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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

ANALOG RECEPTION IS SUPERIOR to digital non-reception



 
 
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  #21  
Old January 18th 08, 05:27 PM posted to uk.tech.digital-tv
Mark Carver
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Default ANALOG RECEPTION IS SUPERIOR to digital non-reception

charles wrote:
In article , Mark Carver


Also interesting about [21,24,27,31] group stations is that BBC 1
occupies Ch21 at most of the relay stations, but Ch31 at all the main
stations. The reason being is that Ch21 requires a sharp roll off at its
lower end to avoid interference to the PMR band below.


This requires a filter on the output of the transmitter, and if it's a
high power Tx, quite a large filter. So of course neither the Beeb nor
IBA grabbed Ch21 at main stations until someone really needed to. Step
forward the IBA in 1982 for C4....


But when the uhf network was planned, well before 1982, it was done on a 4
channel basis.


I never said it wasn't.

The point I'm making is that Ch 21 was not used for high power transmissions,
until C4 came along. Which is fair enough, why spend money when there's an
easier allocation to use, and the 'tricky' one might never see the light of
day anyway.

--
Mark
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  #22  
Old January 19th 08, 12:45 AM posted to uk.tech.digital-tv
Bill Wright
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Posts: 8,408
Default ANALOG RECEPTION IS SUPERIOR to digital non-reception


"Mark Carver" wrote in message
...
Yes. I suspect it's got something to do with the combining arrangements at
each transmitter. Whether it's:-

[[BBC1+BBC2] + [ITV+C4]] or [BBC1+BBC2+ITV+C4]

Also interesting about [21,24,27,31] group stations is that BBC 1 occupies
Ch21 at most of the relay stations, but Ch31 at all the main stations. The
reason being is that Ch21 requires a sharp roll off at its lower end to
avoid interference to the PMR band below.

Didn't know that!


This requires a filter on the output of the transmitter, and if it's a
high power Tx, quite a large filter. So of course neither the Beeb nor IBA
grabbed Ch21 at main stations until someone really needed to. Step forward
the IBA in 1982 for C4....


Didn't know that either. Meanwhile some of the aerial manufacturers (and not
just the no-name ones) went on for years making Gp A aerials that turned out
to be a disaster on ch21. I remember one particular Sheffield rigger
earnestly telling me that C4 was transmitted with ghosting because it was
picked up at Crosspool from Emley Moor and as everyone knows Crosspool is a
bad reception area.

I had a little item about this last year in Wotsat, because the legacy is
that many of these aerials are still causing problems.

A few years ago a NB carrier appeared in the Sheffield area on 469.900MHz.
It was exceeding strong in the city centre and caused some distribution amps
to cough and splutter. It also upset certain TV sets when tuned to ch21. The
problem only lasted a month or two and I never actually got around to
finding out from whence it came, which is a shame really. There was never
any detectable modulation on the carrier.

Is anyone bored with all this yet ?

Far from it. You've answered several questions that I've been wondering
about since the 1970s. Not all the time of course. Sometimes I've wondered
about other things.

Bill



  #23  
Old January 19th 08, 12:47 AM posted to uk.tech.digital-tv
Bill Wright
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Posts: 8,408
Default ANALOG RECEPTION IS SUPERIOR to digital non-reception


"Mark Carver" wrote in message
...
The point I'm making is that Ch 21 was not used for high power
transmissions, until C4 came along. Which is fair enough, why spend money
when there's an easier allocation to use, and the 'tricky' one might never
see the light of day anyway.


Really, there should have been a guard band.

Bill


  #24  
Old January 19th 08, 12:49 AM posted to uk.tech.digital-tv
Bill Wright
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Posts: 8,408
Default ANALOG RECEPTION IS SUPERIOR to digital non-reception


"charles" wrote in message
...
These are all in Planning group C. Crystal Palace is an exception and
uses
ch 30. I don't know why, it might be another station in mainland Europe,
or perhaps the use of ch29 in that part of the country would have
interfered with something else (perhaps military or CAA radar) already in
existance


Many years ago when I used to work in London a lot I used to build up
head-ends in the workshop which were designed for CP. It was convenient to
use Bilsdale as a signal generator, except for ch30.

Bill


  #25  
Old January 19th 08, 10:27 AM posted to uk.tech.digital-tv
Andy Wade
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Posts: 442
Default ANALOG RECEPTION IS SUPERIOR to digital non-reception

Bill Wright wrote:

Really, there should have been a guard band.


There is since the change to System I1 :~) - although perhaps the ch 21
transmissions still have the full -1.25 MHz lower-sideband, there being
no lower adjacent DTT to interfere with.

And there is with DTT. No ch 21 DTT transmissions have negative offsets
so there's a c. 200 kHz gap to the band edge - although they'll still
need an expensive (probably 8-cavity) filter to reduce the transmitter
intermod products.

And there probably is a small (few kHz) guard band on the PMR side, but
since the LSB of the TV transmission could extend over 4 MHz below the
band edge, the need for high-spec filtering can't really be avoided.

Interference goes the other way too. When C4 test transmissions first
started from Sandy Heath there was a horrendous level of herringbone
beat patterning visible on the signal received here in Cambridge - ITU-R
Grade 1, completely unwatchable pictures. Something local was radiating
a fairly high level unmodulated carrier just below 470 MHz. It peaked
up when the aerial was pointed in the direction of a well known local
government site, and putting out some feelers provided a rumour that
this carrier was something to do with protecting us from those Russian
missiles (county emergency something-or-other). Anyway I phoned a
contact in the MPT, as it was in those days, reported the TVI and
mentioned the rumour and within a remarkably short time, a few hours
IIRC, certainly less than a day, the offending carrier disappeared,
never to return.

--
Andy
 




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