Re: BBC2 on UHF38?


"Paul Ratcliffe" <abuse@xxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:slrngphdbb.10ts.abuse@xxxxxxxxxxxxxxxxxx
On Sat, 14 Feb 2009 18:42:38 -0800 (PST), Excitable Jimbo page 84 wrote:

Is there any audio?

No - it's too weak.

Don't be daft. If you can see a picture, you can hear the FM sound. The
latter is the last thing to go.

This used to be generally true when the audio was 6dB below the video. But
since it's been 10dB below the position is rather more indeterminate. In
addition, the ratio between the audio and video carriers actually varies a
lot, in reality. Even where something freaky isn't happening (see examples
of freakiness below) propagation, and mismatches in cables, splitters, etc,
can easily result in the ratio being anything between 4dB and 16dB.

I've also noticed that TV sets vary in their ability to receive weak FM
sound. I have a Grundig multistandard set that is very poor in this respect.
The audio disappears whilst the picture is still present and even watchable.

Of course reception of IF images of VSB broadcast signals will have no
audio, as the spectrum of the whole channel will be reversed.



Extract from
http://www.wrightsaerials.tv/topics/ten-taped-t-junctions-and-a-wailing-woman.html
Today I went to a ten-storey block built in 1970, and found something that
astonished me. There had been constant complaints about the TV system for
years, with the landlord refusing to do anything much about it, but last
week a few residents kicked up about the poor TV reception to the extent of
threatening a rent strike. The system had been installed all those years ago
and everything except the aerial was original equipment. Instead of calling
in a ?proper man? (me!) the managers called in the local aerial man. I just
can?t imagine what this man?s mental processes were, but he carefully
removed the printed circuit boards from the backs of the outlets, and
twisted together the incoming and outgoing cables and the flylead. He?d cut
one plug off the flylead. He did this ten times, and each time he carefully
fitted the outlet over the backbox with the flylead coming through it, and I
bet he thought he was doing a smart thing.
The results were interesting. It wasn?t possible to make a meaningful signal
level measurement anywhere, because the whole thing was a seething mass of
standing waves, but broadly speaking the loss from end to end was 55dB.
Normally it would be about 20dB. In general the signal level and quality
deteriorated from the top to the bottom of the building. The lower four
flats had virtually no reception. On frequencies where the signal returning
from the end of a flylead exactly cancelled out the signal on the main cable
there was a deep notch. One of these coincided with the analogue audio on
ch41, making the sound very weak and ?buzzy? at all the lower flats. There
were all sorts of weird effects, including patterning that I guess came from
TVs on the floors above.

See also
http://www.wrightsaerials.tv/resources/trees-and-uhf-reception.pdf
(page 3)

Bill


.

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