Reasons for quality variation across Freeview channels?

I'm attempting to form a model of quality variation across Freeview
channels. Maybe 'channel' isn't even the correct term. Could someone
point me at a simple introduction to Freeview's use of wavelength bands
and encoding within bands (grasping for the correct terms)?

I use very few channels, BBC4, Dave, Dave +1, ITV4 mostly. Sometimes
BBC1, 2, ITV1, C4, C5 to record (on a cheap Wharfendale receiver /
hard-disc recorder).

I am unaware of BBC1, 2, or 4 ever breaking up; or Dave, Dave +1.

C5 is almost never watchable; ITV4 sometimes breaks up; last week an
ITV1 recording was totally unwatchable. Incidentally, C5 is the weakest
analogue channel too, but I think that is common.

Apart from C5, I think I can link the worst occurrences of poor
reception with heavy rain showers between me and the transmitter.

Just to confuse matters further, I had bad problems with ITV4 which
seemed to be cured simply by moving the receiver/recorder six inches. I
had suspected a SCART connection problem, but I'm now inclined to think
that moving the device has some effect on the RF connection (from the
aerial). But the major variability seems to be independent of the device
position.

I'm 90% confident of the rain shower hypothesis, that C5 is always
unwatchable, and that BBC1, 2, 4 are always okay.

My original Freeview receiver (not the recorder) told me I had a signal
strength of 9/10. I cannot find any indication of signal strength on the
current machine.

In case it matters, I'm in Derry, Londonderry, about 20 miles from the
Limavady transmitter; definitely not in line of sight. I have a tree
that is roughly in line and just about the same height as my aerial and
about 20 metres away. However, I don't think the wetness of the tree has
any effect, nor the density of foliage (it is a sycamore --- deciduous).

TIA,

Jon C.

--
Jonathan Campbell www.jgcampbell.com BT48, UK.

Jonathan Campbell wrote:
: I'm attempting to form a model of quality variation across Freeview
: channels. Maybe 'channel' isn't even the correct term. Could someone
: point me at a simple introduction to Freeview's use of wavelength bands
: and encoding within bands (grasping for the correct terms)?

Freeview uses 6 8MHz UHF analogue "channels" (between channels 21 and 68)
but transmits digital data (a "multiplex" or "mux") in each of these 6
channel positions.
The digital data for each "mux" comprises several Digital TV channels which
are split down further by the tuner ("de-multiplexed") - and then the
tuned one is displayed.

The usual cause of Freeview channel quality variation is that the 6 Freeview
"muxes" are not transmitted at equal power. Also, in some areas, a new aerial
is needed as the DTT channels are not transmitted close to the analogue ones
(for example, in my area, the analogue channels are between 53 and 63 while
the DTT channels are between 28 and 67!)

Freeview STBs always? have some means of displaying the signal strength
and quality on each of the 6 muxes.

....snip...

You might be suffering from any/all/some of...

- wrong aerial which is designed for some but not all of the frequencies
being used for digital where you are
- differences between the different transmissions standards whihc mean BBC
is currently more resilient than others to problems and also more resilient
if the signal strength is right on the edge of good
- notch filtering by the trees (see the thread earlier this week) which can
very effectively block just a small number of frequencies apparently
- an old, worn out aerial and cable causing poor signal which BBC is more
resilient to
- some dodgy cabling/connections giving the same affect.

Paul DS.

"Jonathan Campbell" wrote in message
...
I'm attempting to form a model of quality variation across Freeview
channels. Maybe 'channel' isn't even the correct term. Could someone point
me at a simple introduction to Freeview's use of wavelength bands and
encoding within bands (grasping for the correct terms)?

I use very few channels, BBC4, Dave, Dave +1, ITV4 mostly. Sometimes BBC1,
2, ITV1, C4, C5 to record (on a cheap Wharfendale receiver / hard-disc
recorder).

I am unaware of BBC1, 2, or 4 ever breaking up; or Dave, Dave +1.

C5 is almost never watchable; ITV4 sometimes breaks up; last week an ITV1
recording was totally unwatchable. Incidentally, C5 is the weakest
analogue channel too, but I think that is common.






the only difference in quality is the data rate.
in terms of being unwatchable, that's just down to you not having a suitable
aerial for your location - if you did, you'd never have breakup.



--
Gareth.

that fly...... is your magic wand....

In article ,
Jonathan Campbell wrote:

I am unaware of BBC1, 2, or 4 ever breaking up; or Dave, Dave +1.

These channels are on multiplexes 1, B, C and D which use 16QAM.

C5 is almost never watchable; ITV4 sometimes breaks up; last week an
ITV1 recording was totally unwatchable. Incidentally, C5 is the weakest
analogue channel too, but I think that is common.

These are on multiplexes 2 and A which use 64QAM.

If your signal is poor, reception is much less robust on multiplexes
using 64QAM. So what you report is not surprising. You probably need
a better aerial.

See http://www.ukfree.tv/txdetail.php?a=IC711296 for more information
about your transmitter.

-- Richard
--
Please remember to mention me / in tapes you leave behind.

Richard Tobin wrote:
In article ,
Jonathan Campbell wrote:

I am unaware of BBC1, 2, or 4 ever breaking up; or Dave, Dave +1.

These channels are on multiplexes 1, B, C and D which use 16QAM.

C5 is almost never watchable; ITV4 sometimes breaks up; last week an
ITV1 recording was totally unwatchable. Incidentally, C5 is the weakest
analogue channel too, but I think that is common.

These are on multiplexes 2 and A which use 64QAM.

If your signal is poor, reception is much less robust on multiplexes
using 64QAM. So what you report is not surprising. You probably need
a better aerial.

