Andy Wade wrote:
wrote in message
...

If the picture never breaks up or disappears then the signal is good
enough. It doesn't take measuring equipment to show that.

FSVO "never" (such as "never, ever, ever") perhaps.

The argument applies well for a DIYer even if the picture *does* break
up six months hence. It's no big problem to *then* decide to go up in
the loft (wherever) and improve the aerial, downlead or whatever. As
I said I agree that for a commercial installer this isn't a sensible
approach.


If you are a business installing an aerial then it's a whole
different ball game as you can't afford to prove the installation
is good enough by watching it for a week in varying weather
conditions. So measuring equipment is then useful, however not
nearly so useful/necessary for a DIYer.

A couple of points he firstly, many contributors to this thread have said
that they can get DTT reception, despite the postcode database (which does
err slightly on the cautious side) predicting otherwise. Be aware that

Yes, I'm among them.


signal strength alone is not the only issue. Every UHF channel is re-used
hundreds of times over and coverage is mostly limited by co-channel and
adjacent channel interference from other TV transmitters, rather than by
thermal noise. Now, as many will be aware, UHF propagation is seriously
affected by certain weather conditions which give rise to 'tropospheric
ducting', increasing the signal levels from remote transmitters by large
amounts and causing severe interference[1]. The statistics of this effect
are well-studied and DTT coverage is planned to work for 99% of the time
(c.f. 95% of time for analogue TV). Some of those receiving in areas which
are not officially classed as 'covered' can expect to experience
tropospheric interference for more than 1% of the time.

Yes, so how does that affect my suggestion/comment that 'seeing is
believing'? All I was saying was that for someone who is installing
FreeView (and maybe an aerial) themselves observation of the picture
is a good enough indicator of whether it's working.

If it becomes marginal occasionally due to atmospheric/tropospheric
conditions then one either accepts that it is 'as designed' or one
tries some improvements.


Secondly, in the absence of any test equipment, a simple test of signal
margin is to insert a 3 dB attenuator pad at the receiver input. If this
leads to reception failure then your signal is somewhat marginal.

A good idea. However the on screen diagnostics available with our
Daewoo box are quite good anyway.


OK, but there are not so many areas where the digital MUXs are
transmitted on frequencies a long way away from the analogue ones,

Oh yes there a look at Belmont, Sandy Heath, Wenvoe and Waltham, to pick
four examples of main transmitters which have some of the mux's well outside
the original analogue group.

I did say 'not so many', I didn't say none! :-)


the frequency planning has tried to minimise that.

And has concentrated on population served: thus Crystal Palace, Sutton
Coldfield and Winter Hill get more-or-less in-group DTT channel allocations,
but for the rest of the country it's much more patchy.

Which sort of confirms what I said, for most people a wide band (or
more than one) aerial probably isnn't needed. As it happens, where we
are, served by Sudbury you need a wide[ish] band aerial to get Channel
5 anyway. I think it's Channel 5, I know one of the five analogue
frequencies is a fair way from the others. It's not fully 'wide band'
either but the frequencies spread over more than one letter band.


What I was really trying to say though was that if you can get a
good channel 5 signal then you probably can get good digital
reception with a similar type of aerial, quite likely in most areas
the same aerial.

I think you're just introducing a red herring in suggsting that DTT
reception will be correlated with analogue C5 (which is not even radiated
from many TX sites). In any case very few people upgraded their aerials for
C5, so the existing one will most likely match the original 4-channel plan
group.

.... but as we said above for many people this will be what they need.
We're talking (or I'm talking) about reasonably competant DIYers here,
I'm expecting people to do a little research and check to see what
type of aerial they have and will need. The information is all
available on the 'net and is pretty easy to find and understand.


Is interference a serious problem with digital? Not only that but
does cheap cable *really* let in more noise than expensive cable?
Even at the frequencies involved the mesh of 'cheap' cable must be
pretty well impervious to signal getting onto the downlead, does the
extra aluminium foil of more expensive cable really help in reducing
interference pick up or is it much more to do with reducing
attenuation (or is it even more to do with selling soemthing
expensive unnecessarily)?

