On 16 Sep 2002, at 15:05, Olin Lathrop wrote:

> This is like asking is it possible to heat food without a microwave oven.
> Early TVs are an existance proof that this can be done with analog
> electronics without any integrated circuits.
>
> You first have to detect Hsync.  This is easy as it is the deliberately the
> most negative point of the waveform.  Next low pass filter the Hsync pulses
> and compare to a threshold.  That will be Vsync.  The Hsync pulses are
> stretched during the vertical blanking interval.  If you don't want the lag
> from a low pass filter, just look at the length of Vsync pulses.  Hsync
> starts at the first long Vsync pulse.
>
Good reply. I have just had to do something similar with AVR. The pcb was
already designed! They just used a LM31x (I forget) comparator with a pot to
set the trip level in the centre of the sync pulse. I interrupted on each sync
pulse and checked for a timer overflow - the timer is reset for 40uS so you
can see if time between sync pulses was more or less than this..
The mark 2 pcb *WILL* use an LM1881 or Elantec equivalent to do the work
fom me and remove the preset pot adjustment (user twiddle) problem.


George Smith

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