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Old 10th Jun 2006, 11:48 am   #5
Kat Manton
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Join Date: Jul 2005
Location: West Yorkshire, UK.
Posts: 1,700
Default Re: FOTH RGB combiner problems

Hi Darius,
Quote:
Originally Posted by Darius
Your problem is two transistors Q8 and Q11 are going to saturation. This makes the pulses longer.
I wondered if saturation of one or both transistors might be the problem. I've been thinking about using a FET or two in this area instead, I don't like bipolars much. To be honest, I don't like discrete-component solid-state circuitry much, either. Give me op-amps and nice complex IC's, please

Quote:
Are you sure with D1 and Q6 ?
D1 is there to allow me to set either sync tips at 0V or black level at 0V by adjusting R20. Odd idea I've never seen used anywhere else before, but it seemed to work so I left it.

You've spotted (yet another) mistake on the schematic, both outputs are of course identical circuits; all sorts of errors crept in transferring the drawings from scraps of paper to the computer. I've amended that bit, it now matches what's on my veroboard.

Quote:
I didn't see the minus 5V, I thaught it is ground
this negative voltage rail makes it complicated.
It does, but I wanted to be able to have black level at 0V and negative-going sync.

At the point where sync is inserted, the video signal (on the right-hand end of R17) is somewhere below the 0V rail; hence the need for Q8 as a level-shifter.

To be honest, I'm probably not going to bother refining this circuit any further myself; it was really another lashup that just happened to be better than the last lashup.

Video op-amps are the way forward; component count comes down, performance improves and designing the thing gets a whole lot easier. Jim Beacon's working on a circuit using a video op-amp; I'm likely to take the analogue parts of that but replace the XOR sync combiner with something else; lots of nice digital stuff...

Regards, Kat
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