Re: Composite video via RF modulator, negative image?

isw wrote:

In article <493D19C1.71BD227@xxxxxxxxxxxxx>,
"Michael A. Terrell" <mike.terrell@xxxxxxxxxxxxx> wrote:

Really? Sync doesn't work? Then why have we bothered to transmit TV
for over 60 years? In the US, analog TV visual is transmitted as
'Vestigial Sideband', with the sync tips 'Blacker than Black'. That
means it it is at 100% Visual modulation, with full brightness at the
lowest level. US TVs typically used 'keyed AGC', which measures the
received signal levels during the sync pulses. This prevents changes in
average video from affecting the overall system gain. If the visual
modulation is out of spec, the system doesn't work properly.

Think about how long ago all the specs for RS-170 were laid down. *In
the beginning* things were a bit more difficult, and synchronization was
a bit tricky, what with marginally stable multivibrator oscillators and
all.


A TV station that I visited in Fairbanks in the early '70s still used
their vacuum tube RCA sync & chroma generator. The master monitor in
the control room was based on the RCA CTC4 TV chassis, and the GE film
chain was tube, as well. The equipment worked, but was very temperature
sensitive. Bad tubes were a problem, as well. Some stations had over
1000 vacuum tubes in the studio & control room. That was one reason TV
stations ran a test pattern for a half hour or more each morning.


Using the highest-powered part of the signal for sync made things a
bit easier. "Vestigal sideband" was used instead of true single-sideband
because true SSB is more difficult to receive (since it needs a
synchronous carrier). An interesting benefit of VSB is that for
modulation frequencies near the carrier *where the sync signals are* the
signal is standard double-sideband AM, which *doubles the amplitude of
the detected signal* compared to the single-sideband part; this helps
sync performance. Most of the lower sideband is wiped off on the way to
the antenna, and the characteristics of the filter used must be
carefully specified because there is a complimentary filter in every
receiver.


Not to mention the careful alignment of the diplexer to allow the use
of a single antenna.


I was a RF broadcast engineer at three TV stations. I've built CATV
headends, mobile TV production vehicles, and worked with CARS & STL
systems before I became 100% disabled.

In that case, you may have used some gear that I designed, or helped
design, or managed the design of.


I've worked with a Gates 500 W on Ch 8, a 130 KW Comark UHF on Ch 55,
and a 25 KW RCA TTU-25B that I moved & rebuilt. It had been on Ch 55,
and had to be converted for Ch 58. The RCA was built in 1952.

I've designed a few things for broadcast use, as well. We got a
notice from the FAA & FCC about a change in the laws, regarding tower
lighting. We had 72 hours to let them know if we already had a remote
monitoring system in place, or buy one from an approved equipment list.
I designed and built a system that day, and installed it the next
morning. It used a1024 Hz tone on a spare audio channel in the STL from
our old transmitter site, to the new one. A current transformer
monitored current to the tower lights. The tone was on when the lights
were off, and went off when all the lights were on. A tone decoder at
the other end was mounted in the main console, with a small lamp beside
the master monitor. It was just annoying enough for the operator to
notice, and log the start & stop times each day. I designed and built a
lot of interfaces to connect incompatible hardware, too. The most fun
was transmitting a color station ID at a B&W AFRTS station with no color
equipment, at all. :)


--
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
.

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