CRT Deflection circuit - sync pulses

sp_mclaugh · 2006-11-01T07:41:26+00:00

Hello,I'd like to understand the "guts" of the deflection circuit inside afixed-frequency display. I know the most monitors today are eithertri/quad sync, or truly multi-synchronous, but for learning purposes,I'd rather start with a fixed-frequency display. I'm just learning thebasics of VGA timing (doing a video project on an FPGA), and I feelthat these answers would give me a more rock-solid grasp on what I canand can't do with VGA.At first, I thought that the horizontal deflection circuit would beeven simpler than a normal sawtooth relaxation oscillator. My thoughtwas that rather than using a "threshold" type device to trigger thedraining of the capacitor in the oscillator, we could just use the syncpulse directly to turn a drain transistor on/off. But all theinformation I've read on the net seems to suggest that the sync pulsesin some way add/subtract only slightly to the "normal" frequency. Cansomeone clarify what's actually going on here? Perhaps there isn't onecorrect answer (different companies, different method), but I'mguessing that there's a common method used by most.As for the vertical deflection, it should look like a "staircase",right? Of course, there would be a sharp downward line after each"staircase" for the vertical retrace. How is this waveform generated?My initial guess, again, would be a simple integrator whose input wasthe horizontal sync pulses. It would reset itself on each vertical syncpulse.However, this type of a setup would mean that the *length* of thehorizontal sync pulses would directly affect how much the picture wasvertically stretched. I'm not sure whether this is true or not.Question: So long as the horizontal sync pulses are at the correctfrequency and are close to the correct length, does their *exact*length in fact have a great effect on the picture? I'm a bit more intothe digital circuit world, so excuse the terminology, but another wayof asking the question might be: Is there some part of the deflectioncircuit which is...

Nov 01, 2006 20 Replies

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joseph2k 19 years ago

With all said, i do not think we really disagree technically. Our discussion more seems about how much each of expects less experienced persons to follow. I really appreciated the additions to my historical understanding. I am constantly adapting to explaining for more general knowledge, but i may be a bit behind you on this adaptation.

JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller

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