piclist 2000\12\08\041029a >
Thread: SCENIX VIDEO VIRTUAL PERIPHERAL Design Challenge and Co ntest
www.piclist.com/techref/ubicom/lib/io/index.htm?key=video
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face BY : Michael Rigby-Jones email (remove spam text)



> -----Original Message-----
> From: Sean H. Breheny [SMTP:shb7STOPspamspam.....CORNELL.EDU]
> Sent: Friday, December 08, 2000 8:50 AM
> To:   TakeThisOuTPICLISTEraseMEspam@spam@MITVMA.MIT.EDU
> Subject:      Re: [sx]: SCENIX video VIRTUAL PERIPHERAL Design Challenge
> and Co ntest
>
> Hi Mike,
>
> Could you please explain how that circuit works? I haven't had much time
> to
> study it, but simple as it is, it isn't obvious to me.
>
> Thanks,
>
> Sean
>
>
The diode/resistor combination bias the transistors base to a nominal 0.6
volts.  Any negative going signal on the emitter will switch the transistor
on, effectively stopping the emitter swinging below 0 volts.  The video is
AC coupled to the emitter so this ensures the bottom of the sync pulses
cannot be lower than 0 volts.  Theoretically the sync level is always a
fixed amount lower than the black level ( can't remember exactly by how
much), so the video can be processed by a comparator with a fixed reference.

In practice this circuit definitely works, I build the DCFG, and I was most
impressed at seeing TV pictures (in B/W) on my old 286 many years ago.
However, this is definitely not a "production quality" circuit.  A "proper"
circuit would use the back porch part of the video signal to clamp the true
black level to ground, so the size of the sync pulses wouldn't matter.  If
this circuit was set up with a comparator that gave good operation on one
video source, there is no guarantee it would work 100% on another source

Hope that helps, I'm really quite bad at explaining stuff even when I know
how it works :o)

Mike

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