Blank TV screens as cable firm ditches analogue

"Zero Tolerance" wrote in message
...
On Wed, 16 Dec 2009 10:36:17 -0000, Terry Casey
wrote:

As far as Virgin Media are concerned, this is an exception to the rule.
As I explained in another post, usually ALL subscribers receive the
service using a Set Top Box and VM know where all the old analogue boxes
are. All they need to do is to swap out all the analogue boxes for
digital ones. Once the last customer has been migrated, it is safe to
turn off the analogue service.

This ignores the areas where Virgin Media provides an analogue MATV
service (as a result of agreements made between local councils and the
local cable company, whatever it may have been.) Since end users of
that service connect their TV directly to the cable feed and do not
have a set top box - and indeed probably do not even feature on
Virgin's customer list at all - I suspect this is the problem.

I was under the impression that in some areas the cable Companies *had* to
provide the terrestrial five channels FOC through their cable to every
property as part of the consent. I'm sure this was the case with some
private housing estates around Surrey Heath which were cabled when the
estate was built, with a grey box outside every front door. Also some Local
Authority/Housing Association estates where the intention was to prevent
forests of external aerials in an area where off air signals are not very
strong. The payback for the Cable companies was that many occupants would
buy their pay packages instead of going to Sky.

Has this been repealed? Or did I dream it, as the reality on many of the
estates I'm familiar with is multiple aerials and dishes. This is
understandable as Freeview took off, and Sky+ was available much more
quickly than the cable alternatives.

In article ,
says...

"Zero Tolerance" wrote in message
...
On Wed, 16 Dec 2009 10:36:17 -0000, Terry Casey
wrote:

As far as Virgin Media are concerned, this is an exception to the rule.
As I explained in another post, usually ALL subscribers receive the
service using a Set Top Box and VM know where all the old analogue boxes
are. All they need to do is to swap out all the analogue boxes for
digital ones. Once the last customer has been migrated, it is safe to
turn off the analogue service.

This ignores the areas where Virgin Media provides an analogue MATV
service (as a result of agreements made between local councils and the
local cable company, whatever it may have been.) Since end users of
that service connect their TV directly to the cable feed and do not
have a set top box - and indeed probably do not even feature on
Virgin's customer list at all - I suspect this is the problem.

I was under the impression that in some areas the cable Companies *had* to
provide the terrestrial five channels FOC through their cable to every
property as part of the consent. I'm sure this was the case with some
private housing estates around Surrey Heath which were cabled when the
estate was built, with a grey box outside every front door. Also some Local
Authority/Housing Association estates where the intention was to prevent
forests of external aerials in an area where off air signals are not very
strong. The payback for the Cable companies was that many occupants would
buy their pay packages instead of going to Sky.

Has this been repealed? Or did I dream it, as the reality on many of the
estates I'm familiar with is multiple aerials and dishes. This is
understandable as Freeview took off, and Sky+ was available much more
quickly than the cable alternatives.

There is, of course, another possibility:

That residents have chosen to ignore the local authority's ruling and
the council can't be bothered to enforce it.

In the case of council estates where aerials were banned, did this ban
extend to people who later bought their council house? I think large
numbers where sold off in some areas.

In contrast, some time during the roll-out of cable networks, the word
went round various local authorities that they could make a fortune out
of the cable co by charging hefty amounts for wayleaves (the legal
authority to enter or cross somebodies property).

So they demanded lots of money for the right to run cable into blocks of
council flats. The company I worked for refused to fall for this
blackmail and I know of lots of CATV amplifiers where the subscriber
taps were never fitted for that reason.

--

Terry

On Dec 17, 4:05*pm, Terry Casey wrote:
No filter, including a SAW with extremely channel edge roll-off, could
ever have prevented the problem - the simple answer is that the two
channels OVERLAP!

I've just been playing around with some figures, just to make sure that
my memory isn't playing tricks, so here goes!

This list shows all the relative data for two adjacent channels:

* * * * *MHz
* * * * 566.00 *Channel boundary (limit of vestigial sideband)
E33 * * 567.25 *Vision carrier
* * * * 570.00 *Channel centre
* * * * 573.25 *FM sound carrier
* * * * 573.802 NICAM carrier
* * * * 574.00 *Channel boundary (limit of vestigial sideband)
E34 * * 575.25 *Vision carrier
* * * * 578.00 *Channel centre
* * * * 581.25 *FM sound carrier
* * * * 581.802 NICAM carrier
* * * * 582.00 *Channel boundary

DTT transmissions, of course, occupy the entire 8MHz channel.
Well, not quite. Two adjacent DTT muxes with no offsets will have a
teeny gap between them.


