We`ve had to have our aerial re-done twice in the last couple of years -=20
and of course, the "warranty" these places offer never seems to be enough=
=20
to get them back when the picture goes... Either that, or you were never=20
given a receipt / can`t find it etc.

I was thinking about changing tactics and fitting a loft aerial that I=20
can fit / adjust myself, but i`m not sure whether I might need to=20
consider a higher spec aerial than might normally be required if it was=20
mounted outside.

Would the "top of the range" screwfix (ref 14374) @ =A325 be overkill for=
=20
the Liverpool area ? (as far as I know, we have a decent signal here)

Any comments appreciated :-}

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Some roof materials block the signal. I tried a VHR aerial in my loft
and off a 5 element got worse signal than bits or wire in the house.

I woudl wonder what happens to an aerial in two years. I would expect
a new aerial and cable to last 5 years unless serious wind/weather.
Bag them under sale of good compalin loudly that goods not fit for
purpose if breakdown in that time.


If your signal is very good try a set top aerial.
The following link is the nearest transmitter details to liverpool

http://www.bbc.co.uk/reception/tv_tr...winterhl.shtml

On Sat, 6 Dec 2003 11:10:00 -0000, Colin Wilson
wrote:


We`ve had to have our aerial re-done twice in the last couple of years -
and of course, the "warranty" these places offer never seems to be enough
to get them back when the picture goes... Either that, or you were never
given a receipt / can`t find it etc.

I was thinking about changing tactics and fitting a loft aerial that I
can fit / adjust myself, but i`m not sure whether I might need to
consider a higher spec aerial than might normally be required if it was
mounted outside.

Would the "top of the range" screwfix (ref 14374) @ £25 be overkill for
the Liverpool area ? (as far as I know, we have a decent signal here)

Any comments appreciated :-}

Lawrence

usenet at lklyne dt co dt uk

Some roof materials block the signal. I tried a VHR aerial in my loft
and off a 5 element got worse signal than bits or wire in the house.

I wasn`t sure how well they worked, hence the question about getting a
higher spec model - looking at the orientation of other aerials along the
terrace, it would almost be pointing at the brick divider between ours
and next doors` too, so i`m not sure how it would work out...

If I pick up a reel of co-ax I suppose I could always mount it at the
back of the house - probably wouldn`t look any worse than the redundant
satellite dish i`ve got sat there :-}

I woudl wonder what happens to an aerial in two years. I would expect
a new aerial and cable to last 5 years unless serious wind/weather.
Bag them under sale of good compalin loudly that goods not fit for
purpose if breakdown in that time.

Got no receipt from the last guy... its proving it... i`ve just had a
quick look and its fallen off the chimney completely

If your signal is very good try a set top aerial.
The following link is the nearest transmitter details to liverpool
http://www.bbc.co.uk/reception/tv_tr...winterhl.shtml

I think Winter Hill is about 30 miles away as the crow flies

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Got no receipt from the last guy... its proving it... i`ve just had a
quick look and its fallen off the chimney completely

Blimey, did'nt know some of the Worcester firms went as far as Liverpool
G

Dave

--
And you were born knowing all about ms windows....??

Colin Wilson wrote in
t:



If your signal is very good try a set top aerial.
The following link is the nearest transmitter details to liverpool
http://www.bbc.co.uk/reception/tv_tr...winterhl.shtml

I think Winter Hill is about 30 miles away as the crow flies


There's no real substitute for suck it and see.

If you can get some information from neighbours it would help a lot;
someone may have tried and failed, or succeeded.

If you can get away with a loft aerial it's an excellent choice, cheaper,
no deterioration, easily fit it yourself, but radio wave transmission is
a blackish art, and only with the right signal strength meter.

You say it would look through a brick wall - that might not be3 so bad,
but is it looking through another 5, 10 in the row.

At my last house, a terrace with Crystal Palace, or is it the other place
now? about 20 miles away we got excellent results with a pretty ornery
loft aerial

mike r

You say it would look through a brick wall - that might not be so bad,
but is it looking through another 5, 10 in the row.

