Why is video inverted for transmission?

[Jukka Aho]

It is indeed. Too bad it's going to be moot in 17 months when all the
broadcasters go over to digital only.

Even if broadcast signals go digital, baseband composite video signals
(which is what that page is all about) aren't going anywhere anytime
soon at people's homes.

 

[Don Pearce]

Even if broadcast signals go digital, baseband composite video signals
(which is what that page is all about) aren't going anywhere anytime
soon at people's homes.

But they will be baseband, not amplitude modulated.

d

 

[Jukka Aho]

But they will be baseband, not amplitude modulated.

Exactly. The web page mentioned above doesn't mention modulation at all
(except when discussing the color modulation), so it isn't "going to be
moot in 17 months": on the contrary, baseband composite video signals of
that description will continue their existence at people's homes between
various interconnected devices for years to come. Which was my point.

 

[Don Pearce]

Exactly. The web page mentioned above doesn't mention modulation at all
(except when discussing the color modulation), so it isn't "going to be
moot in 17 months": on the contrary, baseband composite video signals of
that description will continue their existence at people's homes between
various interconnected devices for years to come. Which was my point.

What will be moot in 17 months is the original question about
inversion for transmission. Video baseband is one of the most standard
signals in existence. It is 1V pk-pk with the sync bottoms at 0V.
Every other feature of the signal is defined from that point. I can't
see that vanishing in the short term.

The signal is inverted for transmission so that the sync tips sit at
1V - that way it doesn't matter whether the scene is light or dark,
everything stays in the same place, and the relative positions of the
porches and black level aren't shifted by AGC. This thread has been to
date a heap of the most arrant nonsense about noise and whatever.

d

 

[Joerg]

There is a difference between asking someone that works at WalMart what
they carry, and actually knowing about it yourself.

http://www.walmart.com/catalog/product.do?product_id=5893822

http://www.walmart.com/catalog/product.do?product_id=4995009

Not everything is available AT the store either.

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.

 

[glen herrmannsfeldt]

Jukka Aho wrote:

(snip)

Exactly. The web page mentioned above doesn't mention modulation at all
(except when discussing the color modulation), so it isn't "going to be
moot in 17 months": on the contrary, baseband composite video signals of
that description will continue their existence at people's homes between
various interconnected devices for years to come. Which was my point.

Most likely many of those devices will still have RF modulators
built in. I haven't used the RF output on any device in my home
systems for over 20 years now, but many people still do.

-- glen

 

[Jerry Avins]

What has that to do with anything? Look, it doesn't matter if the
black is at the top or the bottom of the signal, it will have exactly
the same amount of noise on it. Likewise the white.

The same amount of noise, but not the same visibility. Put a few small
pinpricks in a piece of opaque foil and they show up very well when the
foil is held up to light. A few tiny opaque spots on clear film will
disappear under the same viewing conditions even though their contrast
is the same.

Jerry

 

[Joel Kolstad]

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.

Give him... mmm... about a year and a bit: By Christmas session next year, I'm
willing to bet there's going to be a large pile of ATSC->NTSC converters
sitting in a noticeable pile, with price tags $99 or below.

 

[Jerry Avins]

Michael A. Terrell wrote:

...

You've never been around high power transmitter tubes that are water
cooled. One of them had a pair of water cooled power tetrodes with
pairs of 1.5 VAC, 1000 A filaments. 6 KW of heat from the filaments,
plus the plate dissipation. It had a water chillier big enough for a
decent sized office building. Then there was The Comark/Thales 195 KW
(130 KW + 65 KW) UHF transmitter in Orange City, FLorida with multiple
chillers.

I once built a transmitter with a pair of water-cooled 6L6s. I cut away
the outer metal sleeve, exposing a smaller glass envelope than that of a
a glass 6L6. The tubes were the only components mounted on the outside
of the chassis. The plan had been to place the chassis upside down over
a bucket of light oil, but we never got the oil, so we used water
instead. The "bucket" was actually a gallon jug with the top cut off. We
could see the plates glow through the glass. We ran half a KW, replacing
the water as it boiled off.

 

[Don Pearce]

The same amount of noise, but not the same visibility. Put a few small
pinpricks in a piece of opaque foil and they show up very well when the
foil is held up to light. A few tiny opaque spots on clear film will
disappear under the same viewing conditions even though their contrast
is the same.

Jerry

Whether at the top or near the bottom, black will get white spots, and
white will get black spots. There is no difference in the noise.

