Atari ST video capturing & making S-Video out

   Many Atari ST machine users have now problem with displaying device: computer is OK, but monitor is dead. It is especially problem for color modes, since monochrome can be displayed on practically every VGA monitor, and on new LCD ones too - with little adapter (connect monochrome output to all 3 RGB, and H & V sync lines). There are of course monitors capable to display Atari's color signal, but they are not cheap and easy to find.
   Solution may be usage of TV cards, which are now cheap and easy to find in every computer shop. And we can do nice recordings with them.

Attaching composite video output to TVcard's composite input, (+audio out to sound card's line input) - and we have Atari's pic. on every monitor:
This is not bad, but depending on TV card and it's capture driver we may expect problems as shaking picture, freezing. Reason is not standard video timing of Atari ST.
  I have Pinnacle PCTV Pro card, with BT878 chip. Picture from ST shakes in (pre)view mode, but fortunatelly when starting capture it stabilizes. Much better is with Virtual Dub - it is stable always.

 There is no simple solution against timing issues, but we can make sharper and cleaner picture at practically zero cost, in half hours. Solution is S-Video. All newer TV cards have S-Video input.

We need to separate Luma from Chroma signal to get S-Video. It may be done without desoldering whole RF modulator case from ST, what would be not easy. Just remove cover and desolder with vacuum pump following pins of MC1377: 
#10 - chroma to output mixer/amp
#13 - chroma to filter (desoldering will decrease noise)
All 4 pins of Luma delay line (right up on pic)

Connect Luma from IC's pin 6 after resistor R10 direct to pin 8 of IC (black wire on pic)

S-Video outputs are direct from IC: Luma to pin 9 (may add cap. of some 47-100 microF in line by need), and Chroma to pin 13 via ceramic cap. of 100nF and resistor of 47 ohms (not mandatory). There is too much noise on line after transistor Q1, what is on RF modulator's connector, pin 10 (mostleft one down), so it is not usable. I don't see that we need to amplify IC's signals, maybe in case of longer cable...
Note: Bypassing filter between pins 13 and 10 is important for getting sharper picture, colors. It required bypassing of Luma delay line, to get aligned picture.
On pic is marked by Red what is needed to be desoldered. With blue arrows are marked new connections.

 It is much better now...


Medium resolution

This is schematic of ST's PAL/NTSC encoder.

 With Red is marked what needs to be removed - or just desolder pins which lead to IC. Blue arrows are outputs. Thick blue line is new connection.

     Adding PAL encoder to Mega ST:
 I added PAL encoder board from dead 520STM to my Mega ST. But with same modifications as above, to get S-Video out. Difference is that I used here regular video out, after transistor and electrolyt cap. - because removed RF modulator, so no noise problem. I completely desoldered delay line and chroma filter here.
However, one additional thing must be made - because PAL oscillator is not on encoder board, and no such on Mega ST main board too, we need to add it to circuit. Fortunatelly, it is easy and cheap - just take out 4.43MHz quartz from 520 STM (near to shifter chip), and solder it between pin 17 of MC1377 and capacitor of 15pF, which other pin connect to GND. Then add cer. cap. of 220pF between pins 17 and 18, and one of 220pF between pin 18 and GND (according to Motorola ref. circuit from datasheet). Before it remove all capacitors connected to pins 17 and 18, of course.
Hardest part is to find where to solder required lines - R,G,B H and V sync and +5, +12 Volts.
To remove metal case around board some bigger soldering iron is needed (60W or more).
Pic left is illustration, before turning on machine board must be isolated and layed down...

Couple captures:   FSII demo take off (4.3MB)   Nebulus (4MB)

Captured in res. 768x576, YUY2, huffyuv codec, recompressed with XVid.

For comparison here is capturing made from composite video: NebulusComp.avi  (0.5MB)

Note that by Nebulus capture pic is good until die, then it becomes interlaced. Reason is not standard video out of ST (and probably odd Vblank count when restarts).
Solution against may be: deinterlace - not recommended since makes pic unsharp
capturing at half vertical res (288 lines)
usage of BT Tweaker - not simple
Processing captured video with Avisynth:
1: Decomb filter: Telecide() - it will restore progressive frames. Good for Nebulus which is at 25 fps
2: If video is always interlaced use: SeparateFields() - it will give 50 fps with halved vertical res

P. Putnik   Jan-March. 2007.

Video playback on ST

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