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Originally Posted by Darius
Hi Kat, you should use a schmitt after the C1 and C2. The LS86 wants
TTL level.
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Not necessary in my opinion; the sync pulses from the computer are TTL-level already; and the filter time constant should be long enough for the levels after the RC filter to never drop below (or rise above) the respective logic threshold anyway once the charge on the capacitors has stabilised.
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I am not sure understanding your power supply. The -5V is not stabilized?
(+5V -6V3)
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The -5V rail isn't really -5V, it's whatever's left over after 5V (and a diode drop) has been subtracted from the
regulated supply you run it off. (I'm lazy and used the "-5V" library part - only true if you run off 10.6V...) I'm running off a regulated 12V wall-wart, so the -ve rail regulation is as good as the regulation of the wall-wart - good enough with the linear regulated one I'm using.
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"Video op-amps are the way forward; component count comes down, performance improves and designing the thing gets a whole lot easier"
I don't agree. And I want the sync tips on 0V !
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I can get sync tips at 0V if you like; it just takes a tweak of R20 while watching the output on a 'scope...
For what it's worth, my opinion is that any circuit which needs a video signal referenced or clamped such that some part of it is at a fixed level, then it should be the responsibility of that circuit to do it.
The standard for video signals appears to merely specify 1V p-p and 0.3V sync but not that anything is referenced to anything. If I needed sync tips at zero for anything following this (modulator, probably) I'd AC-couple the input and clamp sync tips to 0V myself; thus making the input compatible with anything I feed into it. I have no idea what my VCR produces, for example.
Personally I want black level at 0V as I might end up with several sources of 405-line video at this rate, black=0V makes switching (or even fading if everything's synced together...) nicer.
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"Now there's an idea. I've got several flavours of RS232 line-driver in my junk box."
Do you have some MC145406 in a small smd package?
I got some from RS order code 273-270 and they are big smd's.
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Sorry, no SMD parts here at all; mostly I've got "classic" MC1488/MC1489 and some Maxim parts; all standard 0.1" DIL package.
Regards, Kat