Why is video inverted for transmission?

[Richard Fry]

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.

Consider... when black is being transmitted, the r-f power radiated by
a TV station is greater than when white is being transmitted.
Therefore the field strength produced at a receiving antenna is higher
for black than white, and a given value of r-f noise will be a lesser
percentage of the received signal during transmission of black then
white. In other words, black (and sync, which is blacker than black)
will have a higher signal-to-noise ratio.

However noise voltage tends to drive the detected video waveform about
as far in negative polarity as it does positive. Therefore the
average change in light output from the display (CRT, etc) across a
complete cycle of noise energy can be close to zero, at a normal
viewing distance.

This effect is lost, though, for noise on low luminance video, because
the negative-going noise pulses can extend below reference black where
the display cannot fully show them, whereas the opposite noise
polarity will be fully shown at that luminance level.

RF

 

[Don Bowey]

On 9/21/07 12:38 PM, in article [email protected],

(snip)

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

Oh! You poor (deleted).

What has all that BS to do with your saying:

"So much for the electronic misnamed "good old days".

You're a dishonest person.

 

[Michael A. Terrell]

I know how it works, Richard. My point was, why restore the clamp level
or any other level if the contents of the transmission itself ain't
worth watching? It they did more re-runs of Andy Griffith or Bonanza or
whatever, ok, but not with the average programming these days.

If you don't clamp the video, the brightness and contrast will change
as the overall scene changes. It sets the reference for the black level
in the video amplifier. if it drifts up, the screen gets brighter, and
if it drifts down, it gets darker.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

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.

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.



--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

Arny Krueger wrote:

You got to see more of the world than I did. My time was spent keeping
the North Korean air force from destroying Pittsburgh.

Why? ;-)

I spent a year in Alaska as an AFRTS broadcast engineer where I was
told that if the Russians invaded, I was to keep them from taking the
radio & TV stations. I told the idiot that I would hand them the keys
and watch them try to keep the old equipment on the air. We couldn't
get some repair parts in under six months, so what chance did they have
of finding spare parts for obsolete equipment they had never seen?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Al in Dallas]

Yeah but it has been postponed how many times now? I guess this deadline
is more serious though as some spectra have supposedly been auctioned
off. And at least out here no converters in sight. Oh man, I do not want
to be in the shoes of a local politician on Feb-18, 2009.

Somebody will be selling digital tuners with analog outputs so people
can put the box between their antennas and their old TVs.

 

[Michael A. Terrell]

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.

5692

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

Yup. Gain was expensive (now it's essentially free) and DC coupling
was difficult. And RF noise figures were ghastly, especially at UHF.

John

Early UHF TV tuners and outboard converters used a tube local
oscillator and a 1N82 mixer diode that was anything but a low noise
design.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

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.

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


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

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.

The first NTSC TVs used separate audio and video RF and IF systems,
until the Intercarrier design was developed.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

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

5692
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Al in Dallas]

On 9/21/07 12:38 PM, in article [email protected],

(snip)




Oh! You poor (deleted).

What has all that BS to do with your saying:

"So much for the electronic misnamed "good old days".

You're a dishonest person.

Didn't his first sentence answer your question?

 

[JANA]

The early TV sets did not have any anti-glare coatings on the screen. The
natural colour of the non activated phosphors was a type of greenish colour.
Some phosphors were slightly lighter or darker.

When watching the screen under normal operation, your eyes had the illusion
that the screen was black, because the surrounding white emission was very
bright in relation to the dark or non-illuminated parts.

--

JANA
_____

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

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?
--
% Randy Yates % "My Shangri-la has gone away, fading like
%% Fuquay-Varina, NC % the Beatles on 'Hey Jude'"
%%% 919-577-9882 %
%%%% <[email protected]> % 'Shangri-La', *A New World Record*, ELO
http://home.earthlink.net/~yatescr

 

[Michael A. Terrell]

Somebody will be selling digital tuners with analog outputs so people
can put the box between their antennas and their old TVs.