The difference is that when you place the sync pulses at the top,
every line is the same height regardless of brightness, and the AGC
can return a 1V pk-pk signal. If you put the modulation at the top,
then during dim scenes, the AGC will attempt to turn the brightness up
and consequently raise the sync and black levels.

TV doesn't work with picture mod at the top. Noise doesn't come into
it.

d

 

[Joerg]

Give him... mmm... about a year and a bit: By Christmas session next year, I'm
willing to bet there's going to be a large pile of ATSC->NTSC converters
sitting in a noticeable pile, with price tags $99 or below.

And then the moment of truth cometh. I am a bit concerned that the
chosen system could waffle under really strong multipath conditions like
where we live. Right now we don't know because none of the neighbors has
digital OTA reception. If it doesn't work and people find themselves cut
off from watching the news the situation at the phone desk of the local
politicians may become ugly.

 

[Jerry Avins]

5692

Yes, that's the number. Speaking of numbers, I have a WW I Signal Corps
VT 1. My father bought it surplus around 1920 for $10, half a month's
rent. I made my first regenerative receiver with it. He used is as a
diode to replace a crystal.

Jerry

 

[Jerry Avins]

The 6l6 was a metal, octal based tube. The glass versions were the
6L6G, 6L6GT, 6L6GTA, 6L6GTB series.

I worked on an old experimental 441-line TV transmitter that used metal
tubes. The designer -- damn him -- needed one more tie point than the
birdy boards had, and used socket pins connected to the shells of
several of the tubes. It was grounded for RF, but put 400V on what one
ordinarily would expect to be grounded.

Jerry

 

[Jerry Avins]

Yes, that's the basic idea. That translates to our eyes not having a
high dynamic range, I think.

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

 

[Jerry Avins]

What will be moot in 17 months is the original question about
inversion for transmission. Video baseband is one of the most standard
signals in existence. It is 1V pk-pk with the sync bottoms at 0V.
Every other feature of the signal is defined from that point. I can't
see that vanishing in the short term.

The signal is inverted for transmission so that the sync tips sit at
1V - that way it doesn't matter whether the scene is light or dark,
everything stays in the same place, and the relative positions of the
porches and black level aren't shifted by AGC. This thread has been to
date a heap of the most arrant nonsense about noise and whatever.

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

Jerry

 

[ChairmanOfTheBored]

And then the moment of truth cometh. I am a bit concerned that the
chosen system could waffle under really strong multipath conditions like
where we live.

Digital transmission do NOT suffer, in ANY WAY, SHAPE, OR FORM, from
multipath distortions!

 

[Jerry Avins]

Whether at the top or near the bottom, black will get white spots, and
white will get black spots. There is no difference in the noise.

Correct. There is a difference in how visible the noise is. Try the
experiment I outlines above. All it takes is a pane of glass and a
fine-tip black pen for one sample ans a piece of broiler foil and a
needle for the other. When the size of the "defect" is less than
1/1000th of the viewing distance, the difference is remarkable.

The difference is that when you place the sync pulses at the top,
every line is the same height regardless of brightness, and the AGC
can return a 1V pk-pk signal. If you put the modulation at the top,
then during dim scenes, the AGC will attempt to turn the brightness up
and consequently raise the sync and black levels.

The possibility of DC restoration doesn't depend on the absolute value
of the reference level, although some levels make for simpler circuitry
than others.

TV doesn't work with picture mod at the top. Noise doesn't come into
it.

At the top of what? I can't decode that sentence. DC restoration is done
at baseband, where the sync pulses are at 0V.

Jerry

 

[Don Bowey]

Digital transmission do NOT suffer, in ANY WAY, SHAPE, OR FORM, from
multipath distortions!

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

 

[Bob Myers]

At the top of what? I can't decode that sentence. DC restoration is done
at baseband, where the sync pulses are at 0V.

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.

Bob M.

 

[Don Pearce]

The possibility of DC restoration doesn't depend on the absolute value
of the reference level, although some levels make for simpler circuitry
than others.

Sure you can restore the DC. but that is only half of the issue. If
AGC has caused the signal to be increased in height when it is dark,
the chrominance signal will also get bigger, causing an increase in
saturation. So dark scenes will be highly coloured.

At the top of what? I can't decode that sentence. DC restoration is done
at baseband, where the sync pulses are at 0V.

At the top of the amplitude modulation. It isn't enough simply to be
able to restore sync tips to 0V, the signal must also be of the
correct amplitude. You only have that when the amplitude of the AM is
determined by sync tips, not some random illumination level.

d