"Terry Casey" wrote in message
...
In article ,
says...

"Terry Casey" wrote in message
...
In article ,
lid says...

[1] UK cable systems mostly use HRC at 8MHz spacing but this is
sometimes varied by a carefully calculated amount so that one block of
UHF channels coincides almost exactly with the broadcast frequencies.
This is done on systems with a by-pass facility to allow a few
channels
- usually the local off-airs - to be fed directly to the TV giving the
subscriber direct access from the TV without needing an aerial.

Obviously this block of channels has to be chosen so as not to
conflict
with local transmitters, so the offset will vary from system to system
and can't be fixed as in the US table


In case it isn't clear, I should have pointed out that normal cable
reception is via a set top box and, of course, I was referring to
analogue systems ...

The advent of TV sets that could tune the cable channels all by
themselves
was a game changer.

They are used on the continental cable systems but never have been in
the UK. Possibly because there is much more encrypted subscription
content?

We have TVs that will show encrypted content. They have a slot for a
"Cablecard" which is a decrypter that is supplied by the content provider
(cable company). AFAIK cable company DVRs have two Cablecards inside the
box, and they are available for installation in PCs.

Ian Jackson wrote:
In message , Bill Wright
writes
Michael A. Terrell wrote:


Do you know that the channel combiners in a CATV head end
were wired in odd and even banks, on separate groups to prevent IMD
caused in the passive mixing?

Even now if we use passive filters to combine channels we try to
arrange the filters as follows (for example)
Filter 1: 21, 25, 29, 33 etc
Filter 2: 22, 26, 30, 34 etc
Filter 3: 23, 27, 31, 35 etc
Filter 4: 24, 28, 32, 36 etc
The filter outputs are combined using passive combiners. The idea is
to prevent interactivity between adjacent channel filters.

Ah but, unless they really have to, most large cable TV networks don't
use filter combiners. This keeps the whole of the headend combining
system wideband. Of course, you can only do this if the modulators are
themselves sufficiently clean in respect of out-of-channel unwanted
signals and noise.
The manufacturers of many of the modulators sold for communal TV system
use claim that the output is so clean that filter combiners are
unnecessary. But I use filters anyway. The fact is that I seem to get
better results.


Normally, the losses associated with wideband combining are not a
problem. At least one manufacturer made a purpose-made 8-port (and later
a 16-port) combiner, with insertion losses of around 20 and 24dB
respectively. They were designed specifically to provide high isolation
between input ports. A 48-channel headend could be made using four
16-port combiners followed by a standard 4-way turned-around splitter,
so the total loss was around 28dB. As the modulator outputs were around
60dBmV, the combined spectrum level was 32dB - which was more than
enough to allow splitting to provide multiple feeds for the inputs of
individual trunk-line launch amplifiers.

Typically the loss using four six-way filters and a four way combiner
would be about 15dB. Modulator O/P is usually 25dBmV (analogue mods) so
the combined feed is 10dBmV. That would usually be amplified to
something between 40dBmV and 55dbmV before feeding out.

One advantage of using filters is that if some of the RF sources only
provide low level signals it doesn't hurt, noise-wise, to amplify before
combining.

Bill

In article ,
says...

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



I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.


They would have to be very unusual TV sets!

I don't know about 'unusual', but they were a problem. I think there
were only couple of budget brands which only tuned 'spot-on' to the UHF
channels (xxx.25MHz, in 8MHz steps). One might ask indeed "Why would you
need them to do otherwise?" Of course, even our cable set-top boxes
could normally only tune in 125kHz steps, but at least that got you to
within +/-63kHz of the correct frequency - and that was more than close
enough.


Apologies - I misread what you wrote!

I thought you wrote "the four set-top bypass channels had to be close to
the LOCAL off-air broadcast channels ..."



I had little to do with the system in London (I think I only went there
once - underground, near Shepherds Bush IIRC).

As I said, I didn't actually visit the headend. All of our equipment was
in a room in the basement of a large block not far from Marble Arch
IIRC. Obviously, the headend couldn't have been far away ...

--

Terry

In article , says...

