Why is video inverted for transmission?

[Michael A. Terrell]

Sure, keyed AGC depends on sync being where it is. Keyed AGC came into
existence as an improvement after black-and-white sets came into use. It
wasn't the reason for the original decision. On early sets, getting sync
to work was a major issue. A rolling picture was almost emblematic of a
set tuned by a novice viewer. Stability was the dominant consideration
in setting the sync spec.

Like the 50 year old 17" Philco portable I picked up on Saturday.
Those were never stable, and I doubt this one will be, unless I modify
the sync circuit.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

...

But even "regular glass" appears to have a greenish tint.
Like the mirror in my bathroom. If you look at it from an
acute angle, you can seee the greenish cast from the glass.

True, but if you could look at the edge of a flat CRT faceplate it
would be almost black.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Jerry Avins]

Jerry Avins wrote:
...



That's why the tube databooks listed those pins and N/C. N/C = DO
NOT USE.

NC meant "no connection" and one was advised not to use them because
voltage withstand was not assured. Pin 1 was listed NC on the glass
versions of the 6L6, but it was intentionally connected to the metal
shell (and usually grounded) in the metal version.

Jerry

 

[Joerg]

Its no joke if it was your job to deliver the best video possible.

Ok, sorry, Michael. I just can't find any joy in watching people beat
each other up, the umpteenth rerun of Letterman's show, divorcees
screaming at each other or some never-ending ballgame that pushed away a
nice classic movie. Most annoying these days is the number of times
where a movie in the programming guide ain't showing at all.

Also, OTA transmitters appear to lack the TLC of yesteryear. I can't
count the times when the image froze or the audio went and nobody at the
station seemed to care.

And yes, I've done video projects myself and tried to deliver the best
video signal possible. Designed my own sync separators and so on. But
that was for industrial applications where there are no ladies wrestling
in the mud and stuff.

 

[Jerry Avins]

...

But even "regular glass" appears to have a greenish tint.
Like the mirror in my bathroom. If you look at it from an
acute angle, you can seee the greenish cast from the glass.

Iron impurities are the source of most of the green color you see in
glass, but high-purity glass isn't so expensive that CRTs couldn't be
made of it. I find lead giving a greenish tint counterintuitive. There
is certainly no green in lead "crystal" tableware.

Jerry

 

[Jerry Avins]

Randy Yates wrote:

...

But a sync pulse is really nothing more than the horizontal scan rate,
right?

Horizontal sync occurs at the horizontal scan rate, if that's what you
mean. A pulse can *have* a rate, but it can't *be* one. There are
vertical sync pulses too; those come at the vertical rate. During
vertical retrace, the horizontal oscillator needs to skip or add half a
line.

...

Jerry

 

[Richard Owlett]

You should have seen some of my projects, years ago. I'm still
using one of them: An adjustable battery charger build from a RV power
supply, and a Staco version of a Variac. The output voltage is
adjustable from zero to about 20 volts, and lets you set the charging
current. It is built in the aluminum housing of a WWII aircraft RADAR
receiver.

GADS
I think my Father could top your stories.

He told of making capacitors.
The rule of thumb was "1 micro farad" / "gallon of 'fish tank' "

He also apparently operated a _legal_ land based spark gap transmitter.
[I was once challenged on that statement - Checking with rep of Antique
Wireless Association of Holcomb NY showed claim was _feasible_.]

 

[glen herrmannsfeldt]

Jerry Avins wrote:

(snip)

Iron impurities are the source of most of the green color you see in
glass, but high-purity glass isn't so expensive that CRTs couldn't be
made of it. I find lead giving a greenish tint counterintuitive. There
is certainly no green in lead "crystal" tableware.

Lead gives a yellow tint. I have seen lead-glass bricks used for
radiation shielding that you can see through. They might be more
than 50% lead by weight and only a light yellow color.

There are other requirements for a CRT, though.

-- glen

 

[glen herrmannsfeldt]

Michael A. Terrell wrote:

(snip)

It is built in the aluminum housing of a WWII aircraft RADAR
receiver.

I used to have an oscilloscope built out of a WWII radar
receiver.

-- glen

 

[glen herrmannsfeldt]

Don Pearce wrote:

(snip)

Unfortunately it is all moot because in a couple years we switch off
our nice PAL in favour of digital which is, frankly, shit. The colours
aren't right, there is no resolution to speak of and the sound is so
far out of sync that we are lucky if it accompanies the right
programme. So much for progress.

I don't see why the colors couldn't be right. Resolution is
a complicated question. Also, fast changing scenes may be
too much for the available bit rate.

-- glen

 

[Joel Kolstad]

He also apparently operated a _legal_ land based spark gap transmitter.
[I was once challenged on that statement - Checking with rep of Antique
Wireless Association of Holcomb NY showed claim was _feasible_.]

Absolutely... something a lot of people don't realize is that spark-gap
transmitters did have resonant circuits almost immediately after the first few
were built. Heck, strictly speaking I don't believe there's anything
inherently illegal about a spark-gap transmitter today -- it'd just that you'd
probably have to filter the daylights out of it to meet mordern emission
standards, so it wouldn't exactly be an "authentic" setup.

 

[Joel Kolstad]

I don't see why the colors couldn't be right. Resolution is
a complicated question.