Now, apart from the reality that no filter known to man can pass the
full 8MHz with such a rapid cut-off that nothing above or below it can
get through it, just look at how close the NICAM carrier is to the
adjacent channel - 198kHz!

But the NICAM signal has a 700kHz bandwidth, so the ch 33 NICAM signal
actually extends from about 573.45MHz to 574.15MHz, so approximately
150kHz of the upper sideband is actually inside the upper adjacent
channel!

On an analyser there is very little energy from the nicam carrier in
the upper adjacent channel.

Of course broadcasts make great use of offsets because of these
problems.

Bill

Terry Casey wrote:

DTT transmissions, of course, occupy the entire 8MHz channel.

I think the UK implementation of DVB-T is that only 7.6 MHz of the 8 MHz
channel is used ?


--
Mark
Please replace invalid and invalid with gmx and net to reply.

www.paras.org.uk

On Thu, 17 Dec 2009 16:12:12 -0000, Terry Casey
wrote:

Which part of THIS IS AN EXCEPTION TO THE RULE did you not understand?

Take your seat, young man. If I've overlooked something then I'll
cheerfully say so, but not while someone is acting like a swaggering
buffoon about it.
--

In message , Terry
Casey writes
In article ,
says...

snip

I remember when the trouble started in 1998
(presumably when the DTT test transmissions started).

The main problem was that elderly analogue TV signal processors /
translators were designed to cope only with the possibility of the
presence of adjacent off-air analogue signals. In the IF stage, they had
very deep traps (typically 70dB) to reject the lower-adjacent sound (and
possibly the NICAM) and the upper-adjacent vision. However, slightly
further out, the rejection fell to maybe only 45 to 50 dB.


Possibly, but not the cause of the problem. In any case, 8MHz wide SAW
channel filters were available and widely used in subscribers' STBs as
IF filters, so I'm fairly certain that they would have been used in all
but the most ancient head-end kit as well.

The SAW filters in analogue TV modulators and translators were a minty
bit dearer than those used in STBs (typically £80). However, even these
were not sufficiently 'brick wall', and required a helping hand from
additional tuned traps on the adjacent sound and vision carrier
frequencies. Further out, the rejection of the filter itself tended to
be limited to maybe 50dB, but usually other bandpass tuned circuits
cleaned things up well off-channel. The overall result was that
throughput of the translator had unwanted out-of channel peaks at around
vision minus 3.5MHz and sound plus 3.5MHz. Because of the spectral
content of an adjacent analogue signal (which has most of its sideband
power density close-in to the carrier frequencies), this was sufficient
to ensure no real impairment to the wanted signal. However, when an
adjacent 8MHz band of constant-density digital signal appeared on the
scene, the performance was not quite good enough, and, either side of
the wanted analogue signal, two bumps of digital 'noise' got fed into
the cable network.


This performance was still generally good enough for even equal-strength
adjacent analogue signals, but unfortunately it let part of the adjacent
digital 'haystack' signal through, and this got put out on the cable
network (albeit at a relatively low level).

No filter, including a SAW with extremely channel edge roll-off,

Hence the need for additional L-C traps for adjacent sound.

could
ever have prevented the problem - the simple answer is that the two
channels OVERLAP!

An upper adjacent digital does tend to overlap the NICAM of an analogue.
However, the digital signal is not the full 8MHz wide. It is about 7MHz
'across the top', and has to be well down towards the noise at 8MHz
wide. Also (although I haven't checked) I suspect that an upper adjacent
digital is offset HF (167kHz, isn't it?). If this was not done, I can't
see how (for example) Crystal Palace carries Ch33 analogue and Ch34
digital.

I've just been playing around with some figures, just to make sure that
my memory isn't playing tricks, so here goes!

This list shows all the relative data for two adjacent channels:

MHz
566.00 Channel boundary (limit of vestigial sideband)
E33 567.25 Vision carrier
570.00 Channel centre
573.25 FM sound carrier
573.802 NICAM carrier
574.00 Channel boundary (limit of vestigial sideband)
E34 575.25 Vision carrier
578.00 Channel centre
581.25 FM sound carrier
581.802 NICAM carrier
582.00 Channel boundary

DTT transmissions, of course, occupy the entire 8MHz channel.

Now, apart from the reality that no filter known to man can pass the
full 8MHz with such a rapid cut-off that nothing above or below it can
get through it, just look at how close the NICAM carrier is to the
adjacent channel - 198kHz!

Ch34 HF offset? Yes.
http://www.ofcom.org.uk/static/recep..._guide_4_1.pdf


But the NICAM signal has a 700kHz bandwidth, so the ch 33 NICAM signal
actually extends from about 573.45MHz to 574.15MHz, so approximately
150kHz of the upper sideband is actually inside the upper adjacent
channel!