Depending on how close to the eaves it went, it would probably go through
2-3 max

Thanks for the reply :-)

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On Sat, 6 Dec 2003 11:10:00 -0000, Colin Wilson
wrote:


We`ve had to have our aerial re-done twice in the last couple of years -
and of course, the "warranty" these places offer never seems to be enough
to get them back when the picture goes... Either that, or you were never
given a receipt / can`t find it etc.

I was thinking about changing tactics and fitting a loft aerial that I
can fit / adjust myself, but i`m not sure whether I might need to
consider a higher spec aerial than might normally be required if it was
mounted outside.

Would the "top of the range" screwfix (ref 14374) @ £25 be overkill for
the Liverpool area ? (as far as I know, we have a decent signal here)

Any comments appreciated :-}

Any loft antennas, even in strong signal areas are likely to give poor
results. In poor signal areas, the roof attenuates the signal a
great deal, and in stronger areas the effect of reflected signals will
cause ghosting and general fuzziness on analogue transmissions and
high error rates leading to possible loss of picture on digital TV.

That is not to say that people don't have successful results, but by
and large the results are disappointing.


You would be far better off finding an installer who will fit a good
quality antenna like Antiference, Triax, Televes and not a contract
grade unknown, and who will use satellite grade CT100 cable rather
than cheap TV coax, and proper mountings.
He should also have the correct equipment to align the antenna for
optimum results - which does not necessarily mean strongest signal.
Tell the installer that you want a good quality installation with good
quality materials and ask them what they propose to supply.

Apart from in extreme conditions, this should last for 15 years or so.

Expect something like this to cost £120 to £150; not £50. If you
just go for the cheapest price you will get the cheapest job and it
won't last, as you have found.



..andy

To email, substitute .nospam with .gl

Any loft antennas, even in strong signal areas are likely to give poor
results. In poor signal areas, the roof attenuates the signal a
great deal, and in stronger areas the effect of reflected signals will
cause ghosting and general fuzziness on analogue transmissions and
high error rates leading to possible loss of picture on digital TV.

Any idea if the likes of this would be better at avoiding these pitfalls?
http://www.screwfix.com/app/sfd/cat/...16809&id=14374

You would be far better off finding an installer who will fit a good
quality antenna snip

Yeah... know what you mean

He should also have the correct equipment to align the antenna for
optimum results - which does not necessarily mean strongest signal.

The nearest i`ve ever known an installer around here do as far as
alignment is concerned is to point the thing in the same general
direction as everyone elses :-}

Thanks for the reply !

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On Sat, 6 Dec 2003 13:27:12 -0000, Colin Wilson
wrote:

Any loft antennas, even in strong signal areas are likely to give poor
results. In poor signal areas, the roof attenuates the signal a
great deal, and in stronger areas the effect of reflected signals will
cause ghosting and general fuzziness on analogue transmissions and
high error rates leading to possible loss of picture on digital TV.

Any idea if the likes of this would be better at avoiding these pitfalls?
http://www.screwfix.com/app/sfd/cat/...16809&id=14374

It's not really possible to tell from the photo. Some contract
grade antennas are built in this way (with the X configuration of
directors) but other areas like the cable connection arrangement,
which is important, and the general construction are poor.

That's why I suggested going for a known good branded product.

If you look on-line for products like Triax Unix, Antiference DX,
Televes, you will find them in the £25-35 range.

Proper spec, CT100 cable costs from around £25-30 for a 100m reel -
the cheap £10 satellite cable is not as good. You might be able to
buy cable by the metre of course.

Other materials like mounts and brackets will cost about another £5,
so you can do the job in material costs for about £60.

The situation that you have mentioned, going through walls and being
at 30 miles from the transmitter gives me two negative points about a
loft installation.

You could try it, and then if you have problems ask an installer to
put the antenna that you have bought on the roof. In this scenario,
it will have cost more because he won't have made a margin on the
materials but would charge for the time.




You would be far better off finding an installer who will fit a good
quality antenna snip

Yeah... know what you mean

He should also have the correct equipment to align the antenna for
optimum results - which does not necessarily mean strongest signal.

