[Coco] S-video Prospects...

[Gene Heskett]

On Thursday 26 January 2006 19:59, Stephen H. Fischer wrote:
>Hi,
>
>This is the first I have heard about "A missing HSYNC", I never have
> seen it.
>
>As others clearly know about it, I must be lucky.
>
>I wonder if my TV (and other top of the line sets) keep a oscillator
> running and then phase lock it or similar when the source HSYNC
> arrives.
>
>Gene, perhaps you can educate me on this.

Somebody rang?

Yes Stephan, they do.  The locking of them is done in a bit of trickery 
during the vertical synch pulse.  If you have a tv that you can make 
the picture roll downward a bit, you'll see if you turn the brightness 
up to max, what looks a little like the feather end of an arrow pointed 
to the right between the bottom of the upper pix and the top of the 
bottom pix.

This little cut between the fatter block, and the end of the synch pulse 
to its left is about 2.4 millionths of a second, is put in there so 
that when the synch pulse drops back to active state at the left edge 
of this black block, its exactly in time with the hsync, but twice a 
line for those 3 lines of active v-sync.  You are seeing the one in 
between the active ones in each vertical interval.  There are 3 lines 
(6 pulses) before, and 3 lines (6 pulses) after the 3 lines of vsynch, 
of what we call equalizing pulses, and that block you see is also 
duplicated in the hsynch's time, again so that the leading edge is 
properly aligned in time to maintain h synch lock.  If you can see the 
hsynch and the vsynch at the same time, this block is the hsynchs 
narrow set, with the leading edges matched to hsynch.  This was worked 
out originally in black and white days even before color was shoehorned 
in by some clever math because of the alternate field interlacing done 
to conserve bandwidth without dropping the vertical rate so low that it 
flickers.

In the pro business, we buy monitors that can do this synch display in 
both directions so we can quickly check how well its working just by 
pushing a button.  Handier than a loaf of sliced bread or bottled beer 
to us.

FWIW, if all the timebases are stable, and this has now been mandated by 
the commission for about the last 35 years, if a tv signal is looked at 
with a spectrum analyzer, and the bandwidth of the analyzer is turned 
down to very narrow, one can see that the sidebands generated by the 
video consist of a spike of info every 15,734 hz, and that each of 
these is straddled by lower level signals spaced 59.94 hz away from the 
15,734 hz spikes.  For B&W, those are 15,750 and 60 hz, a never mind 
difference.

In Black and white, there is a huge amount of virtually empty space 
between these 15,734 spaced groups of signals, so the color subcarrier 
frequency was chosen to put the color information into these empty 
areas.  So all these frequencies are in fact locked to each other by a 
very precise set of math formulas turned into strings of divider chips 
and phase locked loop oscillators on our end of the signal.  It works 
surprisingly well most of the time.

Not widely known and not widely enforced is the fact that one of the 
color phases has twice the bandwidth of the other, again because of 
what the eye was sensitive to as much as any other reasons.  I'd say 
that 99% of the color encoders in use today generate a full, 2.4 mhz 
bandwidth color signal, or up to 1.2 mhz either side of the base 3.58 
mhz subcarrier.  It's sent as the summed result of a pair of 4 quadrant 
multipiers, one driven by the subcarrier, and one driven by the 
subcarrier delayed 90 degrees, with the color signals mixed and gained 
into them thru a complex matrix of resistors,  The end result is a 
signal that can be any amplitude and phase when referenced to the burst 
reference we also send on the back porch of the horizontal blanking 
interval.

Since the color is sent as a plus or minus from the black and white, let 
me remind everyone that if your set is miss-adjusted or the crt is 
going sour such that you cannot get a decent grey scale black and white 
image, then no amount of tweaking of the color is going to fix it to 
look really right.

And (offtopic) surprisingly, when watching the encoder output on a 
vectorscope, there is no one alive who can look at the vectorscope and 
tell from that faces color signal, going generally up the -I line on 
the scope, whether it belongs to a white, red, or even a very black 
person.  To the vectorscope, which ignores brightness and only shows 
the color and intensity of the color, they are all exactly the same.

I suspect that a Bell Tel. Fellow named Claude Shannon and his theories 
had a lot to do with that math way back then.  When that was done, I'm 
not sure if the fourier transform had been discovered yet or not, and 
full, realtime  useage of that had to wait many years for both gigabit 
ecl logic and simplfied methods, the most famous of which today is 
probably the butteryfly transform, commonly used because you can check 
your results by running it again on the output data and you should get 
the input data back again.  I was gonna port that to the coco way back 
then, but got busy chaseing my current missus and putting table driven 
crc checking into rzsz instead, which had more benefits for everyone at 
the time.  So much for good intentions.  I hear tell there's a road 
paved with them. :)

I hope I didn't lose too many of you. I don't think I did because the 
folks who read this list are I feel, of well above average IQ's, I've 
seen it demo'd many times.

-- 
Cheers, Gene
People having trouble with vz bouncing email to me should add the word
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Copyright 2006 by Maurice Eugene Heskett, all rights reserved.