If NICAM hadn't been invented, it should have been possible to improve
the filtering in the 750kHz gap between the FM sound carrier and the
upper channel boundary but, with NICAM, it was impossible!

snip


Obviously, additional rejection was required, and the most effective
(and foolproof) way of achieving this was to replace the existing
analogue processors/translators with a demod-remod system. NICAM was
provided either by a demod + re-encode/remod or (in at least one
network) taking it from the demodulator as a subcarrier at 6.552MHz, and
up-converting it the remodulator. Needless to say, none of this
re-engineering was cheap. Also, in order to accommodate the additional
equipment, there was often the problem of finding the additional space
in the already-crowded headend racks.

No argument there!

Well, I don't know. You could well be right. But the crux of the matter
is that the TV channel translators suddenly started causing problems on
the cable network because they lacked adequate selectivity in the middle
of the adjacent channels. This allowed bits of off-air digital 'noise'
to pass through, and get launched onto the cable network. It might have
gone unnoticed if the frequencies had been occupied by analogue signals,
but it definitely affected digital signals. The only practical cure was
the better overall filtering inherent in a demod-remod.
--
Ian

In article 197bc6c1-0018-46cd-86cd-5822d9481715
@c3g2000yqd.googlegroups.com, says...

On Dec 17, 4:05*pm, Terry Casey wrote:
No filter, including a SAW with extremely channel edge roll-off, could
ever have prevented the problem - the simple answer is that the two
channels OVERLAP!

I've just been playing around with some figures, just to make sure that
my memory isn't playing tricks, so here goes!

This list shows all the relative data for two adjacent channels:

* * * * *MHz
* * * * 566.00 *Channel boundary (limit of vestigial sideband)
E33 * * 567.25 *Vision carrier
* * * * 570.00 *Channel centre
* * * * 573.25 *FM sound carrier
* * * * 573.802 NICAM carrier
* * * * 574.00 *Channel boundary (limit of vestigial sideband)
E34 * * 575.25 *Vision carrier
* * * * 578.00 *Channel centre
* * * * 581.25 *FM sound carrier
* * * * 581.802 NICAM carrier
* * * * 582.00 *Channel boundary

DTT transmissions, of course, occupy the entire 8MHz channel.

Well, not quite. Two adjacent DTT muxes with no offsets will have a
teeny gap between them.


Ah, yes - my fault for over simplification - but even the best filters
must be able to pass the full 8MHz channel width.


Now, apart from the reality that no filter known to man can pass the
full 8MHz with such a rapid cut-off that nothing above or below it can
get through it, just look at how close the NICAM carrier is to the
adjacent channel - 198kHz!

But the NICAM signal has a 700kHz bandwidth, so the ch 33 NICAM signal
actually extends from about 573.45MHz to 574.15MHz, so approximately
150kHz of the upper sideband is actually inside the upper adjacent
channel!

On an analyser there is very little energy from the nicam carrier in
the upper adjacent channel.


Quite true. Perhaps I got a bit carried away with my efforts to stress
that two adjacent channel filters must, off necessity, have some degree
of overlap.


Of course broadcasts make great use of offsets because of these
problems.


Yes, also true. This avoids problems in the broadcast environment but
the particular problem we are discussing here is the conflict that
occurs when combining translated off-air analogue signals with
independently generated CATV digital signals.

Bill



--

Terry

In article ,
lid says...

Terry Casey wrote:

DTT transmissions, of course, occupy the entire 8MHz channel.

I think the UK implementation of DVB-T is that only 7.6 MHz of the 8 MHz
channel is used ?

As I said in my reply to Bill, my fault for over simplification - but
even the best filters must be able to pass the full 8MHz channel width.


--

Terry

On Dec 17, 8:13*pm, (Zero Tolerance)
wrote:
On Thu, 17 Dec 2009 16:12:12 -0000, Terry Casey

wrote:
Which part of THIS IS AN EXCEPTION TO THE RULE did you not understand?

Take your seat, young man. If I've overlooked something then I'll
cheerfully say so, but not while someone is acting like a swaggering
buffoon about it.
--

That wasn't zero tolerance. That was a lot of tolerance.

Bill

On 17 Dec, 19:29, "
wrote:

[snip]

Something was amiss on NTL (as was) in Biggleswade - there was an
entire working DTT mux available on cable there - plug-in a Freeview
box and it would happily pick up Mux C or D (I forget which).

It was probably mux c as that was (still is!) adjacent to analogue
five on Sandy Heath - but the filtering must have been pretty poor to
let the whole upper channel through.

Cheers,
David.