The manufacturers of many of the modulators sold for communal TV system
use claim that the output is so clean that filter combiners are
unnecessary. But I use filters anyway. The fact is that I seem to get
better results.


I wonder if they have any active stages AFTER the filter, possibly
because they might be used in installations with less than perfect
matching?

We used to use some tunable modulators, although most of them were
fixed. The idea of the tuneable one was that you had a near instant fix
if one of the modulators developed a fault. The downside was that the
broadband noise from the unfiltered output degraded the headend by 1dB
per modulator ...

The number of tunable modulators was reduced to one - we used it as the
reference for all of our distortion tests which, of course, used a spare
channel, hence no (fixed) modulator ...


One advantage of using filters is that if some of the RF sources only
provide low level signals it doesn't hurt, noise-wise, to amplify before
combining.


Obviously you will add noise to the amplified source but not to the rest
of the system.

--

Terry

Dave Plowman (News) wrote:
In article ,
Bill Wright wrote:
Don't forget that the schools in the UK have many teachers who are
socialist and deny the true
history of the 20th century.

So there are no right wing teachers who deny the history of the 20th
century?
I didn't say that. There must be a few; it stands to reason. But the
teaching profession is overwhelmingly left wing. Tell me why else the
place for teaching job ads is the Guardian?

Tell you what, Bill. Go to a US health care website and get a quote to
cover you and yours.
I'm a massive supporter of the NHS. It provides health care far more
economically that the US system. But that doesn't make other socialist
measures right. The basic problem with the socialist mindset is that it
puts the freedom of the individual below the collective will. If you're
a sheep that's fine; if you're a monkey or a rook it isn't.

You see, you need to think for yourself about each issue, not just
blindly follow the party line.

Incidentally I think the present government has been duplicitous in the
extreme over the NHS, when you compare pre-election promises with the
present reality.

Bill

Ian Jackson wrote:

I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.


They would have to be very unusual TV sets!
They existed. We used to have problems with a system that had crappy
modulators that drifted. Some of the tellys just wouldn't tune the
signals unless they were exactly on the channel. Just skipped over them.


A comb of 7.988636364MHz would allow E25, E27, E29 and E31 to be used
with +/-34kHz error (off-airs being 23, 26, 30 & 33)
When I worked at CBC (near BH) about 20 years ago the cable input was on
channels 29 and 31.

On the subject of adjacent channel working we have several systems that
use almost every UHF channel and the tellys tolerate it perfectly well.
And they mostly cheap Vestels.

Bill

"Bill Wright" wrote

You see, you need to think for yourself about each issue, not just blindly
follow the party line.

That is utterly hilarious Bill! "If you don't agree with me its because you
don't think for yourself but are following a party line"!

Do us all a favour Bill and stop spouting such unutterable crap will you?
Socialism is no more about "following a party line" than any other "ism" is,
including capitalism. But I don't know where you think all these
"socialists" are in Britain these days. Clearly not enough to support a
political party of any size whatsoever. Labour had to abandon socialism back
in the '70s to retain any chance whatsoever of being elected. What do we
have now?, Arthur Scargill's "Socialist Worker's Party"? Even the National
Front gets more votes!

Incidentally I think the present government has been duplicitous in the
extreme over the NHS, when you compare pre-election promises with the
present reality.

The present government has got its knickers in a twist over the
health-service reforms because it failed to properly consult with either the
medical profession or the users of the service (the public), neither of
which are in favour of the government's ideologically-driven plans.

David.

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

The manufacturers of many of the modulators sold for communal TV system
use claim that the output is so clean that filter combiners are
unnecessary. But I use filters anyway. The fact is that I seem to get
better results.


I wonder if they have any active stages AFTER the filter, possibly
because they might be used in installations with less than perfect
matching?

We used to use some tunable modulators, although most of them were
fixed. The idea of the tuneable one was that you had a near instant fix
if one of the modulators developed a fault. The downside was that the
broadband noise from the unfiltered output degraded the headend by 1dB
per modulator ...

The number of tunable modulators was reduced to one - we used it as the
reference for all of our distortion tests which, of course, used a spare
channel, hence no (fixed) modulator ...


One advantage of using filters is that if some of the RF sources only
provide low level signals it doesn't hurt, noise-wise, to amplify before
combining.


