"Simon S Aysdie" wrote
So that the peaks of sync pulses are transmitted 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

And, black dots due to noise spikes in a bright picture annoy much less
than white spots in a night-scene.

Rich.Fry1@gmail.com wrote:

On Thu, 20 Sep 2007 05:31:20 -0700, Rich.Fry1@gmail.com wrote:

...and the mean power output of the TX is lower

--
- René

On Sep 20, 7:19 am, René <rjz~nosp...@xs4all.nl> wrote:
So, the transmitter is max power at black but the CRT receiver is max
power at white. At some number of viewers the power company should see
a constant load. Right?

GG

<stratus46@yahoo.com> wrote in message
news:1190340231.709119.264430@y27g2000pre.googlegr oups.com...
On Sep 20, 7:19 am, René <rjz~nosp...@xs4all.nl> wrote:
So, the transmitter is max power at black but the CRT receiver is max
power at white. At some number of viewers the power company should see
a constant load. Right?

Riiiiiight.

But then consider that LCDs use maximum power when displaying
all black (because they have to turn on ALL the pixels to block the
backlight.

<stratus46@yahoo.com> wrote in message
news:1190340231.709119.264430@y27g2000pre.googlegr oups.com...
On Sep 20, 7:19 am, René <rjz~nosp...@xs4all.nl> wrote:
So, the transmitter is max power at black but the CRT receiver is max
power at white. At some number of viewers the power company should see
a constant load. Right?

GG

Good point. I found that the power drawn from some CRT based sets can double
from full black to full white.

When the standards were developed many decades ago, I doubt power usage at
the receiver end was considered. Noise and picture stability were more
important. Transmitter power was probably a consideration as well.
John

In article <5li9sfF83c16U1@mid.individual.net>, rcrowley@xp7rt.net
says...
<snip>

No, they're just twisted a different direction; same power.

--
Keith

On Thu, 20 Sep 2007 14:42:21 +0200, Andre Lodwig <a.lodwig@lrz.tum.de>
wrote:

I've written a very short piece about analogue ( VSB AM) TV and why it
is broadcast with sync upwards - ie inverted. It has nothing to do
with noise performance but is entirely a system matter. TV will not
work non-inverted.

http://81.174.169.10/odds/analoguetv.ppsx

d

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

nospam@nospam.com (Don Pearce) writes:

I cannot read a "ppsx" file. Can you post it in something more
standard, like pdf?
--
% Randy Yates % "The dreamer, the unwoken fool -
%% Fuquay-Varina, NC % in dreams, no pain will kiss the brow..."
%%% 919-577-9882 %
%%%% <yates@ieee.org> % 'Eldorado Overture', *Eldorado*, ELO
http://www.digitalsignallabs.com

On Mon, 24 Sep 2007 07:10:08 -0400, Randy Yates <yates@ieee.org>
wrote:

OK.

http://81.174.169.10/odds/analoguetv.pdf

d

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

"Don Pearce" <nospam@nospam.com> wrote in message
news:46fd9e98.243977843@news.plus.net...
How is that going to happen. I thought the AGC was keyed, and only worked
during the sync pulses.

Tam

On Mon, 24 Sep 2007 09:42:47 -0400, "Tam/WB2TT"
<t-tammaru@c0mca$t.net> wrote:

AGC which operates on the RF stages to generate the first baseband
signal. All you need is peak detection, and a syncs-up signal will
give you exactly the baseband signal you want.

Keying the AGC is a refinement (which incidentally couldn't possibly
work if the signal were non-inveretd, because there would be no signal
to key on during sync).

But keying is mainly used for determining an exact black level after
sync separation. A specific location in the back porch, clear of the
colour burst, is chosen as the measuring point, and that level is
clamped to zero.

d

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

On 24 sep, 13:25, nos...@nospam.com (Don Pearce) wrote:
Although the reasoning in that paper is correct,
the conclusion that positive modulation cannot be used is not.

In the video as it was once (a long time ago we are all digital here
now)
in the vertical interval _ALWAYS_ a transmission test signal (steps
0-100%)
was added in one line.
This *would* have allowed a peak detector scheme to work 100% correct,
if the sync pulses were inverted.
The real reason was noise = black, and max sync (sync before picture).

On Sep 24, 11:28 am, pantel...@yahoo.com wrote:

There are as many elements of dumb as elements of truth in that little
paper. The implication is that the carrier power would go to 0 during
sync. This would cause out of band splatter and would not be allowed
by the FCC. Sync tip would have been assigned some minimum power level
like 10% carrier (just a guess) and the AGC would then reference the
back porch which would be around 30% power. It could work but I think
inverted is better.

As far as vertical interval test signals go, they did not exist in the
early days so the system had to be functional without them. The early
RCA sync generator was a full 6 foot rack of tube to make monochrome
sync. The early receivers didn't have the capability to deal with the
test signals. They barely worked as it was.

GG

On a sunny day (Mon, 24 Sep 2007 13:12:59 -0700) it happened
stratus46@yahoo.com wrote in
<1190664779.043820.66420@57g2000hsv.googlegroups.c om>:

You are absolutely right, I even worked in the studio maintaining
the big tube sync 'factory' we called it.
The vertical test signal was inserted at the transmitter site IIRC,
it has been there since the late sixties here.
I used it at times to align stuff.
Old receivers were not that bad:-)
My first one had round CRT, PL81, PY81, 70 degrees defection IIRC.
Once you tuned the H and V sync it worked perfectly :-)