Why is video inverted for transmission?

[Green Xenon [Radium]]

Prolonged blacks can damage television transmitters, however (video
is inverted for transmission, so black requires full power from the
transmitter).

Why is video inverted for transmission?

 

[Simon S Aysdie]

Why is video inverted for transmission?

Blackness is darkness. Sith Lords were and are in control of the FCC.

"All slaves have a transmitter placed somewhere in their body." --
Shmi Skywalker

 

[John Larkin]

Why is video inverted for transmission?

So black, which is the sync pulse, which is the retrace blanking, gets
lots of transmit power, so things tend to stay in sync. Max-black also
make dc restoration work nicely.

John

 

[Charles]

Why is video inverted for transmission?

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

 

[Jamie]

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

Really..
I always thought it was the perfect solution since the Video is
AM and thus, the signal is varying in level. Kind of hard to
regulate the video information signal for proper balance.
So thus, the use of Sync signals being at their MAX amplitude
which are constant in level allows for the receiver to adjust it's
AGC at that point and hold the receive gain steady while video is
being detected between the sync pulses which is a varying level of
DC after the detector.
It's obvious that varying levels of DC isn't going to work well
in generating a stable signal gain in the receiver.

In most receivers, the signal is gated(disconnected) from the AGC and
sync detectors on each scan line once lock is established. This is done
to reduce the effects of signal gain and Sync drift, from the scan
line information.
There has been lots of advancement over the years in making receivers
better in this aspect.

 

[Richard Crowley]

...

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

Yes, that is my recollection of the original explanation by the
engineers & scientists who devised the system.

We tend to forget how primitive things were back then (>50
years ago) by modern standards. I can't believe what they
did with vacuum tubes ("valves") in those days.

 

[robert bristow-johnson]

Why is video inverted for transmission?

when i took a course on TV engineering 3 decades ago, it was explained
to me like this:

while the sound for TV is FM, the video is AM (actually VSB-AM). that
means the amplitude of the envelope of the video portion of the RF
signal is proportional to the (inverted) video signal.

now, when a spike of noise (like lightning or something) is added to
an RF signal at some totally random time, it is added to the RF at a
totally random phase. probabilistically, that spike, being
uncorrelated to the RF, will increase the short-time energy of the RF
and thus increases the amplitude envelope of the RF and then the
inverted video signal. an instantaneous spike in time will much more
likely cause the envelope to INcrease rather than cause it to DEcrease
(but it *could* cause it to decrease if the polarity of the spike
where opposite of the instantaneous polarity of the RF and of the
correct relative amplitude).

so then ask yourself: if you were to get a spike of noise added to the
video signal (which increases the envelope amplitude) which would you
rather see, a spurious bright dot or a spurious dark dot?

that is the reason i was taught for that engineering decision made 7
decades ago.

r b-j

 

[Frank Raffaeli]

Why is video inverted for transmission?

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)

 

[Unclaimed Mysteries]

...


Yes, that is my recollection of the original explanation by the
engineers & scientists who devised the system.

We tend to forget how primitive things were back then (>50
years ago) by modern standards. I can't believe what they
did with vacuum tubes ("valves") in those days.

A LOT of maintenance.


--
It Came From Corry Lee Smith's Unclaimed Mysteries.
http://www.unclaimedmysteries.net

In a time of deception telling the truth is a revolutionary act. -
George Orwell

 

[Kevin McMurtrie]

John Larkin wrote:

(someone wrote)


That is the reason I always heard.

In addition, sync pulses are blacker than black to make sure that
they are not visible on retrace.


Why does it make it easier?

-- glen
||
---||----+-----+--->

|| | |
--- /
/ \ \
| /
| \
| |
----------+-----+--- GND

To be really fancy, clamp to ground every time the sync fires.

 

[Charles]

...


Yes, that is my recollection of the original explanation by the
engineers & scientists who devised the system.

We tend to forget how primitive things were back then (>50
years ago) by modern standards. I can't believe what they
did with vacuum tubes ("valves") in those days.

Some of us were around back then. <

 

[glen herrmannsfeldt]

John Larkin wrote:

(someone wrote)

So black, which is the sync pulse, which is the retrace blanking, gets
lots of transmit power, so things tend to stay in sync.

That is the reason I always heard.

In addition, sync pulses are blacker than black to make sure that
they are not visible on retrace.

Max-black also make dc restoration work nicely.

Why does it make it easier?

-- glen

 

[Don Pearce]

John Larkin wrote:

(someone wrote)


That is the reason I always heard.

In addition, sync pulses are blacker than black to make sure that
they are not visible on retrace.


Why does it make it easier?

-- glen

With the signal inverted, the tips of the sync pulses are the biggest
part of the signal. They are also always exactly the same height,
regardless of whether the scene is bright or dark. This means you can
always rely on the black level being 2/3 (or whatever, I forget) of
the way up the signal. Anything above that is sync, anything below is
picture. If the signal were broadcast "right way up", with syncs at
the bottom, you would recover a signal which varied in amplitude
depending on the brightness of the scene, and it would be impossible
to determine the exact location of either black or the sync front and
back porches.

d

 

[Joerg]

So black, which is the sync pulse, which is the retrace blanking, gets
lots of transmit power, so things tend to stay in sync. Max-black also
make dc restoration work nicely.

Other than brief portions of the evening news the question arises:
What's the whole point in restoration these days? If OTA-TV really goes
digital some distant day we might not even bother buying a new set.

 

[Arny Krueger]

Why is video inverted for transmission?

White noise is less visible than black noise.

 

[Arny Krueger]

"Unclaimed Mysteries"

Especially if your name was Muntz. ;-)

A LOT of maintenance.

Agreed. I once maintained equipment with about 400 "high reliability" tubes
in it. MTBF: less than a day.

 

[Richard Fry]

Other than brief portions of the evening news the question arises:

What's the whole point in restoration these days? If OTA-TV really goes
digital some distant day we might not even bother buying a new set.

_________

Accurate colour reproduction requires accurate transmission of the
luminance (brightness) value of each colour, which is set by a
specific, DC-coupled voltage. If the video signal was transmitted
using AC coupling, then luminance values would be a function of the
average voltage of the video waveform.

For that matter accurate monochome reproduction also requires DC
coupling, but it is not as objectionable if not used (cheap TV set,
etc).

Also - the purpose of inverting video for transmission is to transmit
the peaks of sync pulses at +100% modulation, which allows TV
receivers to show the most stable picture in the presence of noise
(eg, fringes of the coverage area of the TV station).

RF
RCA Broadcast Field Engineer, retired

 

[John Larkin]

John Larkin wrote:

(someone wrote)


That is the reason I always heard.

In addition, sync pulses are blacker than black to make sure that
they are not visible on retrace.


Why does it make it easier?

-- glen

The thing you clamp to is not in the noise floor.

John

 

[John Larkin]

...


Yes, that is my recollection of the original explanation by the
engineers & scientists who devised the system.

We tend to forget how primitive things were back then (>50
years ago) by modern standards. I can't believe what they
did with vacuum tubes ("valves") in those days.

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

John

 

[Allen]

"Unclaimed Mysteries"


Especially if your name was Muntz. ;-)


Agreed. I once maintained equipment with about 400 "high reliability" tubes
in it. MTBF: less than a day.

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.
Allen