Why is video inverted for transmission?

[Don Pearce]

This thread continues to make little sense.

The sync tips are NEVER at "0V". If you're looking
at the signal prior to restoration, then the DC reference has
not yet been established, and so the sync tips are wherever
they happen to be. If after DC restoration, it is the
blanking level (not the sync tips OR the black) which is
considered to be the "0V" reference for the rest of the signal.
IF the signal is then to be shipped around at RS-343 levels
(from which most current analog video standards were derived),
then the nominal white is 0.714V above that reference, and the
sync tips are 0.286V below it; if the older RS-170 levels are
used, white is +1.000V, and the sync tips are -0.400V. And
at least in U.S. practice, black is set slightly above blanking
("setup"), about 54 mV in the 1.000 Vp-p RS-343 specs.

You are talking about the situation post sync-separator. In the
composite video signal, sync tip is 0V. Black level is 0.3V and peak
white is 1V.

d

 

[Don Pearce]

All other shortcomings aside, synch tips at 0V permit DC restoration
just as well 1V. The inversion is for other reasons.

Jerry

The transmission is AM. It must use AGC to get a video signal of the
correct amplitude (forget DC restoration for just one moment). When
sync is at the top, every line has the same amplitude, so the AGC
works. If some indeterminate illuminance level is at the top, the AGC
will force whatever level that may be to peak white by changing the
gain. The system can not work that way.

d

 

[Don Pearce]

The amount of light reflected from a lump of coal in sunlight far
exceeds what is reflected from a sheet of white paper indoors. Our
ability to make sense of what we see depends on that restriction of
perceived dynamic range.

Jerry

http://www.freshgasflow.com/ourcrazyplanet/illusions/checker_illusion.htm

illustrates this very well.

d

 

[glen herrmannsfeldt]

Joerg wrote:

(snip)

That is not what the masses need. The masses on living room couches need
something that turns their old analog TV into a digital TV. Walmart used
to have a converter for around $250, about the same amount our whole TV
had cost (!). Maybe they still do. What I wanted is to test the Walmart
guys. If a technology is supposed to be widespread I would at least
expect the manager to know. He didn't.

I believe DVD recorders with ATSC tuners are now available for around
$200 or less. They at least have composite video output, probably
RF out, too.

Otherwise the government is supposed to subsidize the converters.

-- glen

 

[ChairmanOfTheBored]

Move out to the signal fringe and check again. You missed something.

There is a difference between bit error rate and multipath distortion.
Big difference.

With digital, and of course, the FEC that gets included, it is either
ALL there or not displayable. There is NO distortion ever.

 

[Randy Yates]

Bullshit. If the sync pulse was the weakest signal, it would be lost
long befroe a the picture became unusable.

At first I wanted to agree with you - it makes sense in one way.

But a sync pulse is really nothing more than the horizontal scan rate,
right? You can build a PLL to pull this way out of the noise by making
a very narrow-band loop filter. The video information is a different
matter since you need a wide bandwidth to utilize it.

So I'd say the sync pulse power can be lower than the video power, by the
ratio of the bandwidths, and still be appropriate for a usable video signal.
--
% Randy Yates % "She has an IQ of 1001, she has a jumpsuit
%% Fuquay-Varina, NC % on, and she's also a telephone."
%%% 919-577-9882 %
%%%% <[email protected]> % 'Yours Truly, 2095', *Time*, ELO
http://www.digitalsignallabs.com

 

[Richard Crowley]

At first I wanted to agree with you - it makes sense in one way.

But a sync pulse is really nothing more than the horizontal scan rate,
right? You can build a PLL to pull this way out of the noise by making
a very narrow-band loop filter. The video information is a different
matter since you need a wide bandwidth to utilize it.

So I'd say the sync pulse power can be lower than the video power, by
the
ratio of the bandwidths, and still be appropriate for a usable video
signal.