He's probably complaining about how -- somewhat ironically -- as technology
advances and becomes easier to use, those using it tend to be less educated
and therefore more likely to get something wrong (such as colors)! Perhaps
the relevant example here was the "wall of CRTs" seen in big box stores before
LCDs/plasma displays took over -- inevitably they were all cranked up so
bright to "look good" that there was pretty bad geometric distortion,
blooming, and awful color rendition. But the average consumer cared about
little more than brightness, it would seem, and voted with their wallet
accordingly...

 

[Frank ess]

No wonder Thunderbird's spell checker flagged it. I assumed the
word was just too big. Silly me!

Ayuh.

--
Frank ess

Forecasting is difficult.
Particularly about the Future.
—Deepak Gupta

 

[glen herrmannsfeldt]

Randy Yates wrote:
(snip)

But a sync pulse is really nothing more than the horizontal scan rate,
right? You can build a PLL to pull this way out of the noise by making
a very narrow-band loop filter. The video information is a different
matter since you need a wide bandwidth to utilize it.

How many vacuum tubes does it take to build a PLL?

-- glen

 

[glen herrmannsfeldt]

Joerg wrote:
(snip)

Now you've got it. I was not talking about distortion. In heavy
multipath your analog picture might look yucky (out here it does) but
it's there. You can still watch the news. When the digital gets hit with
too much multipath the picture ain't there at all. Huge difference.
Grandma Miller and uncle Leroy won't have any tolerance for that.
They'll fly off the handle.

Isn't multipath usually in big cities, where you probably have
good access to cable?

In far fringe areas where the signal might be low, and where cable
isn't easily available it could be harder. With a big antenna
and a good amplifier it might still work.

-- glen

 

[glen herrmannsfeldt]

Jerry Avins wrote:

(snip)

The TV of a century ago -- Nipkow, 1884 patent -- has absolutely no
influence on today's standard. There was one on-air broadcaster[1] and
one maker of bare chassis and kit receivers[2] (build your own cabinet)
in 1939.

Some years ago I had the book "Television To-day and To-morrow"
from the Caltech library. It was written in 1930. There were
descriptions of spinning disks and neon lamps. No CRTs.

It wasn't useful for questions about the color subcarrier,
the reason I went looking in the first place.

http://clas.caltech.edu/record=b1018287

-- glen

 

[Al in Dallas]

4: Riot in the streets.

I doubt there are enough people still using antennas to populate a
riot.

 

[Al in Dallas]

"Richard Crowley" writes:
[...]
How easy it is for us to do arm-chair engineering here 50+
years after the original development *with the technology of
the day*! This entire "discussion" reeks of revisionism and
has turned just completely silly.
Richard, don't look now, but your old-fartness is showing.
Forgive me if I would rather take the word of the people
who actually did it over a bunch of prognostigations from
random Usenet users 50 years after the fact.
I don't recall challenging anything the original designers said or
developed, unless Michael Terrell is one such person.
What does "prognostigation" mean? I can't find it in my dictionary.

Try "PrognostiCation", or "PrognostiCate". Of course, if you looked up
the original spelling you would have seen this immediately the blank
space that you looked at, so you have probably identified yourself as
a troll.

Allen,

Is it that satisfying to you to sit behind your computer and write
nasty little messages like this to total strangers?

If you would read the adjacent post I made on this very subject
a few minutes ago, you would see my reasoning.

Or lack thereof.

 

[Al in Dallas]

That's unfortunately been the trend in the U.S. as well, at
least as far as digital cable/satellite providers are concerned.
I get horrible compression artifacts on many programs via
the digital satellite service.

My cable provider has claimed that the digital artifacts that I've
seen are due to squirrels chewing on cables and as soon as they get
around to replacing the lines, the artifacts should go away.

 

[Michael A. Terrell]

Ok, sorry, Michael. I just can't find any joy in watching people beat
each other up, the umpteenth rerun of Letterman's show, divorcees
screaming at each other or some never-ending ballgame that pushed away a
nice classic movie. Most annoying these days is the number of times
where a movie in the programming guide ain't showing at all.

I don't watch that crap. Basically, it's local and national news,
and about 10 hours a week of the Sci-Fi channel for a little escapism.
I listen to radio more than I watch TV. Still, after being an engineer
at three TV stations it has become ingrained to insist on quality. If
you have ever seen NTSC video on a $7,000 RGB studio monitor, fed from
a $100,000 studio camera and lens, you would puke at the digital TV
crap. I have seen it in Circuit City and Best Buy. I already have
problems with analog TV, because Brighthouse's digital to analog
converters are always freezing the video with a garbage display, and a
loud roar from the speakers for several seconds at a time.

I was given a two year old 32" Sony HDTV 'Capable' TV a few weeks
ago, and have no idea what I'm going to do with it. I have considered
converting it to a computer monitor.

Also, OTA transmitters appear to lack the TLC of yesteryear. I can't
count the times when the image froze or the audio went and nobody at the
station seemed to care.

You can blame the cheap ass networks who switched to KU band digital
feeds with barely enough bandwidth to supply a fixed image without
problems. After they piggyback all the control and timing data, its no
wonder the system freezes.

And yes, I've done video projects myself and tried to deliver the best
video signal possible. Designed my own sync separators and so on. But
that was for industrial applications where there are no ladies wrestling
in the mud and stuff.

None of the stations I worked at broadcast that crap. One was
military, and the other two were Christian TV stations. In fact, they
didn't even have commercials.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida