Why is video inverted for transmission?

[Richard Fry]

It has nothing to do with sync pulses. The sync occurs during the
blanking time at the beginning of each picture frame.

________

Note that sync pulses are transmitted between each horizontal LINE
(not frame), and in the vertical interval between interlaced fields.

The peak amplitude of the sync and equalizing pulses is transmitted
with the maximum ERP allowed for the TV station, which makes for more
stable pictures on receivers in weak signal areas.

The power ratio of peak-of-sync to reference black video in normal r-f
transmission is about 1.68 (NTSC). In fact, the peak output power of
an NTSC TV transmitter is determined by measuring the average power
dissipated in a matched dummy load when the transmitter is modulated
by reference black + sync, and multiplying that value by 1.68.

If the peak of sync was transmitted at 100% NEGATIVE amplitude
modulation, as it would be without inverting the polarity of the
composite video waveform from the studio, then the ERP of the TV
station would be zero at the peaks of those pulses, with which TV
receivers could have a problem.

 

[Tam/WB2TT]

Reverse video to give higher transmission power during black was chosen,
because noise is more visible to the eye in the black levels of video.
With the whites, noise is less visible to the eye. Therefore the higher
transmission power levels in black levels allow for less visible noise
due to any RF carrier signal loss.

There is a rather logical reason for this. For maximum stability, you want
the most power to occur during the sync pulses. That means the sync pulses
have to be either blacker than black, or whiter than white. But, you want
the screen to be black during sync pulses/retrace. Remember, we are talking
1940s technology here; and no color.

It has nothing to do with sync pulses. The sync occurs during the
blanking time at the beginning of each picture frame. The total
horizontal blanking is approximately 10.6 to 10.8 usec. The sync pulse
approximately 4.2 to 4.8 usec wide. The colour colour burst pulse sits
on the back-porch of the blanking.

Excellent web page about the structure of a video signal:
http://zone.ni.com/devzone/cda/tut/p/id/4750

Supposedly, the average scene is more light than dark, and transmitting
black actually saves power.
Tam

 

[kevin93]

Why is video inverted for transmission?

There are multiple reasons including lower effect of impulse noise
(black spots vs white spots) but a very important aspect is that
negative modulation was introduced at the same time as FM sound.

It was not easy to achieve the required LO stability to receive the
sound carrier directly at the high carrier frequencies used by the
time colour came into use so most systems used "intercarrier sound"
where the difference between the vision and sound carrier (at 4.5MHz,
5.5MHz or 6MHz depending on system) was picked off after video
demodulation.

If positive video modulation was used there would tend to be bad video
modulation of the sound carrier (the buzz that often could be heard if
the receiver was not tuned in correctly) as the RF power at the sync
tips was only about 3% or so modulation.

Using negative video modulation avoided this problem by ensuring
adequate video RF at all (most!) times to mix well with the sound
carrier.

kevin

 

[Jamie]

There are exceptions. System L, still used in France, for example, has
peak white at max RF power. The analog audio sub-carrier is amplitude
modulated.

There is a digitally modulated sub-carrier for stereo audio (NICAM)

Well, That's france, what else would you expect!

 

[John Monro]

Reverse video to give higher transmission power during black was chosen,
because noise is more visible to the eye in the black levels of video.
With the whites, noise is less visible to the eye. Therefore the higher
transmission power levels in black levels allow for less visible noise
due to any RF carrier signal loss.

It has nothing to do with sync pulses. The sync occurs during the
blanking time at the beginning of each picture frame. The total
horizontal blanking is approximately 10.6 to 10.8 usec. The sync pulse
approximately 4.2 to 4.8 usec wide. The colour colour burst pulse sits
on the back-porch of the blanking.

Excellent web page about the structure of a video signal:
http://zone.ni.com/devzone/cda/tut/p/id/4750

What has not been considered so far in this thread is the TYPE of noise.

1. White noise is roughly symmetrical about zero and, regardless of the
video modulation polarity, will appear as a mixture of white spots and
black spots.

2. Impulse noise (from electrical appliances and so on) produces a video
signal which is asymmetrical, being relatively unlimited in the
direction corresponding to more carrier and of course limited to zero in
the direction corresponding to less carrier. So, impulse noise can
produce either intense white spots or very black spots, depending of the
vide modulation polarity.

Negative video modulation has no effect on perceived white noise, but
does make impulse noise less obtrusive, so was chosen for this reason.

Regards,
John

 

[glen herrmannsfeldt]

Tam/WB2TT wrote:
(snip)

Supposedly, the average scene is more light than dark,
and transmitting black actually saves power.

Maybe even more after gamma correction.

-- glen

 

[Jerry Avins]

I can top that. I helped to maintain a 1500-tube (all but three of which
were dual or triple purpose) analog computer back a little over 50 years
ago. It was part of a Nike Surface-to-air missile system. The standard
first try repair was to kick it in the area where we suspected the
problem lay, which was effective more often than not. One day, though, a
general was making an announced inspection tour; on that morning, of
course, the computer failed. We applied the standard fix, but this time
the fixer kicked too hard and caved in one of the doors. The general
came in, looked around and asked "Did that fix it?" We told that it had,
and he said "good!" and walked out. We hadn't even told him what had
happened. And, incidentally, we called the van it was in the "pizza
oven". You can't believe how much heat 1500 tubes put out unless you've
been there. Viva la Solid State.

There was no heating system in Harvard University's computer building in
the 1950s. Plenty of air conditioning for use in warm weather, but in
winter, they took air in through vents in the roof -- it was cleaner up
there -- and blew it out through gratings in the sidewalk. It was a
welcome relief when walking down the street in sub-zero weather to stand
in the flow of 60-degree air that emerged. The tubes in the computer
were special red (high reliability with red Bakelite bases) mostly 6SN7
dual triodes.

Jerry

 

[Jerry Avins]

Noise in the black portion shows up more than in the white. Black is
transmitted at higher power, more received signal. less noise.

No. Noise is noise and is just as visible whatever the level. What
matters is signal-to-noise ratio, especially for synchronizing. Black
level is naturally used for blanking, and sync is "blacker than black"
so it can't be seen. That part of a line wants the most power.

Jerry

 

[Jerry Avins]

Reverse video to give higher transmission power during black was chosen,
because noise is more visible to the eye in the black levels of video.
With the whites, noise is less visible to the eye. Therefore the higher
transmission power levels in black levels allow for less visible noise
due to any RF carrier signal loss.

It has nothing to do with sync pulses. The sync occurs during the
blanking time at the beginning of each picture frame. The total
horizontal blanking is approximately 10.6 to 10.8 usec. The sync pulse
approximately 4.2 to 4.8 usec wide. The colour colour burst pulse sits
on the back-porch of the blanking.

It makes sense to use black for blanking. If you use whiter than white
for sync, how would you blank that?

Excellent web page about the structure of a video signal:
http://zone.ni.com/devzone/cda/tut/p/id/4750

Jerry

 

[Jerry Avins]

While we're on the subject of TVs, here's a question that had me
going for awhile several years back.

In the old days the television screen, when turned off, was a dark
greenish color. Why is it (or was it), then, that areas of "black" in
a picture look(ed) black and not green?

When your TV is turned off, you can see (but generally ignore) specular
reflections from the picture tube. Those don't get darker when the set
is on, but like the green color, they go completely unnoticed.

Jerry

 

[Bob Myers]

Supposedly, the average scene is more light than dark, and transmitting
black actually saves power.

Not so. The average video/movie scene, ignoring
oddities such as static frames of text as might appear at
the end of a commercial or some such, generally has an
average overall luminance well under 50% of that of a
full white screen. Most often, in fact, you're down
around 25% or under.

Bob M.

 

[Bob Myers]

In the old days the television screen, when turned off, was a dark
greenish color. Why is it (or was it), then, that areas of "black" in
a picture look(ed) black and not green?

What you see as "black" at any given moment really just
means "an area that's a whole lot darker than its surroundings."
With the TV off, you see the screen "as it is," with no bright
light sources in your field of view to make it look darker.
With the TV on, those parts of the screen which aren't making
light are the darkest thing by far in your field of view, and look
"black."

Vision in general doesn't work in terms of absolutes, but
instead deals with differences and change; it's all relative.

By the way, the screen itself (in a color CRT) is still a
greenish-gray, if you could see it directly. In modern
color tubes, though, you're typically looking at the phosphor
screen through a dark glass faceplate, which is made so to
enhance the perceived contrast. (Up really close, the
phosphor screen is stripes or rectangular dots of the red,
green, and blue phosphors - each of which in the "off"
state typically has a pale tinge of that color - with each
individual dot surrounded by a black material, again put
there as a contrast-enhancement measure.)

Bob M.

 

[Don Pearce]

Reverse video to give higher transmission power during black was chosen,
because noise is more visible to the eye in the black levels of video.
With the whites, noise is less visible to the eye. Therefore the higher
transmission power levels in black levels allow for less visible noise
due to any RF carrier signal loss.

So do you think the noise gets smaller during the higher parts of the
signal (the blacks)? Let me assure you it doesn't. The amplitude of
the noise in the black area is identical whether it is placed at the
top of the modulation or the bottom. Noise adds linearly to the
signal, it doesn't multiply. It makes not a jot of difference to the
noise whether black it at the top, or whit is.

The polarity of the signal is all about making every frame identical
in height so that each component of the signal can be identified
unambiguously.

d

 

[Allen]

Jerry Avins wrote:

There was no heating system in Harvard University's computer building in
the 1950s. Plenty of air conditioning for use in warm weather, but in
winter, they took air in through vents in the roof -- it was cleaner up
there -- and blew it out through gratings in the sidewalk. It was a
welcome relief when walking down the street in sub-zero weather to stand
in the flow of 60-degree air that emerged. The tubes in the computer
were special red (high reliability with red Bakelite bases) mostly 6SN7
dual triodes.

