded or processed in any manner.
The clock accuracy you are referring to, is it the soundcard clock? The PC clock? Or the clock the generates the SSTV signal itself? I have been assuming the 'knob' for this is within MMSSTV, maybe I'm wrong there.
Steve Anderson wrote:As per your suggestion I recorded 60 seconds of the generated SSTV signal at 12000Hz sample rate using 'Audacity' on a Windows XP machine. If you can quickly play it and let me/us know what you find. Please don't spend much time on this. This .wav file has been nowhere near MMSSTV at all.
M3DVQ wrote:If I play the file in with the clock rate set to 12000 then adjust the slant in MMSSTV until the bars are vertical MMSSTV reckons to have resampled it (if my understanding of how slant correction works is correct!) to 11962.8Hz.
Steve Anderson wrote:Yes, very strange indeed! That's an error of -3100ppm in the sample rate which far greater than any crystal frequency tolerance. Even the cheapest ones are spec'd at +/-100ppm, yer average common-or-garden variety as used in PCs are around +/-30ppm. So where that error is coming from is beyond me.
Steve Anderson wrote:More interestingly (to me) is did it sync up? Both line and frame?
Steve Anderson wrote:Yes, very strange indeed! That's an error of -3100ppm in the sample rate which far greater than any crystal frequency tolerance. Even the cheapest ones are spec'd at +/-100ppm, yer average common-or-garden variety as used in PCs are around +/-30ppm. So where that error is coming from is beyond me.
Steve Anderson wrote:More interestingly (to me) is did it sync up? Both line and frame?
M3DVQ wrote:AHA! I've finally worked out what's going on...The 8 second mode MMSSTV implements is the the Robot Research model 1200 variation with the 4:3 aspect ratio, longer line syncs, and no vertical sync tone, not the original "Macdonald" 1:1 format. Doh
Harry Dalek wrote:...I think its time to find something that plays the original SSTV system .
Steve Anderson wrote:Thanks for the links Harry, but all are based on the MMSSTV engine, i.e. it's gonna require the VIS signal at the start of each frame.
There is one site I found a few weeks ago that looked promising at first - forget the complexity for the moment, but I have my reservations as to the claims made. I hope that I'm proven wrong...
http://www.qsl.net/kd2bd/sstv.html
..and the site specific to his converter...
http://www.qsl.net/kd2bd/TriplePIC.html
...but at least we have a.n.other who's also interested.
I have been in contact via e-mail with him, but he's giving nothing away, perhaps as a result of his agreement with 'ATV Quarterly' which is understandable.
Steve Anderson wrote:Interesting the weather-fax system, I wonder if that AM system is still used? I would have thought it all digital these days.
As for your question regarding syncs within a recent PM...I think I understand what you're thinking of. A fixed frequency (1200Hz) oscillator that is switched to when you require some sync tone then back to the video sub-carrier signal after the appropriate length of time. Is that correct?
If it is I don't recommend that method as at the switching point it will introduce a discontinuity in the FM waveform. 'Traditional' demodulators using tuned circuits and the like would probably cope with this gracefully, but a software-based version might get a bit upset, it depends on how the software filtering is done (if any).
If you plan to use a VCO, say a 4046, an arrangement as shown below will do the trick. If for example the VCO produces 1kHz per volt of input the sync pulses disconnect the video amp via a 4066 during sync-time and and the 1.2V input provides the sync tone. At other times the 4066 is on and the vieo amp with an output impenance of much less than 100k dives the VCO. The video amp needs to be configured to produce +1.5V on black, and 2.3V on white.
Of course you can use any sensible voltage, this was just an example.
I don't know what you are using/planning for the video amp but it should be possible to make it fit in with this method.
Steve A.
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