The nearest i`ve ever known an installer around here do as far as
alignment is concerned is to point the thing in the same general
direction as everyone elses :-}


I just took a quick look at yell.co.uk with a search on Aerial
Services and Liverpool and there are the usual range of businesses
with names like Low Cost, ABC and so on - basically to get your
attention or appear near the top of a list.

If you want to find a good installer, one thing to look for is if they
reference that they also do communal, SMATV and commercial work.
Although not a guarantee, this is usually a pointer to an organisation
with some level of competence and the right test equipment.

Unfortunately the proliferation of Sky dish installations has
attracted a lot of cowboys into the business, who do many per day to a
mediochre standard.

In part, it also depends on what you are prepared to accept in terms
of quality of picture.





Thanks for the reply !

..andy

To email, substitute .nospam with .gl

Unfortunately the proliferation of Sky dish installations has
attracted a lot of cowboys into the business, who do many per day to a
mediochre standard.

We had Telewest cable in several years ago, and to test the line was in,
one bloke dropped to all fours and put his tongle across the phone line.
I was shocked - I asked him where his multimeter was, and he said they
weren`t supplied with any.

Would have been great if they`d nicked the electricity service cable in
the process and got 240V down there instead !

This wasn`t a "local" cable installer problem, as a friend 15 miles away
under another Telewest contract area saw the same "test" carried out on
his installation too.

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On Sat, 6 Dec 2003 23:54:23 -0000, Colin Wilson
wrote:

Unfortunately the proliferation of Sky dish installations has
attracted a lot of cowboys into the business, who do many per day to a
mediochre standard.

We had Telewest cable in several years ago, and to test the line was in,
one bloke dropped to all fours and put his tongle across the phone line.
I was shocked - I asked him where his multimeter was, and he said they
weren`t supplied with any.

Would have been great if they`d nicked the electricity service cable in
the process and got 240V down there instead !

This wasn`t a "local" cable installer problem, as a friend 15 miles away
under another Telewest contract area saw the same "test" carried out on
his installation too.

50 volts across his 'tongle', or even tongue, (sorry, couldn't pass
that one up :-) ) plus another 75ac if it rang would be pretty
exillerating..........





..andy

To email, substitute .nospam with .gl

We had Telewest cable in several years ago, and to test the line was in,
one bloke dropped to all fours and put his tongle across the phone line.

LOL I only just noticed my spelling mistake :-} - "tongue" :-}
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Any loft antennas, even in strong signal areas are likely to give
poor
results. In poor signal areas, the roof attenuates the signal a
great deal, and in stronger areas the effect of reflected signals
will
cause ghosting and general fuzziness on analogue transmissions and
high error rates leading to possible loss of picture on digital TV.

I live in an area where the signal is rubbish and we are not allowed
to have external aerials. It was a new house and the builder put in
the wiring for 3 rooms which terminated in the loft. I went to a
reputable installer and they fitted a fairly small aerial but a
whopping big signal booster attached to it and the picture is great.
Cost me £130 which I didn't think was too bad having seen the rubbish
pictures some of my neighbours have. He didn't have to do anywiring
but I guess he used quality products.

Angela

On Sun, 7 Dec 2003 14:35:27 +0000 (UTC), "Angela"
wrote:

Any loft antennas, even in strong signal areas are likely to give
poor
results. In poor signal areas, the roof attenuates the signal a
great deal, and in stronger areas the effect of reflected signals
will
cause ghosting and general fuzziness on analogue transmissions and
high error rates leading to possible loss of picture on digital TV.

I live in an area where the signal is rubbish and we are not allowed
to have external aerials. It was a new house and the builder put in
the wiring for 3 rooms which terminated in the loft. I went to a
reputable installer and they fitted a fairly small aerial but a
whopping big signal booster attached to it and the picture is great.
Cost me £130 which I didn't think was too bad having seen the rubbish
pictures some of my neighbours have. He didn't have to do anywiring
but I guess he used quality products.

