On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in
http://groups.google.com/group/sci.e...89f0d1e8eff770 :

Why is video inverted for transmission?

On Sep 19, 5:58 pm, "Green Xenon [Radium]" <gluceg...@excite.com>
wrote:
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:

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:


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 ...

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:

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:
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:
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

In article <YcudnSn31vrJbGzbnZ2dnUVZ_sDinZ2d@comcast.com>,
glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:

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

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

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


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

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

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

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:

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
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
Especially if your name was Muntz. ;-)

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

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:

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:

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:
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
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.