UHF filters and the digital switchover

Here's one for ya! What's gonna happen to all the communal aerial
systems that use filters designed for the current DTT frequencies
after switchover, when most frequencies will change? Surely it must
be difficult to decide what to do, filter wise, when installing a
coomunal aerial system now knowing that when the frequencies change
after switchover, the system may not work.

Marky P.

"Marky P" wrote in message
...
Here's one for ya! What's gonna happen to all the communal aerial
systems that use filters designed for the current DTT frequencies
after switchover, when most frequencies will change? Surely it must
be difficult to decide what to do, filter wise, when installing a
coomunal aerial system now knowing that when the frequencies change
after switchover, the system may not work.

Most main stations will use the channels presently used for analogue for
DTT, plus others. In many cases DTT and analogue are adjacent. The filters
are retunable with care.

So it could be worse. But yes, it is a worry.

Bill

"Marky P" wrote in message
...
Here's one for ya! What's gonna happen to all the communal aerial
systems that use filters designed for the current DTT frequencies
after switchover, when most frequencies will change? Surely it must
be difficult to decide what to do, filter wise, when installing a
coomunal aerial system now knowing that when the frequencies change
after switchover, the system may not work.

As far as I can tell - I'm no expert - channel filters are not all that
precise. A digital mux will still get through a communal system if it is
anywhere close to an analogue channel frequency. In fact the biggest
obstacle to digital reception where I live is "swamping" of the digital
carriers by highly amplified analogue signals. Once these are switched off,
digital reception should improve immensely. In addition to that, it is a
simple screwdriver job to either retune the existing channel filters to
permit digital muxes or else remove the filter blocks completely. Many of
the reasons why the filters were installed in the first place will have
disappeared once analogue transmissions end. The downside is it will be the
year 2011 before that happens around here. I also learned only this
afternoon that the contractor responsible for maintaining our communal
system does a lot of installation work for Sky so has a vested interest in
degrading the quality of reception for anyone not subscribed to the latter.

(kim)

"kim" wrote in message
...
"Marky P" wrote in message
...
As far as I can tell - I'm no expert - channel filters are not all that
precise. A digital mux will still get through a communal system if it is
anywhere close to an analogue channel frequency.
Ordinary passive filters are three stage and will impose a slope on the
adjacent channels that goes from 0dB to about 15dB across the channel.
Obviously rejection is better beyond that. Five channels away it's oh -- a
lot, I dunno, maybe 40dB. It gets to the point ten channels away where the
tiny bit of signal is getting across the filter without actually passing
through the tuned stages, so the rejection doesn't vary if the tuning is
altered.

Filters are now available (at about £120 per channel) that are, in effect,
perfect. The response across the channel is flat and the sides are like the
white cliffs of Dover.

In fact the biggest obstacle to digital reception where I live is
"swamping" of the digital carriers by highly amplified analogue signals.
Once these are switched off, digital reception should improve immensely.
If you mean that the amplification talkes place in the TV distribution
system where you live then the system is incorrectly adjusted or is using
inadequate equipment, if the analogue channels are affecting DTT more than
they would if received direct from an aerial.

In addition to that, it is a
simple screwdriver job to either retune the existing channel filters to
permit digital muxes
It is quite tricky to adjust filters so the response is flat across the
channel but drops off steeply at each side.

or else remove the filter blocks completely. Many of the reasons why the
filters were installed in the first place will have disappeared once
analogue transmissions end.
Given the lower signal levels of DTT it will become possible to increase
system gain. But then out-of-channel interference (strong signals from
mobile transmitters, etc) will be a major issue, just as they are now.
Furthermore, all out-of-channel signals add to the amplifier load and thus
reduce, if slightly, the maximum output possible. There's a lot of stuff out
there that can cause all sorts of weird problems. Don't forget that TV amps
can be affected by 1GHz signals, as an example. Keep your filters.

Bill

On Wed, 18 Apr 2007 02:40:28 +0100, "Bill Wright"
wrote:


"kim" wrote in message
...
"Marky P" wrote in message
...
As far as I can tell - I'm no expert - channel filters are not all that
precise. A digital mux will still get through a communal system if it is
anywhere close to an analogue channel frequency.
Ordinary passive filters are three stage and will impose a slope on the
adjacent channels that goes from 0dB to about 15dB across the channel.
Obviously rejection is better beyond that. Five channels away it's oh -- a
lot, I dunno, maybe 40dB. It gets to the point ten channels away where the
tiny bit of signal is getting across the filter without actually passing
through the tuned stages, so the rejection doesn't vary if the tuning is
altered.