Angela


I am glad that you ended up with a good result.

Generally, adding amplification to a poor signal doesn't help because
the amplifier itself introduces its own noise to the signal; and
usually this is worse at higher levels of amplification.

A good quality commercial grade amplifier will have a better noise
figure than a cheap one so should be used when a scenario like this
happens.

Normally though, it is better to install a larger antenna and not need
to add amplification......



..andy

To email, substitute .nospam with .gl

On Sun, 07 Dec 2003 15:45:14 +0000, Andy Hall
wrote:



I am glad that you ended up with a good result.

Generally, adding amplification to a poor signal doesn't help because
the amplifier itself introduces its own noise to the signal; and
usually this is worse at higher levels of amplification.

A good quality commercial grade amplifier will have a better noise
figure than a cheap one so should be used when a scenario like this
happens.


Indeed a cheap bubble pack "Booster" for what? £7.50? Is unlikely to
have a better noise figure than a £300 TV.

Normally though, it is better to install a larger antenna and not need
to add amplification......


Lets agree on a "better" antenna (from a proper manufacturer) rather
than "bigger".

DG

On Sun, 07 Dec 2003 16:07:48 +0000, derek
wrote:

On Sun, 07 Dec 2003 15:45:14 +0000, Andy Hall
wrote:



I am glad that you ended up with a good result.

Generally, adding amplification to a poor signal doesn't help because
the amplifier itself introduces its own noise to the signal; and
usually this is worse at higher levels of amplification.

A good quality commercial grade amplifier will have a better noise
figure than a cheap one so should be used when a scenario like this
happens.


Indeed a cheap bubble pack "Booster" for what? £7.50? Is unlikely to
have a better noise figure than a £300 TV.

Normally though, it is better to install a larger antenna and not need
to add amplification......


Lets agree on a "better" antenna (from a proper manufacturer) rather
than "bigger".

DG

Yes, exactly.




..andy

To email, substitute .nospam with .gl

Andy Hall wrote:

On Sun, 7 Dec 2003 14:35:27 +0000 (UTC), "Angela"
wrote:


Any loft antennas, even in strong signal areas are likely to give

poor

results. In poor signal areas, the roof attenuates the signal a
great deal, and in stronger areas the effect of reflected signals

will

cause ghosting and general fuzziness on analogue transmissions and
high error rates leading to possible loss of picture on digital TV.

I live in an area where the signal is rubbish and we are not allowed
to have external aerials. It was a new house and the builder put in
the wiring for 3 rooms which terminated in the loft. I went to a
reputable installer and they fitted a fairly small aerial but a
whopping big signal booster attached to it and the picture is great.
Cost me £130 which I didn't think was too bad having seen the rubbish
pictures some of my neighbours have. He didn't have to do anywiring
but I guess he used quality products.

Angela



I am glad that you ended up with a good result.

Generally, adding amplification to a poor signal doesn't help because
the amplifier itself introduces its own noise to the signal; and
usually this is worse at higher levels of amplification.

A good quality commercial grade amplifier will have a better noise
figure than a cheap one so should be used when a scenario like this
happens.

Normally though, it is better to install a larger antenna and not need
to add amplification......



Totally agree. Boosting a 40dB S/N ratio signal gives at best - a 40dB
S/N ratio!

You muts have reaosnbable signal strenght to start with. However ofetn
its not height you want, but accurate location and adirectionality -
both of which are as easily achieved with a loft aerial as one up a pole.

Only if you REALLY have to get the height to get any signal at all are
externals indicated.





.andy

To email, substitute .nospam with .gl

On Sun, 07 Dec 2003 19:49:30 +0000, The Natural Philosopher
wrote:

Andy Hall wrote:

On Sun, 7 Dec 2003 14:35:27 +0000 (UTC), "Angela"
wrote:


Any loft antennas, even in strong signal areas are likely to give

poor

results. In poor signal areas, the roof attenuates the signal a
great deal, and in stronger areas the effect of reflected signals

will

cause ghosting and general fuzziness on analogue transmissions and
high error rates leading to possible loss of picture on digital TV.

