ghosts on digital OTA TV

On Jan 20, 2:31 am, "N_Cook" <dive...@tcp.co.uk> wrote:

So what you were seeing were fringes introduced in overdriven RF
stages or
harmonics/sub-harmonics/beats within the IFs or combinations
between

I would rather have ghosts instead of  "no signal" blank screen and
sound in
bad reception environment but unfortunately not an option these
days

Not how it works at all Nigel. Anything you're describing would
prevent proper demodulation of the data. With enough errors it would
fall off the 'cliff' and give no useful data. I don't doubt he saw
'ghosts' but they weren't RF multipath problems. I've been seeing a
bit more 'enhancement' artefacts on some HD signals which makes me a
little sad to see a good system get mis-used. OR, it may have been SD
material again 'enhanced' to cause the cartoon-like outlines. I have
seen some commercials that clearly (well, to me) came from a composite
source (likely a D2 machine) with decoding artefacts. Fortunately THAT
variety is fading away.

G˛

 

[whit3rd]

On Jan 19, 10:35 pm, mm <NOPSAMmm2...@bigfoot.com> wrote:

Before the advent of mandatory digital over-the-air TV, it was said
that the picture would be near perfect, and that there woudl be no
ghosts.   Tonight I saw one, for a full half hour if the scene had the
right kind of lines, including vertical lines.

Digital broadcast TV doesn't support ghosts from transmission
artifacts. The link from DTV adapter to analog TV, or the
video amplifiers inside a vacuum-tube CRT television, can
support some kinds of ghost/ringing that might be perceptible.

Could the ghost be in the coaxial connection between my TV tuner (a
DVDR) and the tv.  There is 15 feet of co=ax, an RF amp/splitter, 8
more feet of co-ax, a splitter, another RF amp, another splitter, 30
feet of co-ax, then another splitter, then 10 more feet of co-ax.

But the important question is, is this connection from tuner
an analog NTSC (USA old TV standard) connection, or a digital ATSC
connection? Does your TV get this signal over channel 3, or is it
a digital channel, like 7.3, that it tunes? I presume it's
analog (i.e. your DVDR is converting ATSC input to NTSC output).

If it's really a poorly-terminated bunch of coaxial wiring, or even a
loose
connection somewhere in the coaxial knotwork, your sixty-feet
of wire can cause a resonance with circa 120 ns of delay (that's
a signal reflection doing the sixty feet of travel in round-trip
style).
Full screen-width, NTSC, is about 55 uS, so 120 ns is a couple of
percent
of the screen size. Whether that's consistent with '1/8 inch', I
can't say.

Reflections in wiring are caused by
(1) stub transmission lines - like splitters with long cables
connected
that DO NOT have a receiver attached. You can fix that by
disconnecting
long cables, or by putting a 75 ohm terminator on the unused end.
This applies to cables that are NOT part of the connection to your
target
television, as well as to the connecting channel; the echo from the
cable can be related to wiring to the spare bedroom. It all connects.
(2) loose connections. While watching the signal, adjust/loosen/
tighten
any and all connectors in the coaxial maze.
(3) faulty or overdriven amplifiers- For most use, a simple
splitter is better than
an amplifier; the signal/noise is not improved by amplification on the
OUTPUT of a tuner, you don't NEED any amplifier to drive a bit
of cable from that output. If the amplifier(s) cause signal
overload, or
support oscillation, they're a problem rather than a solution.

None of these reflection issues is related to the digital
transmission. It is
also possible that the TV has a weak video amplifier that has started
to
create dark shadows behind bright vertical lines because of aging
of the video amplifier's filter capacitors. Plug a video game into
the
TV and see if it has the same artifacts (if it does, it's TV repair
time).