Hi Darius,
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Originally Posted by Darius
Kat, thanks for explaining, but I know this stages from monitors.
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Fair enough.
The purpose of the Mk2 combiner was to be a better lash-up than the Mk1 lash-up; which had appalling DC level stability issues; black-level was all over the place and varied with scene content. The Mk2 circuit as produced works well enough; one requirement was to keep component count down and be able to build the thing in time for NVCF; something I only just managed; I was still soldering bits on veroboard at 8pm the night before, and was up at 4am to drive down...
I don't consider it a final design and I'm basically going to abandon it completely and work on something different and better.
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I am sure this solves your sync problems.
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The problems with this circuit are:
- Level-shifter transistors both operated as saturated switches, hence stretching the sync pulse. (Workaround - reduce sync pulse length in modeline as covered above.)
- Sync is XORed; so VBI is all wrong and produces line-pairing on my Pye monitor.
- Some DC level drift as the whole mess warms up.
- Overall component count too high for my liking. I prefer a few ICs to a lot of transistors.
New version (Mk3):
- Op-amp for RGB summing.
- Active clamp to set DC level wherever you want it (sync tips or black level at 0V at the flick of a switch) and deal with the DC drift problems.
- Either a PLL run at twice line-rate and some other digital electronics or maybe even a small microcontroller, synced to line-sync and generating proper sync for whatever standard the PC is producing.
- Probably all surface-mount and on a PCB as well.
This would likely have any buffering on the inputs necessary; run entirely off the 5V rail present on pin 9 of most graphics cards (generating a -ve rail internally, ICL7660 take a bow...) with an option to plug a wall-wart in on the offchance the card doesn't have 5V on pin 9.
A microcontroller of some sort is in the lead here as it gets the sync generation and timing under
software control, where I like it. I can then detect what TV standard the PC is producing then generate correct syncs as required in software; which suits my way of working. It might also be worth my while looking at the smaller FPGAs as an alternative to a microcontroller.
Regards, Kat