See http://www.ukfree.tv/txdetail.php?a=IC711296 for more information
about your transmitter.


Thanks, that site and the information you give are most helpful and
informative --- just what I wanted.

Incidentally, as a post on the website mentions, the aerial location on
the map is incorrect; it is at least four times further from
Derry-Londonderry than indicated. It is interesting how far west it
reaches --- I thought my friends in Donegal were dreaming when they
claimed to receive Freeview.

Also, just in case readers think my aerial installation is really bad,
Derry is notoriously difficult for TV reception. We have the Sheriff's
Mountain transmitter no more than four miles from my house, yet nearly
everyone in this area uses Limavady --- except, I see out of my window,
a house opposite. (I realise Sheriff's Mt. will not have digital until
the last moment.) As the map shows, there are parts of the city that
cannot receive Limavady. In fact, there are houses in the low-lying
southern part of the city that seem to have monstrous aerials pointing
at Strabane --- but maybe that is historical legacy.

I have little doubt that my cabling, at least, could be improved.

Should I be thinking of a Freesat dish? One problem I can see with that
is that the dish would have to be fitted to the front of the house.

Best regards,

Jon C.

--
Jonathan Campbell www.jgcampbell.com BT48, UK.

"Jonathan Campbell" wrote in message
...
I'm attempting to form a model of quality variation across Freeview
channels. Maybe 'channel' isn't even the correct term. Could someone point
me at a simple introduction to Freeview's use of wavelength bands and
encoding within bands (grasping for the correct terms)?



Four main reasons: -

1. Raw power - not all transponders emit the same power

2. Bit rate - not all channels have the same bit rate. Second stringer
channels (like QVC + 1) tend to be lower rate.

3. Encoding - BBC etc. on 16QAM, whereas ITV on 64QAM, which is more likely
to drop out.

4. Aerial - if you have a banded aerial (as opposed to a wide band) and some
channels are in other bands then the aerial gain will be unity (or worse) as
compared to 10dB in band.

Nonsense, break-up is often caused by impulse interference such as
'dirty' switching by neighbouring central-heating thermostats, the
motor scooters that children have nowadays, etc. It's been the plague
of reception of everything here, from FM through Analogue TV to DTT,
whether using professionally fitted aerials (initially) or my own
improvements using advice here (more recently), specifically designed
to counter it. The latter have brought definite improvements, but
certainly have not cured the problem completely.

On Tue, 28 Jul 2009 17:08:03 +0100, "The dog from that film you saw"
wrote:

the only difference in quality is the data rate.
in terms of being unwatchable, that's just down to you not having a suitable
aerial for your location - if you did, you'd never have breakup.

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Jonathan Campbell wrote:
I'm attempting to form a model of quality variation across Freeview
channels. Maybe 'channel' isn't even the correct term. Could someone
point me at a simple introduction to Freeview's use of wavelength bands
and encoding within bands (grasping for the correct terms)?

I think it more the word quality that is wrong, but yes Mux is a more
correct term for channel these days (others have explained). Quality is
normally used to describe how well the content is compressed, so mostly
quality is controlled by the broadcaster. Higher bit rates=higher
quality=less channels. Although mux capacity can be varied by using a
more robust modulation scheme, as others have explained the BBC Muxes
are a bit more robust that the others.

Signal reception problems cause breakup, Freeview either works perfects
or not atall (almost). You can have a very weak signal that will work
perfectly most of the time but when it rains, or someone opens the
fridge you have problems.

A higher RF signal generally gives you more margin against interference
and weather conditions, but you won't notice any improvement in picture
quality when there is no interference.

I use very few channels, BBC4, Dave, Dave +1, ITV4 mostly. Sometimes
BBC1, 2, ITV1, C4, C5 to record (on a cheap Wharfendale receiver /
hard-disc recorder).

I am unaware of BBC1, 2, or 4 ever breaking up; or Dave, Dave +1.

C5 is almost never watchable; ITV4 sometimes breaks up; last week an
ITV1 recording was totally unwatchable. Incidentally, C5 is the weakest
analogue channel too, but I think that is common.

Apart from C5, I think I can link the worst occurrences of poor
reception with heavy rain showers between me and the transmitter.

Just to confuse matters further, I had bad problems with ITV4 which
seemed to be cured simply by moving the receiver/recorder six inches. I
had suspected a SCART connection problem, but I'm now inclined to think
that moving the device has some effect on the RF connection (from the
aerial). But the major variability seems to be independent of the device
position.


Moving it should not affect the reception unless there is something
wrong, usually with the RF plugs or shielding. If the plug is not well
connected to the cable shield that can happen, or if you have a cheap
'accessory' cable in the link to the STB, these normally do not have
proper shielding where the plug join the cable and what you describe is
a classic symptom of this. Also the problems on certain muxes is
indicative.

If you have wall sockets then you probably need to remove the connector
and pass the cable through, making a proper F type join if necessary.

If you have standard 'low loss' TV cable with minimal shielding, again
you may notice problems with positioning, particularly close to other
metal objects (eg metal cased products). Check your RF cable has a foil
shield as well as braid.

Some of the original Pace boxes had problems with earthing inside and
needed a bag of sugar place on top to help the contact with the top cover.

--
Tony
(Co. Down)

In article ,
Jonathan Campbell wrote:
Should I be thinking of a Freesat dish? One problem I can see with that
is that the dish would have to be fitted to the front of the house.

Things will almoist certainly improve after the switchover - the power
is increasing from 800W to 15kW. But that's more than 3 years away.

-- Richard
--
Please remember to mention me / in tapes you leave behind.