I live in an area where the signal is rubbish and we are not allowed
to have external aerials. It was a new house and the builder put in
the wiring for 3 rooms which terminated in the loft. I went to a
reputable installer and they fitted a fairly small aerial but a
whopping big signal booster attached to it and the picture is great.
Cost me £130 which I didn't think was too bad having seen the rubbish
pictures some of my neighbours have. He didn't have to do anywiring
but I guess he used quality products.

Angela



I am glad that you ended up with a good result.

Generally, adding amplification to a poor signal doesn't help because
the amplifier itself introduces its own noise to the signal; and
usually this is worse at higher levels of amplification.

A good quality commercial grade amplifier will have a better noise
figure than a cheap one so should be used when a scenario like this
happens.

Normally though, it is better to install a larger antenna and not need
to add amplification......



Totally agree. Boosting a 40dB S/N ratio signal gives at best - a 40dB
S/N ratio!

You muts have reaosnbable signal strenght to start with. However ofetn
its not height you want, but accurate location and adirectionality -
both of which are as easily achieved with a loft aerial as one up a pole.

Only if you REALLY have to get the height to get any signal at all are
externals indicated.





.andy

To email, substitute .nospam with .gl


Another point to consider is that Yagi aerials are high "Q" which
means they can be detuned by nearby solid (especially metal) objects.
So try and mount the aerial as far away from dividing walls, water
tanks etc as possible.


Paul

Colin Wilson wrote:

We`ve had to have our aerial re-done twice in the last couple of years -
and of course, the "warranty" these places offer never seems to be enough
to get them back when the picture goes... Either that, or you were never
given a receipt / can`t find it etc.

I was thinking about changing tactics and fitting a loft aerial that I
can fit / adjust myself, but i`m not sure whether I might need to
consider a higher spec aerial than might normally be required if it was
mounted outside.

Would the "top of the range" screwfix (ref 14374) @ £25 be overkill for
the Liverpool area ? (as far as I know, we have a decent signal here)

Any comments appreciated :-}



Do it.

I have a decent loft aerial mounted as high as possible - I am some 15
miles Line of sight from a not particularly strong transmitter (Sudbury)
and get excellent recption off it, and the best bit is the connections
and aerial are easily accesiible and not corrdng away in the weather.

Get teh bets and largest aerial you can firt. Mine is about 1.5m long,
and just fitrs a few feet below the ridge. Sadly the signal is at right
angles to teh ridge line, or I would have gone longer...it cots nme 11
quid from Tony Sayers favorite Cambridge shop.

Get teh bets and largest aerial you can firt.

The number of elements make a difference though from what I can make
out... the mid-range one has 18 and the top of the range has 43 - i`m not
sure about physical dimensions though

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Colin

My outside aerial was pointing to Winterhill, but when it fell off, I
mounted a loft aerial pointing to the Storeton repeater. The orientation
must be vertical. I get a perfect analogue picture, channel 5 is superb,
digital is fine too.

Andrew

My outside aerial was pointing to Winterhill, but when it fell off, I
mounted a loft aerial pointing to the Storeton repeater. The orientation
must be vertical. I get a perfect analogue picture, channel 5 is superb,
digital is fine too.

Where`s Storeton ? (is that the wirral somewhere ?)

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Where`s Storeton ? (is that the wirral somewhere ?)



Yes, it's a hill in the Bebington area.

On Sat, 6 Dec 2003 14:01:09 -0000, Colin Wilson
wrote:

Get teh bets and largest aerial you can firt.

The number of elements make a difference though from what I can make
out... the mid-range one has 18 and the top of the range has 43 - i`m not
sure about physical dimensions though


Take a good look at the picture in the link you posted. They claim 43
ele's but I count 10 directors (each apparently counted 4 times!) the
dipole and reflector.

That's not strictly Kosher!

The ele's might well be compound elements, but being co-located I
can't see them being "just as good as" a "Bona- Fide" 43 element beam
if such a thing existed, (beyond a certain limit diminishing returns
sets in anyway). Better IMO to use 2 x proper 18 ele. beams and a
phasing cable.

