Re: some questions relating television

Bob Myers wrote:
<tapwater@xxxxxxxxxxxxxxxxxxx> wrote in message
news:7J9yg.3852$gF6.528@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Couldn't happen to a nicer bunch. Serves them all right. Broadcast
media,
infomercial producers, people who watch infomercials, jerks who
set up
"airwave pollution" with high power spam, they all deserve to get
screwed.

You seem to be missing the point here - the only person that
reallyt "gets screwed" in this scenario is the poor viewer, whose
television suddenly starts emitting a very nasty ~60 Hz buzz
whenever this sort of stuff appears on the screen.

Surely you've seen this happen; at the end of a commercial, or
even during some "local" programming (where those nice
folks at the local station - usually, this would happen at one
of the smaller operations where the engineering staff might not
be paying as close attention to their video levels, AGC, etc. -
slap up a title or some such produced by an in-studio
"character generator" box), you seem some bold text
appear on the screen, and suddenly you're getting this
really loud and annoying buzz in the audio?

What's happening has to do with a potential problem that
turned up when the NTSC color encoding system was added
to the original "black and white" broadcast standard. As
noted earler, the broadcast form of TV video employs
negative modulation - increasing the video level toward
white results in a reduction of carrier amplitude, not an
increase. It was already standard that the maximum "white"
level (100 IRE) would result in a carrier amplitude of 12.5%
(relative to the unmodulated video carrier). But the
color system puts addition components "on top of" the
video signal prior to modulation, and can as defined produce
peak video signal levels slightly in excess of 130 IRE. A
level of about 120 IRE results in complete cancellation of
the carrier, so clearly anything greater is to be avoided.
Levels in excess of 120 would happen with fully-saturated
yellow or cyan colors - fortunately, such colors are very
rare in camera-generated video, but could happen with
"generated" video as might come out of a color character
generator or similar device. If the studio or transmitter
engineer isn't watching for this (or has already taken care
of it by placing limits into the system), such video DOES
cause momentary carrier cancellation and the resulting
buzz at the receiver.

Bob M.

Like Terrell, I too am a broadcast engineer and used to do transmitter
alignments at a CBS affiliate. I ASSURE you, broadcasters do not let
visual carrier current go to 0. That causes out of channel splatter
which gets the attention of the FCC real fast. Michael also points out
that a very likely culprit is AGC setting in the receiver which can
cause carrier clipping in the receiver (made worse by frequency
response errors in the receiver) which is exactly the same as
transmitter overmodulation.. A much less likely issue would be ICPM of
the visual carrier. ICPM is Incidental Carrier Phase Modulation. The PM
of the visual carrier adds to the FM of the aural carrier and can show
up as the field rate (60Hz) buzz. If that were the case, a separate
carrier receiver (Sony VTX-1000) would have no buzz but an intercarrier
receiver (almost all receivers) WOULD have the buzz. If 1 TV has no
problem, the broadcaster is not at fault.

Your description of the color is correct but nobody allows subcarrier
over 110 IRE. The Harris (Gates) transmitter I worked with would not go
below 3% carier regardless of the input signal and would default to 70%
power with no video at all. I'm sure other manufacturers do this as
well. Nobody wants to be on the bad side of the FCC.

Get a DTV receiver and have a feed in your living room as good as I see
in a commercial video duplication house from Digital Betacam (std def
component digital)and Panasonic D-5 (HD). 100% colors are legal in the
digital world. You'Il see color ranges you cannot have with analog
transmission. I promise you won't miss analog.

GG

.

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