🚀 go-pugleaf

In article <1992Aug31.132411.8008@ms.uky.edu>, billq@ms.uky.edu (Billy Quinn) writes:
> What does one have to do to a composite video signal to make the TV
> do a verticle retrace?  This is for BLACK & WHITE video and it does
> not need to be broadcast quality!  My only referrences to diagrams of   
> this sort are for color video.  It seems that with out color one could
> just keep the horizontal sync low for a while after the last line of 
> video.  Is this true?  If so, how long does it take?  Any suggestions,
> references etc?
> 

Yes, just taking the signal to the sync level (0v) for a couple of line
periods will do it. Its crude, but is done on many of the cheap
computers & TV games.

Within the TV there is usually some sort of integration circuit (a
resistor/capacitor pair). The short sync pulses at the start of each line
don't charge the capacitor up very much, and it discharges before the
next line, so nothing happens. A long pulse at the end of the frame allows
the capacitor to charge up enough to trigger the vertical retrace.

A real broadcast-quality signal uses a series of long pulses, each almost one
full line in duration, over about 10 lines. They are long enough to allow
the integration capacitor to charge, but the fact that there is still a
rise/fall at each line period allows the horizontal circuitry to stay in sync.
In practice a modern TV will stay synched anyway, so just using one
long pulse of 600mS or so will work OK.

Steve