Forums

Inverted Video

Started by Unknown September 20, 2007
On Mon, 24 Sep 2007 07:10:08 -0400, Randy Yates <yates@ieee.org>
wrote:

>nospam@nospam.com (Don Pearce) writes: > >> On Thu, 20 Sep 2007 14:42:21 +0200, Andre Lodwig <a.lodwig@lrz.tum.de> >> wrote: >> >>>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: >>>> "Simon S Aysdie" wrote >>>>> Why is video inverted for transmission? >>>> >>>> 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 >>>> >> >> 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 > >I cannot read a "ppsx" file. Can you post it in something more >standard, like pdf?
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...
> On Mon, 24 Sep 2007 07:10:08 -0400, Randy Yates <yates@ieee.org> > wrote: > >>nospam@nospam.com (Don Pearce) writes: >> >>> On Thu, 20 Sep 2007 14:42:21 +0200, Andre Lodwig <a.lodwig@lrz.tum.de> >>> wrote: >>> >>>>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: >>>>> "Simon S Aysdie" wrote >>>>>> Why is video inverted for transmission? >>>>> >>>>> 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 >>>>> >>> >>> 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 >> >>I cannot read a "ppsx" file. Can you post it in something more >>standard, like pdf? > > OK. > > http://81.174.169.10/odds/analoguetv.pdf > > d
How is that going to happen. I thought the AGC was keyed, and only worked during the sync pulses. Tam
> > -- > Pearce Consulting > http://www.pearce.uk.com
On Mon, 24 Sep 2007 09:42:47 -0400, "Tam/WB2TT"
<t-tammaru@c0mca$t.net> wrote:

> >"Don Pearce" <nospam@nospam.com> wrote in message >news:46fd9e98.243977843@news.plus.net... >> On Mon, 24 Sep 2007 07:10:08 -0400, Randy Yates <yates@ieee.org> >> wrote: >> >>>nospam@nospam.com (Don Pearce) writes: >>> >>>> On Thu, 20 Sep 2007 14:42:21 +0200, Andre Lodwig <a.lodwig@lrz.tum.de> >>>> wrote: >>>> >>>>>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: >>>>>> "Simon S Aysdie" wrote >>>>>>> Why is video inverted for transmission? >>>>>> >>>>>> 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 >>>>>> >>>> >>>> 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 >>> >>>I cannot read a "ppsx" file. Can you post it in something more >>>standard, like pdf? >> >> OK. >> >> http://81.174.169.10/odds/analoguetv.pdf >> >> d > >How is that going to happen. I thought the AGC was keyed, and only worked >during the sync pulses. >
First you need to know where the sync is. I'm talking here about the 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:
> On Mon, 24 Sep 2007 07:10:08 -0400, Randy Yates <ya...@ieee.org> > wrote:
> http://81.174.169.10/odds/analoguetv.pdf
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:
 > On 24 sep, 13:25, nos...@nospam.com (Don Pearce) wrote:
 >
 > > On Mon, 24 Sep 2007 07:10:08 -0400, Randy Yates <ya...@ieee.org>
 > > wrote:
 > >http://81.174.169.10/odds/analoguetv.pdf
 >
 > 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).

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.com>:

>On Sep 24, 11:28 am, pantel...@yahoo.com wrote: > > On 24 sep, 13:25, nos...@nospam.com (Don Pearce) wrote: > > > > > On Mon, 24 Sep 2007 07:10:08 -0400, Randy Yates <ya...@ieee.org> > > > wrote: > > >http://81.174.169.10/odds/analoguetv.pdf > > > > 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). > >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
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 :-)