But, I'm not an expert, (I'm an old radio amateur), so take a look
here :

http://www.wrightsaerials.tv/articles.html

FWIW I've had a good experience with loft aerials I'm in an an
elevated position in Leeds LS27 and at one time I was getting good
results from Bilsdale West Moor 40 miles away directly North over flat
countryside, but an aerial for Emley Moor 10 miles away needed to be
higher up since I was looking straight into the hill behind me.


DG

In message , derek
writes

FWIW I've had a good experience with loft aerials I'm in an an
elevated position in Leeds LS27 and at one time I was getting good
results from Bilsdale West Moor 40 miles away directly North over flat
countryside, but an aerial for Emley Moor 10 miles away needed to be
higher up since I was looking straight into the hill behind me.

I'm in Leeds LS16 one of the highest bits of Leeds, we have a direct
line of sight to Emley Moor- maybe 15-20 miles as the crow flies?

When we moved in the aerial was a settop aerial blanched in the loft on
a box. The picture was ok really, but not brilliant. I replaced it with
larger but fairly cheap aerial mounted on the internal wall in the loft-
nothing but a few roof timbers ad the felt and tiles in the way.

Get a decent picture now.
--
Chris French, Leeds

In article ,
Colin Wilson writes:
Get teh bets and largest aerial you can firt.

The number of elements make a difference though from what I can make
out... the mid-range one has 18 and the top of the range has 43 - i`m not
sure about physical dimensions though

In theory, more elements increases the gain on-axis and reduces
it off-axis, so it should be better if you can accurately perform
the more critical alignment it requires.

However, I'm sceptical that many aerials are really designed that
accurately, at least as far as the physics goes. I suspect much of
the design is to 'look' higher quality so they can charge more;-)

--
Andrew Gabriel

Andrew Gabriel wrote:

In article ,
Colin Wilson writes:

Get teh bets and largest aerial you can firt.

The number of elements make a difference though from what I can make
out... the mid-range one has 18 and the top of the range has 43 - i`m not
sure about physical dimensions though


In theory, more elements increases the gain on-axis and reduces
it off-axis, so it should be better if you can accurately perform
the more critical alignment it requires.

However, I'm sceptical that many aerials are really designed that
accurately, at least as far as the physics goes. I suspect much of
the design is to 'look' higher quality so they can charge more;-)



IIRC the number of elements increases teh gain slightly, but the real
gain is in drectionality and freedom from ghosting.

If the transmitter is behind hills/houses you do need LOT of height,
but its easy to check yoiurloft with a set top aerial and a small TV up
there. If you can get reaosnable quality from that, and move around to
find teh best spot, a decent aerial in the same spot will do about 4-10
times better than the set top aerial. 4 stomes stronger and maybe 10
times better on ghosting.

Final alignment is done by ot looking at signal strength, but picture
quality. You want to make sure your main ghosts are coming into the
antenna best rejection lobes, this is usually a few degrees off maxiumum
signal.

In the end you fiddle around with te antenna and a TV in the loft, til
you are pretty sure its as good as you are going to get, then bolt it
down and enjoy whatever you can get.

Frankly it will be better than a quick external installation by a cowboy
rigger in 99 cases out of 100.

Doing it this way also allows you to assess the best place within the
loft. Metal tanks and metal in the house will make big differences, so
its not always obvious where the best place actually is till you try em
all..

but

"Andrew Gabriel" wrote in message ...

In theory, more elements increases the gain on-axis and reduces
it off-axis, so it should be better if you can accurately perform
the more critical alignment it requires.

Overall length is a better guide to gain than the element count, especially
when the vendors cheat and don't count the elements properly.

--
Andy

I was thinking about changing tactics and fitting a loft aerial that I
can fit / adjust myself, but i`m not sure whether I might need to
consider a higher spec aerial than might normally be required if it was
mounted outside.




Take a look at the TLC aerials, they have one high gain version, just make
sure you buy the correct band .... check the IBA site to find out what your
transmitter is - and whether you need to set for horizontal or vertical
polarisation.

