Hi,
I thought I'd post the results of some measurements and calculations after further refinement of the System A X Modeline.
The card I'm using is an MSI card which uses the nVidia GeForce 4 MX4000 chip. This card supports interlaced modes, I bought it (for £17.71 including postage off eBay) to replace a GeForce2 card which doesn't support interlaced modes. The pixel-clock range of this card can be specified in increments of 10kHz from 12MHz to 350MHz (determined by experiment), and as was suggested earlier in the thread, this is one of the cards which needs horizontal timings specifying in multiples of 8 pixel clocks. However, in spite of having to make a few approximations, I think I've got pretty close!
My source of System A timing information was
http://www.pembers.freeserve.co.uk/W...Standards.html
The results of my calculations give the following; I'm compiling a .pdf document showing how I arrived at this which I'll post here later.
- Pixel-clock: 12.07MHz
- Vertical resolution: 377 pixels
- Horizontal resolution: 968 pixels
The modeline I've ended up with in my /etc/X11/xorg.conf file is as follows:
Code:
Modeline "SystemA" 12.07 968 992 1104 1192 377 377 385 405 -hsync -vsync interlace
This gives the following results:
- Line rate: 10.1279kHz (measured - 2.9Hz high - spec is 10.125kHz)
- Frame period: 20.00ms (measured - within spec - spec is 20.0ms)
- Active line period: 80.2us (calculated - 0.1us low - spec is 80.3us)
- Horizontal front porch: 1.99us (calculated - 0.29us high - spec is 1.7us)
- Horizontal sync pulse length: 9.3us (calculated - within spec - spec is 8-10us)
- Active field period: 188.5 lines (as spec - 377 active lines/frame)
- Field sync duration: 4 lines (as spec and confirmed by observation)
By twiddling with the trigger level control on my Gould OS250 I can get it to lock to alternate field sync pulses with the line sync on the other trace - and indeed, the field sync duration is correct and we definitely have an interlaced signal.
Apart from the slight timing errors noted above, about the only thing "wrong" is that the VBI won't contain the double-line-rate serrations as per the specification. However, if I XOR the vertical and horizontal syncs to get composite there will be line-rate serrations, basically horizontal sync pulses inverted during the VBI.
I suspect all this is close enough to work.
Now I'm at the stage of having near-correct System A timings; I'm happy enough with it to feel there's some point in building the electronics to get RGB+syncs combined into monochrome composite. I'll try to photograph some waveforms once I've got that far
Regards, Kat