Why is video inverted for transmission?

On Wed, 19 Sep 2007 21:08:28 -0800, glen herrmannsfeldt
<gah@ugcs.caltech.edu> wrote:

>John Larkin wrote:
>
>(someone wrote)
>>>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.
>
>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

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

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

John Larkin 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.e...89f0d1e8eff770 :
>>
>>
>>>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?
>
>
> 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.

--
Regards, Joerg

http://www.analogconsultants.com

"Green Xenon [Radium]" <glucegen1@excite.com> wrote in
message news:46f1c33d$0$32547$4c368faf@roadrunner.com
> On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in
> http://groups.google.com/group/sci.e...89f0d1e8eff770
> :
>> 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?

White noise is less visible than black noise.

"Unclaimed Mysteries"
<theletter_k_andthenumeral_4_doh@unclaimedmysterie s.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.

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.

> 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

On Wed, 19 Sep 2007 21:08:28 -0800, glen herrmannsfeldt
<gah@ugcs.caltech.edu> wrote:

>John Larkin wrote:
>
>(someone wrote)
>>>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.
>
>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

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

John

On Wed, 19 Sep 2007 19:04:31 -0700, "Richard Crowley"
<rcrowley@xp7rt.net> 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.
>

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

John

Arny Krueger wrote:
> "Unclaimed Mysteries"
> <theletter_k_andthenumeral_4_doh@unclaimedmysterie s.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.
>
> 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

"Allen" <allen@nothere.net> wrote in message
news:46f283e5$0$17145$4c368faf@roadrunner.com
> Arny Krueger wrote:
>> "Unclaimed Mysteries"
>> <theletter_k_andthenumeral_4_doh@unclaimedmysterie s.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.
>>
>> 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. ]

My only comeback is that I had two of these 400 tube monsters and another
smaller one with only about 200 tubes. Even so my total of 1,000 tubes falls
short of 1,500. BTW, this was the Hawk system.

Do I get bonus points if my monsters were in the open air (sun, rain and
hurricane-force winds), on the top of 60-80 foot towers in the Everglades,
complete with alligator-infested scenery that was mostly under water?

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.

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@unclaimedmysterie s.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.