They are supposed to already be availible, but try to buy one.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

Jerry Avins wrote:


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

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


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Randy Yates]

The early TV sets did not have any anti-glare coatings on the screen. The
natural colour of the non activated phosphors was a type of greenish colour.
Some phosphors were slightly lighter or darker.

When watching the screen under normal operation, your eyes had the illusion
that the screen was black, because the surrounding white emission was very
bright in relation to the dark or non-illuminated parts.

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

--Randey

--

JANA
_____




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?
--
% Randy Yates % "My Shangri-la has gone away, fading like
%% Fuquay-Varina, NC % the Beatles on 'Hey Jude'"
%%% 919-577-9882 %
%%%% <[email protected]> % 'Shangri-La', *A New World Record*, ELO
http://home.earthlink.net/~yatescr

--
% Randy Yates % "Watching all the days go by...
%% Fuquay-Varina, NC % Who are you and who am I?"
%%% 919-577-9882 % 'Mission (A World Record)',
%%%% <[email protected]> % *A New World Record*, ELO
http://www.digitalsignallabs.com

 

[JosephKK]

glen herrmannsfeldt [email protected] posted to
sci.electronics.design:

Bob Myers wrote:
(snip)




No, you can't recover RGB without all three.
Yes, Y is a good "black and white" image signal, but
it isn't the correct one. The R, G, and B in the
equations above have been gamma corrected.
If R=G=B then Y is appropriately gamma corrected,
otherwise it isn't the exact gamma corrected Y.
(Hint: (X+Y)**2 is not equal to X**2+Y**2.)

Since RGB has three degrees of freedom, and Y has one
(luminance) that leaves two for chrominance.
That is, chrominance is the amount of information that,
when combined with luminance gives the appropriate color
signal.

(I believe it is usual to use R' G' B' and R G B
gamma corrected and uncorrected signals, though I forget
which one gets the prime and which doesn't.)

-- glen

About the matching of the gamma curves, when i was young and had very
good eyes, and color TV was just becoming popular. There were still
plenty of broadcasts in monochrome and i could watch them on B&W sets
and color sets "side by side" on the showroom floors. There was
scant (if any, i sure did not notice) difference in the viewing
experience (at a reasonable distance from the screen). If there were
obvious/significant differences in the gamma correction they would
have showed up at that time. The changes to better match YUV were
rather minor tweaks, and the monochrome shows that i watch have no
observable color artefacts as a result of the change.

 

[JosephKK]

Joerg [email protected] posted to
sci.electronics.design:

I know how it works, Richard. My point was, why restore the clamp
level or any other level if the contents of the transmission itself
ain't worth watching? It they did more re-runs of Andy Griffith or
Bonanza or whatever, ok, but not with the average programming these
days.

True, but a lot of the best stuff is B&W (notably much of Andy
Griffith, Bonanza, The Outer Limits, Masterpiece Theater, and so on).
Often a lot of the old good stuff is available on cable or satellite
that OTA stations cannot use because the audience is too small.

 

[JosephKK]

Al in Dallas [email protected] posted to sci.electronics.design:

Somebody will be selling digital tuners with analog outputs so
people can put the box between their antennas and their old TVs.

They are already available, just google. Watch the price spike, and
then the sheeple bitch, and then the regulators step in. Time your
investments carefully.

 

[Richard Crowley]

"JANA" wrote ...

The early TV sets did not have any anti-glare coatings on the screen.

And the latest computer screens (including laptops) are switching
back to the very shiny surface apparently because the "non-glare"
treatment also reduces the clarity of the image you are viewing
from the screen.

The natural colour of the non activated phosphors was a type
of greenish colour. Some phosphors were slightly lighter or darker.

Some of that may have been from the thick glass envelope
over the front of the screen. Most glass looks slightly greenish
under conditions where you can see the glass itself vs. whatever
is on the other side.

When watching the screen under normal operation, your eyes had the
illusion
that the screen was black, because the surrounding white emission was
very
bright in relation to the dark or non-illuminated parts.

Indeed, one of the many psychological effects that modern
technology depends on to interface with humans.