Jerry

I don't think that people who haven't been there can comprehend how much
heat a large number of vacuum tubes can generate. The Nike systems had
separate trailer-mounted air conditioners that were ducted to the van
with the computer. Even with four feet of snow on the ground, the
failure of the air conditioner meant failure of the computer.
Incidentally, I'm surprised that the Harvard computer had so many
6SN7s--an octal-base tube. In the Nike computer there were only a few
octals, mostly MILSPEC equivalents of 6L6s; the rest were 7- and 9-pin
mini tubes. So much for the electronic misnamed "good old days".
Allen

 

[Don Bowey]

Jerry Avins wrote:

I don't think that people who haven't been there can comprehend how much
heat a large number of vacuum tubes can generate. The Nike systems had
separate trailer-mounted air conditioners that were ducted to the van
with the computer. Even with four feet of snow on the ground, the
failure of the air conditioner meant failure of the computer.
Incidentally, I'm surprised that the Harvard computer had so many
6SN7s--an octal-base tube.

I'm curious about why you are surprised. The 6SN7 predated the 12AX7. Both
were good tubes. And the octal tubes seated better (more solid) in their
sockets than did the 7 and 9 pin min tubes.

In the Nike computer there were only a few
octals, mostly MILSPEC equivalents of 6L6s; the rest were 7- and 9-pin
mini tubes. So much for the electronic misnamed "good old days".

In what manner were they NOT the good old days? Because you didn't like the
tubes? I think they were were the good old days, and I was there.

 

[Jerry Avins]

So do you think the noise gets smaller during the higher parts of the
signal (the blacks)? Let me assure you it doesn't. The amplitude of
the noise in the black area is identical whether it is placed at the
top of the modulation or the bottom. Noise adds linearly to the
signal, it doesn't multiply. It makes not a jot of difference to the
noise whether black it at the top, or whit is.

The polarity of the signal is all about making every frame identical
in height so that each component of the signal can be identified
unambiguously.

It's easy to see even fairly dim stars against a dark nighttime sky.
Seeing black specks against a bright daytime sky is much harder. The
contrast ratio may be the same, but size matters.

Jerry

 

[Don Pearce]

It's easy to see even fairly dim stars against a dark nighttime sky.
Seeing black specks against a bright daytime sky is much harder. The
contrast ratio may be the same, but size matters.

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.

d

 

[Jerry Avins]

Jerry Avins wrote:

I don't think that people who haven't been there can comprehend how much
heat a large number of vacuum tubes can generate. The Nike systems had
separate trailer-mounted air conditioners that were ducted to the van
with the computer. Even with four feet of snow on the ground, the
failure of the air conditioner meant failure of the computer.
Incidentally, I'm surprised that the Harvard computer had so many
6SN7s--an octal-base tube. In the Nike computer there were only a few
octals, mostly MILSPEC equivalents of 6L6s; the rest were 7- and 9-pin
mini tubes. So much for the electronic misnamed "good old days".

Do you really mean 6L6? That is a beam power tetrode, practically the
same as a 5881, and KT66s could be used for a little more allowed
dissipation. IIRC, the classic Williamson amplifier used 6L6s in the
output stage.

Jerry

 

[Allen]

Jerry Avins wrote:

Do you really mean 6L6? That is a beam power tetrode, practically the
same as a 5881, and KT66s could be used for a little more allowed
dissipation. IIRC, the classic Williamson amplifier used 6L6s in the
output stage.

Jerry

Yes, I meant 6L6. I don't recall what their use was, but they were
definitely, hidden under a MILSPEC number. I was in a strange situation,
incidentally. I was drafted, but our techs had to enlist for three
years, of which they spent 54 weeks in training. After they completed
that period of a year and two weeks, they were specifically NOT
authorized to make a soldered connection, although our TO&E included
soldering irons and solder. I wound up being assigned to the supply van,
which I loved because I was not on the KP or guard rosters. I had been
building electronic equipment off and on for ten years, so if soldering
was needed the techs had a choice: get me to solder, or take the part to
an ordnance depot several miles away and wait while a part-time high
school student fixed it. Ah, the army and its mysterious ways.
Allen

 

[Allen]

I'm curious about why you are surprised. The 6SN7 predated the 12AX7. Both
were good tubes. And the octal tubes seated better (more solid) in their
sockets than did the 7 and 9 pin min tubes.


In what manner were they NOT the good old days? Because you didn't like the
tubes? I think they were were the good old days, and I was there.

Well, for starters, I wouldn't want to have a four-story building
housing a computer that cost perhaps $40,000,000,000 and also paying a
utility bill in the tens of thousands every month to keep it running and
cool, just to do what my 3-year-old Dell will do. Nor do I want to go
back to LPs and 78s for music. Nor do I want to go back to film as my
only choice for photography. Nor do I want everyone who has gall bladder
surgery to have a foot-long scar like mine. Nor do I want every child in
the country to be at risk of having polio (of which I had a mild case
when I was seven years old. Nor do I want to see segregated schools,
water fountains, restrooms, etc. If you want to go back to the "good old
days", please leave me here, in spite of the bad things happening now.
Allen