Why is video inverted for transmission?

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?

On Sep 19, 5:58 pm, "Green Xenon [Radium]" <gluceg...@excite.com>
wrote:
> On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote inhttp://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8e...:
>
> > 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?

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

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.

John

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?


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

"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.

On Sep 19, 8:58 pm, "Green Xenon [Radium]" <gluceg...@excite.com>
wrote:
> On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote inhttp://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8e...:
>
> > 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?


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

On Sep 19, 7:58 pm, "Green Xenon [Radium]" <gluceg...@excite.com>
wrote:
> On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote inhttp://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8e...:
>
> > 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?

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)

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.
>

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

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.
>


Some of us were around back then. <:-)

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