Rick

Take a look at the TLC aerials, they have one high gain version, just make
sure you buy the correct band .... check the IBA site to find out what your
transmitter is - and whether you need to set for horizontal or vertical
polarisation.

whoosh ok you lost me :-}

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Colin Wilson wrote in
t:

Take a look at the TLC aerials, they have one high gain version, just
make sure you buy the correct band .... check the IBA site to find
out what your transmitter is - and whether you need to set for
horizontal or vertical polarisation.

whoosh ok you lost me :-}

That means whether it stands up or lies flat, and where it points.

Hint - check the neighbours.

Also if you're buying from a reasonable dealer, he'll be able to tell you.

There are web sites that will, or you could ring bbc information
information, but I've got a suspicion thats gone down the tubes like the
rest of the Beeb.

I can't find a site, but this being the ng that it is, I very much doubt if
we'll wallow in ignorant bliss for long.

mike r

At the moment my aerial gets a reasonable analogue picture, but with
some ghosting. Would I be right in thinking that if I got a digital
set-top box this setup would be more forgiving of ghosting? I think
the signal itself is fairly strong, but there's just a bit of
reflection off nearby trees and things.

On 7 Dec 2003 14:32:09 -0800, (Martin
Pentreath) wrote:

At the moment my aerial gets a reasonable analogue picture, but with
some ghosting. Would I be right in thinking that if I got a digital
set-top box this setup would be more forgiving of ghosting? I think
the signal itself is fairly strong, but there's just a bit of
reflection off nearby trees and things.

Ghosting is the result of signals being reflected from some other
object between the transmitter and your receiving antenna. Because
the signal takes a longer time to reach you (think of it as corners of
a triangle, you get a ghosted image offset to the right on an analogue
picture.

You can roughly work out the extra distance by measuring the offset as
a percentage of the total screen width. On a standard 625line/50Hz
TV (which is what you have, one line takes just under 64 microseconds
in total, but thre is about 53 microseconds worth visible because of
synchronisation time. So your extra delay is roughly the proportion
of the screen for the ghosted image multiplied by 53 uS.

The TV signals travel at approx. the speed of light (299,792,458
metres/sec); so from this you can work out the extra distance.
Generally the transmitter is a lot further from you than the
reflecting object, so to a good approximation, the distance you
calculate is the distance the object is from you.
Sometimes, if the objects are very close, e.g. surrounding walls and
other objects, the ghosted image will be so close that it just appears
to be a fuzziness of the verticals of the picture.

With digital TV, you would not see ghosting directly, since the
digital information is placed on an analogue signal and the decoder
uses that. The decoder is taking the signal and making decisions
millions of times per second (the data bit rate) about whether the
signal is 1 or 0 and uses that. However it is doing that by
sampling the level of the signal at that rate. This works well in
principle, but there are problems with it. Interference to the
signal or poor signal will mislead the decoder about whether given
bits should be 0 or 1. Currently digital TV transmissions use a
very low power in comparison with the analogue TV, which is one of the
reasons why people are having to use better antennas for digital TV.

If you factor in an additional delayed signal (which is what ghosting
on analogue is), the decoder will see it as interference to the wanted
signal. Depending on how strong the delayed signal is relative to
the wanted one, the decoder may make mistakes as it will with
interference.

Up to a point, the decoder can take care of this because the
transmitted digital signal has extra data added that is used to help
the decoder detect and correct such errors.
Beyond a certain rate of errors, these mechanisms fail and the result
is the the picture and sound will freeze or the picture breaks into
small blocks (called macroblocks). Obviously you don't want this.

Therefore, when an antenna is set up for digital reception, it is not
a case of pointing and hoping or looking on the screen for best
picture or minimum ghosting. A proper installer will have a piece
of test gear which measures the rate of errors coming in the signal.
This takes account of all the interference and multiple reception
paths, and generally the objective is to minimise the error rate.

