[mythtv-users] Video Noise and VHS-to-DVD conversion (long - only read if interested)
gull at gull.us
Sun Jan 7 20:45:33 UTC 2007
Brian Wood wrote:
> The best connectors for video are actually the PL-259 "UHF"
> connectors (don't know why they call them that, they are not rated
> for actual UHF frequencies (defined as 300-3000 Mhz.)). They are
> hardly used at all anymore though, because they are a pain to install
> and use, and take up a lot of space.
They're called "UHF" connectors because they were invented in the 1930s,
when "ultra-high frequencies" were what we call VHF now. They're pretty
much obsolete now, except for shortwave-band stuff like amateur radio
and CB. They don't work well above 300 MHz and they don't present a
constant impedance, like more modern connector types.
Fortunately, baseband composite video (like what comes out of your VCR's
"video out" jack) only has frequency components up to 10 MHz, so none of
that really matters. As you point out, professional gear generally uses
BNC connectors. Besides having better electrical characteristics than
RCA connectors, they lock in place, which makes them a lot more reliable.
By the way, cabling problems tend to show up in three ways:
1. Ghosting (caused by improper impedance, which lets some of the signal
reflect off the far end of the cable)
2. Hum bars (caused by poor shielding)
3. Poor contrast (caused by excessive losses)
If you make your own video cables, try not to use regular cable TV coax.
It's meant for high frequencies and the shielding is inadequate for
baseband video. You want cable with at least 95% braid coverage. This
is available from places that cater to security camera installers. It's
more expensive, but will pick up far less hum on long runs. You
probably won't see a visible difference on a cable that's only a few
feet long, but in my job I regularly deal with runs of over 100 feet,
and this can make the difference between video that's usable and video
that won't even sync properly.
> You might have to violate some electric codes by lifting the third-
> wire ground from one or more power cables to eliminate AC hum. The
> best solution is an isolation transformer but few consumers have one
> of those. "Humbuckers" and optical isolation units can break up
> ground loops, but again not many consumers have access to those.
If the incoming cable TV line is the culprit -- and it often is, since
they're usually grounded separately from the power circuit -- you can
sometimes isolate it by wiring two 75-to-300 ohm baluns together to make
a 75 ohm isolation transformer. You *will* lose a fair bit of signal
across an arrangement like that, though, so it depends on how much
signal strength you have to spare. You can't isolate baseband video
this way because it has DC components which get lost when transformers
Even when all the external noise issues are cleaned up, video from VCRs
tends to capture to digital poorly because it's not very stable. Due to
tape stretch and other mechanical issues there tends to be a lot of sync
jitter which drives some capture cards up the wall. (The professional
solution to this is a time-base corrector, but as you would say, "not
many consumers have access to those.") Slow-speed VHS recordings are
also full of random color noise, which really messes with video
compression. With a variable-bitrate codec, I've seen a capture from
videotape take up *ten times* the space of "clean" video from a CCTV camera.
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