No, it's not a con. What Andy L was talking about there is 'impulsive
interference' - interference from sparking contacts - to which DTT is quite
sensitive. Impulses propagate on, and are radiated from, mains wiring, and
thus are much stronger in the house and loft than above the roof (which
provides some screening). A well-screened coax downlead from antenna to
receiver will largely prevent impulses entering the signal path. With poor
screening, a coupling mechanism exits to allow impulse energy into the
receiver, resulting in the familiar momentary 'freezes' and clicks.

Using well screened coax is necessary, but not sufficient. Any gaps in the
screening will allow the interference to get in, and poorly screened outlet
plates and flyleads are the other betes noir. At the other end of the coax,
the lack of a balun on the aerial will also provide a coupling mechnanism.

I've never seen a balanced feeder aerial input on any domestic TV
(or set top box) here in the UK, are you saying that the internal
bodge (which is probably what it is) can pick up impulse interference?
This is true enough but it's not the sort of thing that most people
are willing to do anything about as it would involve
digging around inside the FreeView box. Or are you saying something
else and I've misundestood?

This is all sound theory, and has been verified experimentally in tests done
under controlled conditions. Oh, and an aerial in the loft is likely to be
close to mains wiring, so is asking for trouble.

With a really well-screened system and a decent receiver you can get solid
reception with signal levels down to 35 dBuV, even on the 64-QAM muxes
(ITV/C4 and SDN). With grotty brown open-weave coax and a typical outlet
and flylead, you may need 45 or even 50 dBuV before the signal is strong
enough to swamp the impulse interference.

Thanks for the details re. impulse interference, it suggests that it
might be worth improving my downleads though that isn't an easy or
cheap option as they're "in the walls" as it were. It might actually
be cheaper (for me) to improve the distrubution amplifier in the loft
and make very sure that the lead from the aerial to the amplifier is as
near perfect as possible.

--
Chris Green

In article ,
wrote:
I'd be most surprised if the VCR looped through RGB. And has your set
two RGB SCARTS?

Not sure about the first, as the VCR (very recently bought Panasonic) has
naff-all documentation. It's vaguely plausible that it does, as I've told
the Freeview box to push out RGB rather than composite video, and there's
a signal visible on the TV; however it's also possible that the menu lies
and the RGB is an "also" rather than an "either".

The easy way to check is by slowly removing/connecting one of the SCART
cables at an angle - you can usually make one of the RGB feeds break up
first.

It's often difficult to tell - there isn't always a vast difference
depending on programme material.

But if the VCR is looping through RGB, I'd expect it to also have to
convert RGB internally as well for recording purposes. Now some Philips
ones do or did, but it's not common.

An RGB SCART also carries composite in and out. It's a devilish system to
get your head round at times.

I built an RGB DA to feed the ONDodgy box to the set in the kitchen as
well as the main set. The STB obviously is powered all the time, but the
DA is powered off my Audio/TV supply that all switches from the AV amp
on/off. If I power down the DA, and leave everything else on, the kitchen
set reverts to composite off the SCART cable since this is looped straight
through - it's only supplying the syncs so I didn't bother with 'DAing'
it. And often you can't tell the difference. Seemed like a good idea at
the time...

--
*Windows will never cease *

Dave Plowman London SW 12
RIP Acorn

"IMM" wrote in message
...

"Owain" wrote in message
...

There's nothing to stop you asking for something to be brought out
before
you buy it in Argos; a lady was giving a CD/radio an exhaustive
going-over
whilst I was in the queue behind her the other day.

They don't have too.

How would that work for wrapping? Surely the next customer (if it's
rejected) won't be happy about getting a box that's clearly been opened?

I always assumed that was _why_ they had to follow distance-selling rules.
Non-catalogue shops have the article on display, or at least will have a
store model. Argos don't have a lot of floor space, which only really gets
used for promotions.

F.

"R W" wrote in message
...

Just buy it online or by telephone order, then you're covered by the
distance selling regulations that apply to any retailer. You can then
return it within 7 days for any reason you like.

Tiny niggle, but I thought 28 days was the magic number?

F.

"tony sayer" wrote in message
...

Now there is a school of thought with digital transmission that sez
either it works or doesn't which isn't exactly true there is a halfway
stage with the picture lockup and blocking.


