In article ,
"Michael A. Terrell" wrote:

isw wrote:

In article ,
"Michael A. Terrell" wrote:

isw wrote:

In article ,
(Samuel M. Goldwasser) wrote:

writes:

On Dec 4, 3:42 pm, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help
fromhttp://ask.metafilter.com/56826/now connects an old DVD player
to
an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out,
and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something
with
audio, and the last is a dragging one that does some w/the
video.
The
switch seems to be for a testpattern with two vertical bars?)

Berend

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that
requires
sync positive. It's not like audio where the absolute phase doesn't
matter. (OK some audio nuts swear they can tell the difference but
most of us can't).

Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.


That was done so that any 'snow' in a weak picture was white, instead
of black dots.

But mostly because sync circuits didn't work very well, and since video
is transmitted as AM (well, the sync frequencies are, anyhow), sync up
gave even a weak signal the best chance of getting a lock.


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. 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.

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.

Isaac