How easy it is for us to do arm-chair engineering here 50+
years after the original development *with the technology of
the day*! This entire "discussion" reeks of revisionism and
has turned just completely silly.

 

[Don Pearce]

At first I wanted to agree with you - it makes sense in one way.

But a sync pulse is really nothing more than the horizontal scan rate,
right? You can build a PLL to pull this way out of the noise by making
a very narrow-band loop filter. The video information is a different
matter since you need a wide bandwidth to utilize it.

So I'd say the sync pulse power can be lower than the video power, by the
ratio of the bandwidths, and still be appropriate for a usable video signal.

The sync pulse is defined in the standard as occupying 30%. The video
occupies 70%, So the video has more than 5 times the power of the sync
(70/30)^2.

And of course putting the sync pulse at the top of the modulation
down't make it stronger any more than putting it at the bottom makes
it weaker. It is precisely the same amplitude wherever it is.

d

 

[Randy Yates]

[...]
How easy it is for us to do arm-chair engineering here 50+
years after the original development *with the technology of
the day*! This entire "discussion" reeks of revisionism and
has turned just completely silly.

Richard, don't look now, but your old-fartness is showing.
--
% Randy Yates % "She's sweet on Wagner-I think she'd die for Beethoven.
%% Fuquay-Varina, NC % She love the way Puccini lays down a tune, and
%%% 919-577-9882 % Verdi's always creepin' from her room."
%%%% <[email protected]> % "Rockaria", *A New World Record*, ELO
http://www.digitalsignallabs.com

 

[Randy Yates]

The sync pulse is defined in the standard as occupying 30%. The video
occupies 70%, So the video has more than 5 times the power of the sync
(70/30)^2.

My comment was just an observation based on first principles in
response to the "weak video/strong sync" argument. I wasn't trying to
rewrite the spec.

And of course putting the sync pulse at the top of the modulation
down't make it stronger any more than putting it at the bottom makes
it weaker. It is precisely the same amplitude wherever it is.

I don't know what you mean.
--
% Randy Yates % "...the answer lies within your soul
%% Fuquay-Varina, NC % 'cause no one knows which side
%%% 919-577-9882 % the coin will fall."
%%%% <[email protected]> % 'Big Wheels', *Out of the Blue*, ELO
http://www.digitalsignallabs.com

 

[Don Pearce]

The sync pulse is defined in the standard as occupying 30%. The video
occupies 70%, So the video has more than 5 times the power of the sync
(70/30)^2.

Oops, my error, I was forgetting timing. For UK PAL, the sync area
occupies 12uSec, and the video 52uSec. So the sum should be (.7
*52)/(.3*12) or roughly 10:1 video to sync power ratio.

The idea was that sync should start dropping at about the same time as
the picture started becoming unwatchable because of noise. This ratio
achieved that balance. There would be no point in having a picture
that was clear but rolling, or one that was locked solid and looking
like boiling rice.

d

 

[Richard Crowley]

[...]
How easy it is for us to do arm-chair engineering here 50+
years after the original development *with the technology of
the day*! This entire "discussion" reeks of revisionism and
has turned just completely silly.

Richard, don't look now, but your old-fartness is showing.

Forgive me if I would rather take the word of the people
who actually did it over a bunch of prognostigations from
random Usenet users 50 years after the fact.

 

[Don Pearce]

My comment was just an observation based on first principles in
response to the "weak video/strong sync" argument. I wasn't trying to
rewrite the spec.

I know.

I don't know what you mean.

There seems to be an idea floating around that if the sync tips are at
the 100% modulation end rather than the 0% modulation end (ie
inverted, not non-inverted) they are represent a somehow stronger
signal. In AM that just isn't so.

d

 

[Richard Owlett]

At first I wanted to agree with you - it makes sense in one way.

But a sync pulse is really nothing more than the horizontal scan rate,
right? You can build a PLL to pull this way out of the noise by making
a very narrow-band loop filter. The video information is a different
matter since you need a wide bandwidth to utilize it.

So I'd say the sync pulse power can be lower than the video power, by the
ratio of the bandwidths, and still be appropriate for a usable video signal.

Yeah BUT
What techniques and hardware were commonly used/available almost a
century ago. Cost would be important.

 

[Randy Yates]

[...]
How easy it is for us to do arm-chair engineering here 50+
years after the original development *with the technology of
the day*! This entire "discussion" reeks of revisionism and
has turned just completely silly.

Richard, don't look now, but your old-fartness is showing.

Forgive me if I would rather take the word of the people
who actually did it over a bunch of prognostigations from
random Usenet users 50 years after the fact.

I don't recall challenging anything the original designers said or
developed, unless Michael Terrell is one such person.

What does "prognostigation" mean? I can't find it in my dictionary.
--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% <[email protected]> % 'Waterfall', *Face The Music*, ELO
http://www.digitalsignallabs.com

 

[Randy Yates]

Yeah BUT
What techniques and hardware were commonly used/available almost a
century ago. Cost would be important.

That could very well be the case, but then that is a different reason
(for the structure of the sync pulse), isn't it?
--
% Randy Yates % "I met someone who looks alot like you,
%% Fuquay-Varina, NC % she does the things you do,
%%% 919-577-9882 % but she is an IBM."
%%%% <[email protected]> % 'Yours Truly, 2095', *Time*, ELO
http://www.digitalsignallabs.com

 

[Jerry Avins]

This thread continues to make little sense.

The sync tips are NEVER at "0V". If you're looking
at the signal prior to restoration, then the DC reference has
not yet been established, and so the sync tips are wherever
they happen to be. If after DC restoration, it is the
blanking level (not the sync tips OR the black) which is
considered to be the "0V" reference for the rest of the signal.
IF the signal is then to be shipped around at RS-343 levels
(from which most current analog video standards were derived),
then the nominal white is 0.714V above that reference, and the
sync tips are 0.286V below it; if the older RS-170 levels are
used, white is +1.000V, and the sync tips are -0.400V. And
at least in U.S. practice, black is set slightly above blanking
("setup"), about 54 mV in the 1.000 Vp-p RS-343 specs.

You're describing the ideal spec. In practice, the clamping diode is
referenced to ground, but the narrow sync tips don't have as much effect
as the wider blanking porch (except during vertical sync. You can see
this on some sets as a slight black-level shift at the top of the
screen. Overscan often masks that anyway.)

Jerry

 

[Jerry Avins]

The transmission is AM. It must use AGC to get a video signal of the
correct amplitude (forget DC restoration for just one moment). When
sync is at the top, every line has the same amplitude, so the AGC
works. If some indeterminate illuminance level is at the top, the AGC
will force whatever level that may be to peak white by changing the
gain. The system can not work that way.

Sure, keyed AGC depends on sync being where it is. Keyed AGC came into
existence as an improvement after black-and-white sets came into use. It
wasn't the reason for the original decision. On early sets, getting sync
to work was a major issue. A rolling picture was almost emblematic of a
set tuned by a novice viewer. Stability was the dominant consideration
in setting the sync spec.

Jerry

 

[Joerg]

There is a difference between bit error rate and multipath distortion.
Big difference.

With digital, and of course, the FEC that gets included, it is either
ALL there or not displayable. There is NO distortion ever.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^


Now you've got it. I was not talking about distortion. In heavy
multipath your analog picture might look yucky (out here it does) but
it's there. You can still watch the news. When the digital gets hit with
too much multipath the picture ain't there at all. Huge difference.
Grandma Miller and uncle Leroy won't have any tolerance for that.
They'll fly off the handle.

 

[Don Bowey]

There is a difference between bit error rate and multipath distortion.
Big difference.

With digital, and of course, the FEC that gets included, it is either
ALL there or not displayable. There is NO distortion ever.

And what, among other things, do you think causes it to be *not there*?
Magic?