Why is video inverted for transmission?

"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.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.

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

> 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 ...
______

Luminance, hue and saturation information are required to define every
color, so it isn't strictly accurate to write that "color information
comes through on a 3.58.. MHz subcarrier." Chrominance information
does, but luminance information is contained in the standard
monochrome video spectrum from about 60 Hz to about 4.2 MHz (NTSC). It
takes both the luminance and the chrominance signals to convey
accurate color.

For example, if the luminance information is removed from a standard
color bar test signal, the color bars will not look the same as before.

"Don Pearce" <nospam@nospam.com> wrote in message
news:46f790a7.202443468@news.plus.net...
> 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.

Not in the case of video, though - the reason that the blacks
are "more sensitive to noise" than the white is actually because
our eyes are more sensitive to small changes in dark regions
than in light regions. As you noted, though, the noise is going to
hit the blacks just as much as the whites within the signal - but
as others noted, it's better that a brief high-amplitude noise spike
make a "black dot" in a white region than the other way around.
The only part of the standard I'm aware of that was specifically
put in to handle the increased sensitivty to black-region noise
was a modification to the assumed standard response ("gamma")
curve.

And, of course, the other (and possibly the primary) reason for
the inverted transmission was to put the most "oomph" behind
the sync pulses - which means that even those in the fringe areas
can get a stable, albeit snowy, picture.

Bob M.

On 9/20/2007, Allen posted this:
> 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

OT (I guess)

My first thought at the start of this thread was "OMG, another radium
thread", but I read a few posts anyway. So:

1. I'm learning something about why black is full signal - actually a
meaningful question after all.

2. I'm enjoying the tales in this part of the thread about pizza ovens
and so forth. I even had a couple of LOLs...

--
Gene E. Bloch (Gino)
letters617blochg3251
(replace the numbers by "at" and "dotcom")

"Richard Fry" wrote ...
>> 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 ...
> ______
>
> Luminance, hue and saturation information are required to define every
> color, so it isn't strictly accurate to write that "color information
> comes through on a 3.58.. MHz subcarrier." Chrominance information
> does, but luminance information is contained in the standard
> monochrome video spectrum from about 60 Hz to about 4.2 MHz (NTSC). It
> takes both the luminance and the chrominance signals to convey
> accurate color.
>
> For example, if the luminance information is removed from a standard
> color bar test signal, the color bars will not look the same as before.

Yes, you are correct, one cannot decode a full NTSC composite
color signal without the "Y" luminance part. OTOH, the backwards-
compatibility feature of NTSC guarantees that the "Y" luminance
part will stand on its own without the "C" chrominance portion.

>
> 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.
>
>
Some distant day in the US is Feb 17, 2009.

Mark

"glen herrmannsfeldt" wrote ...
> Richard Crowley wrote:
>
> (snip)
>
>> 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.
>
> On tape, luminance is usually an FM signal, which could be
> considered to preserve DC. (If you are careful with the
> calibration.)

Right. But even professional equipment relied on downstream
DC restoration and/or clamping rather than absolute accuracy
of the FM calibration.