DIYbanter - View Single Post - Audio Precision System One Dual Domani Measuirement Systems

Terry Casey

In article ,
says...

In message , Geoffrey S.
Mendelson writes
David Looser wrote:

Of course if you are only looking for local coverage you could run several
networks in the available spectrum. But the argument was that VHF gave
better national coverage than UHF. If that is the aim then, I suggest, you'd
need both Bands 1 and 3 to give truly national coverage of just one network.
Its probable that it would be possible to add a second network that only
covered the main population centres, as Analogue Channel 5 did on UHF.

I don't know how well UK sets worked in the 1960's, but US TV sets were
not capable of receiving adjcent channels at one time, so they were not
used. For example, channel 2 was used in New York City, while the nearest
channel 3 station was in Philadelphia, 90 miles away and too far to be
received without a large antenna.

I think the next one up was 5 in NYC and 6 in Philly.

Generally, UK (and even European) TV sets had a hard time with adjacent
channels. Like the USA, the off-air broadcast channels were arranged so
that, within the normal service area, there would never be an adjacent
channel which was anything like as strong as the channel(s) intended for
that area.

The same was true of cable systems. As TV sets were incapable of
operating with adjacent channels, they carried only alternate channels.

SNIP

The UK UHF band plan specifically avoided the use of channels n, n + 5
and n + 9 in any transmitter group (n + 5 = n + IF; n + 9 = n + 2*IF) to
prevent interference.

I was quite surprised not to find any problems with a cable system I
started work on in 1969 which used 22 adjacent VHF channels (45 -
228MHz). As the system provided financial information only, there were
no sound carriers.

All the receivers used were modified domestic receivers using the ITT/KB
VC100 chassis. This chassis was effectively the old dual standard
chassis that had gone through at least five iterations that I can
remember - VC1, VC2, VC3, VC51, VC52 - in the previous four or five
years, with all the 405-line bits left out. Consequently it was really
quite an old design.

The GPO (which was just starting to transform itself into BT) were
responsible for the RF generation and trunk distribution and had chosen
a non-standard 8.3MHz channel spacing to ensure that the local
oscillator never clashed with a vision channel. This was possibly
inherited from the ILEA schools CCTV system they'd run because the
tuning errors they'd allowed for were a joke as our receiver tuning
always had to be spot-on because of the high frequency component of the
video - think CEEFAX in vision but with 48 character[1] lines.

Despite the adjacent channel traps in the receivers still being aligned
for 8MHz spacing(!) we never encountered any problems.

All later (broadcast) CATV and SMATV systems I've encountered, though,
have always used alternate channels, as described by Ian, for channels
intended for direct reception by a domestic receiver (i.e.: without
first being received by an STB).

[1] The worst characters in the special set used in these pre-decimal
days were 10 and 11 (for tenpence and elevenpence). Of these, ten was
the worst, producing a 10101 pixel sequence for most of its height -
tuning really had to be spot on for this!

--

Terry