Why is video inverted for transmission?

On 9/20/07 7:29 AM, in article 46f283e5$0$17145$4c368faf@roadrunner.com,
"Allen" <allen@nothere.net> wrote:

> Arny Krueger wrote:
>> "Unclaimed Mysteries"
>> <theletter_k_andthenumeral_4_doh@unclaimedmysteries.net>
>> wrote in message news:13f3v3cdjqesj66@corp.supernews.com
>>> Richard Crowley wrote:
>>>> "Charles" wrote ...
>>>> 
>>>>>> 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.
>>>> 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.

In less than 50 years ignorant people will be commenting on how primitive
things were in the early 2000s.  Better educated people will recognize that
they are fortunate that engineers in early electronics history created the
basis for everything they enjoy.

>> 
>> 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.
>> 
>> 
> 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


Our first test was always to darken the room and look for dead tubes.

"Richard Fry"  wrote ...
>> 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.

Well, the luminance ("Y") is defined by the instantaneous
DC voltage of the signal, but none of the color information
is carried there.  The color information comes through on
a 3.579545MHz subcarrier and the color hue is defined by
the phase of the signal, while the saturation of the color is
defined by the amplitude of that same signal.  A black, white,
or gray area has no associated subcarrier amplitude.

> If the video signal was transmitted using AC coupling,
> then luminance values would be a function of the
> average voltage of the video waveform.

When it comes right down to it, ALL analog video is AC
coupled. Through many stages of electronics, to be sure,
but even if all the electronics were DC coupled, the trip
through the air (or written to and read back from a tape)
is AC coupled.  The sync parts of the signal (particularly
the blanking/ "back porch", etc.) make convienent reference
points to "clamp" the signal and perform "DC restoration".

>
> 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
> 

On Thu, 20 Sep 2007 01:19:35 GMT, Charles <ckraft@SPAMTRAP.west.net>
wrote:

>On Wed, 19 Sep 2007 17:58:23 -0700, "Green Xenon [Radium]"
><glucegen1@excite.com> wrote:
>
>>On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in 
>>http://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8eff770 :
>>
>> > 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?
>
>
>Noise in the black portion shows up more than in the white.  Black is
>transmitted at higher power, more received signal. less noise.

Huh? This is just nonsense. The noise doesn't live at the bottom of
the signal where the white is. The noise accompanies the signal
whatever level it is at. The black will have just as much noise in it
as the white. The reason why small signals appear noisier is that you
have to apply gain to make them bigger - making the noise bigger too.
This is not the case with a TV signal - you don't apply more gain when
the signal drops into the white.

d

-- 
Pearce Consulting
http://www.pearce.uk.com

Arny Krueger wrote:
<snip>
> Once I got *tired* of that, they moved me to Germany, where the radars were 
> still in the open air (sun, sleet, rain, waist-deep snow and subzero temps), 
> on individualized hilltops.  The scenery in Germany included a 270 degree 
> vista of a large valley, complete with farms, little towns, and a castle or 
> two.
> 
> 
You got to see more of the world than I did. My time was spent keeping 
the North Korean air force from destroying Pittsburgh.
Allen

Richard Fry wrote:

>>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
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com

Richard Fry wrote:

>>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
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com

Richard Fry wrote:

>>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
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com

Richard Fry wrote:

>>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
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com

Richard Fry wrote:

>>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
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com

Richard Fry wrote:

>>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
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com