Filters are now available (at about £120 per channel) that are, in effect,
perfect. The response across the channel is flat and the sides are like the
white cliffs of Dover.

In fact the biggest obstacle to digital reception where I live is
"swamping" of the digital carriers by highly amplified analogue signals.
Once these are switched off, digital reception should improve immensely.
If you mean that the amplification talkes place in the TV distribution
system where you live then the system is incorrectly adjusted or is using
inadequate equipment, if the analogue channels are affecting DTT more than
they would if received direct from an aerial.

In addition to that, it is a
simple screwdriver job to either retune the existing channel filters to
permit digital muxes
It is quite tricky to adjust filters so the response is flat across the
channel but drops off steeply at each side.

or else remove the filter blocks completely. Many of the reasons why the
filters were installed in the first place will have disappeared once
analogue transmissions end.
Given the lower signal levels of DTT it will become possible to increase
system gain. But then out-of-channel interference (strong signals from
mobile transmitters, etc) will be a major issue, just as they are now.
Furthermore, all out-of-channel signals add to the amplifier load and thus
reduce, if slightly, the maximum output possible. There's a lot of stuff out
there that can cause all sorts of weird problems. Don't forget that TV amps
can be affected by 1GHz signals, as an example. Keep your filters.

Bill


Round'ere (Sandy Heath) the current analogue frequencies are group A
and digital mux's are all group B except for one at ch67. After
switchover, I've heard that the PSB mux's will use thr analogue group
A frequencies but the COM mux's will be group C/D! That's gonna be
bad enough for domestic installations let alone communal systems.

Marky P.

On Apr 18, 9:43 am, Marky P wrote:

Round'ere (Sandy Heath) the current analogue frequencies are group A
and digital mux's are all group B except for one at ch67. After
switchover, I've heard that the PSB mux's will use thr analogue group
A frequencies but the COM mux's will be group C/D! That's gonna be
bad enough for domestic installations let alone communal systems.

Yes, but the three PSB muxes, that will carry between them BBC, ITV,
C4 and C5 will occupy UHF Chs 21, 24, 27. Therefore no one with a
communal system should be worse off after DSO. It's true to say that
modifications will need to be made to enable reception of the three
COM muxes, but by and large they will only carry junk, so why worry
too much ?

"Bill Wright" wrote in message
...

"kim" wrote in message
...
"Marky P" wrote in message
...
As far as I can tell - I'm no expert - channel filters are not all that
precise. A digital mux will still get through a communal system if it is
anywhere close to an analogue channel frequency.
Ordinary passive filters are three stage and will impose a slope on the
adjacent channels that goes from 0dB to about 15dB across the channel.
Obviously rejection is better beyond that. Five channels away it's oh -- a
lot, I dunno, maybe 40dB. It gets to the point ten channels away where the
tiny bit of signal is getting across the filter without actually passing
through the tuned stages, so the rejection doesn't vary if the tuning is
altered.

Filters are now available (at about £120 per channel) that are, in effect,
perfect. The response across the channel is flat and the sides are like
the white cliffs of Dover.

In fact the biggest obstacle to digital reception where I live is
"swamping" of the digital carriers by highly amplified analogue signals.
Once these are switched off, digital reception should improve immensely.
If you mean that the amplification talkes place in the TV distribution
system where you live then the system is incorrectly adjusted or is using
inadequate equipment, if the analogue channels are affecting DTT more
than they would if received direct from an aerial.

The system is 40 years old and has never been rewired. It uses thin VHF
quality cable from the days when the only UHF channel was BBC2 at the lower
end of the channel group. The wiring shares a conduit pipe with the external
lighting system. I often get sparklies when the lighting system is switched
on the evenings. An employee of the aerial contractor told me 15 years ago
it needed completely replacing. Nothing has been done to it since except an
extra filter added to permit reception of Channel 5 and a slight boost
across all channels.

(kim)

"kim" wrote in message
...
"Bill Wright" wrote in message
...
The system is 40 years old and has never been rewired. It uses thin VHF
quality cable from the days when the only UHF channel was BBC2 at the
lower end of the channel group. The wiring shares a conduit pipe with the
external lighting system. I often get sparklies when the lighting system
is switched on the evenings. An employee of the aerial contractor told me
15 years ago it needed completely replacing. Nothing has been done to it
since except an extra filter added to permit reception of Channel 5 and a
slight boost across all channels.

It's pointless thinking about an upgrade. Start again.

Bill