Obviously you will add noise to the amplified source but not to the rest
of the system.

Using unfiltered combiners, the tuneable ('frequency agile' in American
parlance) modulators which I worked with were specced to give a CNR of
better than 60dB after 48 headend channels had been combined. Typically,
it was 63dB.

I recall visiting one particular headend in order to prove to that we
met the spec, and to explain why 'Brand X's' modulators gave more like
70dB or better. I spent the day making lots of measurements on a goodly
selection of the 48 channels, and in the end, everyone was satisfied
that the usual 63dB was being obtained.

The reason that the Brand X units were so much better was quite simply
that they incorporated individual channel filters. The disadvantage was,
of course, that they were not instantly tuneable to any network channel.
But, as Terry says, what a headend operator could do was to have a few
fully tuneable modulators to do system measurements with, and to use as
backup units.

Although the competitor's equipment undoubtedly gave around 10dB better
SNR than ours, individual channel filtering was something we had ditched
two generations of equipment earlier, as it had been decided that there
was absolutely no advantage in getting a 70dB CNR at the headend when
the rest of the network was going to degrade this to something like 48dB
(or even less). The overall impact of changing the headend launch CNR
from better than 70dB to our typical 63dB would have been negligible.
Nevertheless, I believe that later on, all our gear was replaced with
the Brand X units.
--
Ian

On Fri, 10 Feb 2012 16:04:22 +0000, Ian Jackson
wrote:

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

The manufacturers of many of the modulators sold for communal TV system
use claim that the output is so clean that filter combiners are
unnecessary. But I use filters anyway. The fact is that I seem to get
better results.


I wonder if they have any active stages AFTER the filter, possibly
because they might be used in installations with less than perfect
matching?

We used to use some tunable modulators, although most of them were
fixed. The idea of the tuneable one was that you had a near instant fix
if one of the modulators developed a fault. The downside was that the
broadband noise from the unfiltered output degraded the headend by 1dB
per modulator ...

The number of tunable modulators was reduced to one - we used it as the
reference for all of our distortion tests which, of course, used a spare
channel, hence no (fixed) modulator ...


One advantage of using filters is that if some of the RF sources only
provide low level signals it doesn't hurt, noise-wise, to amplify before
combining.


Obviously you will add noise to the amplified source but not to the rest
of the system.

Using unfiltered combiners, the tuneable ('frequency agile' in American
parlance) modulators which I worked with were specced to give a CNR of
better than 60dB after 48 headend channels had been combined. Typically,
it was 63dB.

I recall visiting one particular headend in order to prove to that we
met the spec, and to explain why 'Brand X's' modulators gave more like
70dB or better. I spent the day making lots of measurements on a goodly
selection of the 48 channels, and in the end, everyone was satisfied
that the usual 63dB was being obtained.

The reason that the Brand X units were so much better was quite simply
that they incorporated individual channel filters. The disadvantage was,
of course, that they were not instantly tuneable to any network channel.
But, as Terry says, what a headend operator could do was to have a few
fully tuneable modulators to do system measurements with, and to use as
backup units.

Although the competitor's equipment undoubtedly gave around 10dB better
SNR than ours, individual channel filtering was something we had ditched
two generations of equipment earlier, as it had been decided that there
was absolutely no advantage in getting a 70dB CNR at the headend when
the rest of the network was going to degrade this to something like 48dB
(or even less). The overall impact of changing the headend launch CNR
from better than 70dB to our typical 63dB would have been negligible.
Nevertheless, I believe that later on, all our gear was replaced with
the Brand X units.

There is always a downside to individual channel filtering, and that
is the group delay that gets screwed at the edges, unless you also
have all-pass equalizers. It all tends to get a bit messy.

d

J G Miller wrote:

With the switch off of analog TV, all TV transmissions in Germany are now
on UHF channels. In Western Europe, only Danmark and Letzebuerg have
transmitters with DVB-t on VHF Band III.

http://www.ukwtv.DE/sender-tabelle/

And Finland (if you count that as Western Europe ?)

http://www.dvb.org/about_dvb/dvb_worldwide/finland/

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

www.paras.org.uk