You could try with your existing antenna, but don't be surprised if
you have to get a new, more directional one which will give a stronger
signal and because it is more directional, reject unwanted reflected
signals.





..andy

To email, substitute .nospam with .gl

Andy Hall wrote:

On 7 Dec 2003 14:32:09 -0800, (Martin
Pentreath) wrote:


At the moment my aerial gets a reasonable analogue picture, but with
some ghosting. Would I be right in thinking that if I got a digital
set-top box this setup would be more forgiving of ghosting? I think
the signal itself is fairly strong, but there's just a bit of
reflection off nearby trees and things.


Ghosting is the result of signals being ..



snip long treatise on ghosting to summarise the relevant bits


(i) All digital transmission methids have one theing in common. They
work better at high signals, and tend to stop working altogether on low
ones. They do not, for the most part, degrade gradually and gracefully.

(ii) Andy sez, and I have no reason to douvbt it, that digital signals
are at lower signal strength..

There are two points here. The first is that if the signals are weaker
so are the ghosts. So unless you have almost double images now - in
which case a better antenna pointing in a different direction slightly,
is called for, that ought to work OK.

But the second point is I don't know what the modulation method is. I do
know that it invloves digital compression to get teh data rate down, but
how that data is encoded is not somethig I am aware of - however it
would seem strange indeed if it were done in such a way as to make te
signal peculiarly sensitive to low apmlitude time delayed analogues.
Ghosts. Given the amount of computing power and the bitrate, it would
alos seem that adaptive filtering to detect static ghposts and eliminate
them would in fact be pretty simple. So my guess is that digital signal
qre bothe encoded to elminintae ghost effects, and probably filtered to
remove even more - at leats in better designed sets.

The technology of all this gubbins has come on very fast with mobile
phones - fast and stable chips to do FFT stuff etc exist, and
de-ghosting is pretty much basic algorithims for DSP's.

On Mon, 08 Dec 2003 10:17:44 +0000, The Natural Philosopher
wrote:



But the second point is I don't know what the modulation method is. I do
know that it invloves digital compression to get teh data rate down, but
how that data is encoded is not somethig I am aware of - however it
would seem strange indeed if it were done in such a way as to make te
signal peculiarly sensitive to low apmlitude time delayed analogues.
Ghosts. Given the amount of computing power and the bitrate, it would
alos seem that adaptive filtering to detect static ghposts and eliminate
them would in fact be pretty simple. So my guess is that digital signal
qre bothe encoded to elminintae ghost effects, and probably filtered to
remove even more - at leats in better designed sets.

There is compression anyway (MPEG-2 for the video) and forward error
correction. Modulation is Coded Orthogonal Frequency Division
Multiplexer (COFDM).


More that you ever wanted to know can be found by Google searching
using 'DVB-T' as a keyword.



The technology of all this gubbins has come on very fast with mobile
phones - fast and stable chips to do FFT stuff etc exist, and
de-ghosting is pretty much basic algorithims for DSP's.

..andy

To email, substitute .nospam with .gl

Martin Pentreath wrote:

At the moment my aerial gets a reasonable analogue picture, but with
some ghosting. Would I be right in thinking that if I got a digital
set-top box this setup would be more forgiving of ghosting? I think
the signal itself is fairly strong, but there's just a bit of
reflection off nearby trees and things.


I would have THOUGHT so, but....I don't know for sure.

You may need a slightly different aerial too.

On Mon, 08 Dec 2003 10:06:43 +0000, The Natural Philosopher
wrote:

Martin Pentreath wrote:

At the moment my aerial gets a reasonable analogue picture, but with
some ghosting. Would I be right in thinking that if I got a digital
set-top box this setup would be more forgiving of ghosting? I think
the signal itself is fairly strong, but there's just a bit of
reflection off nearby trees and things.


I would have THOUGHT so, but....I don't know for sure.

You may need a slightly different aerial too.


The critical parameters are the carrier to noise ratio and the error
rate........

..andy

To email, substitute .nospam with .gl

Many thanks for the advice, I guess it's a try-it-and-see situation,
but at least I'm better informed now :-)