There is no halfway stage. If you get lockup and blocking at all, then it's
not working!! ;-)

The data has degenerated to the point where it is useless, and the TV loses
it.

F.

"Fraser" wrote in message
...

"IMM" wrote in message
...

"Owain" wrote in message
...

There's nothing to stop you asking for something to be brought out
before
you buy it in Argos; a lady was giving a CD/radio an exhaustive
going-over
whilst I was in the queue behind her the other day.

They don't have too.

How would that work for wrapping? Surely the next customer (if it's
rejected) won't be happy about getting a box that's clearly been opened?

I always assumed that was _why_ they had to follow distance-selling rules.
Non-catalogue shops have the article on display, or at least will have a
store model. Argos don't have a lot of floor space, which only really gets
used for promotions.

Argos don't have to show you the goods.


---
--

Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.558 / Virus Database: 350 - Release Date: 02/01/2004

In article , Fraser
writes

"tony sayer" wrote in message
...

Now there is a school of thought with digital transmission that sez
either it works or doesn't which isn't exactly true there is a halfway
stage with the picture lockup and blocking.


There is no halfway stage. If you get lockup and blocking at all, then it's
not working!! ;-)

The data has degenerated to the point where it is useless, and the TV loses
it.

F.



Humm...a lot of people in the broadcast industry would disagree with
that statement. Still, its an opinion and its mine:--)))
--
Tony Sayer

On Thu, 8 Jan 2004 08:53:51 +0000, "Andy Luckman (AJL Electronics)"
wrote:

In article , Niall
wrote:


So if it happens, change the cable. But freeview *can* work perfectly
well without an enormous aerial array and CT100 all over the place.
If you have a reasonable analogue picture, it will probably work fine.

Believe whatever makes you happy. I can only give you the benefit of 16
years in the trade and thousands of digital installations carried out.

I believe that my Freeview installation works *perfectly*. I believe
this because I can see it with my own eyes.

You are telling people that they need to replace their aerial and coax
before it will work. This is not neccesarily true. It is not true in
my case.

I am saying why not try it first? If results are disappointing, yes,
you need CT100, gold plated connectors and an enormous outside aerial.
But you may not.

I also understand why you will always quote for this when asked for a
freeview installation. This is not a criticism. But it's not
applicable for advice to this group.

--
Niall

"Niall" wrote in message
...
On Thu, 8 Jan 2004 08:53:51 +0000, "Andy Luckman (AJL Electronics)"
wrote:

In article , Niall
wrote:


So if it happens, change the cable. But freeview *can* work perfectly
well without an enormous aerial array and CT100 all over the place.
If you have a reasonable analogue picture, it will probably work fine.

Believe whatever makes you happy. I can only give you the benefit of 16
years in the trade and thousands of digital installations carried out.

I believe that my Freeview installation works *perfectly*. I believe
this because I can see it with my own eyes.

You are telling people that they need to replace their aerial and coax
before it will work. This is not neccesarily true. It is not true in
my case.

I am saying why not try it first? If results are disappointing, yes,
you need CT100, gold plated connectors and an enormous outside aerial.
But you may not.

My Freeview set up also works perfectly. I didn't expect it to, as the
aerial (roof) cable is rather manky with some holes in it (!). I was quite
happy that it might not work but knew I had a strong signal, so I tried it.
It has never failed or dropped signal and the quality is always excellent.
We also have a TV up in the loft conversion, which I use with a cheap multi
element aerial in the loft. The signal here is not very good. Here the
Freeview box struggles and the signal drops out occasionally.

I would agree with Niall that signal strength is the most important factor
and that, although one should not expect a Freeview box to work with the
existing aerial/cable set up, if a strong signal exists, it's worth a try.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

I also understand why you will always quote for this when asked for a
freeview installation. This is not a criticism. But it's not
applicable for advice to this group.

--
Niall

Bob Mannix wrote:


I would agree with Niall that signal strength is the most important factor
and that, although one should not expect a Freeview box to work with the
existing aerial/cable set up, if a strong signal exists, it's worth a try.




Signal strength relative to interference is the ONLY important factor
actually.

All the aerial does is collect and selectively amplify the wanted signals

whilst not selectively amplifying unwanted signals.



If you have enough of the wnated signals, you don't need an aerial at
all :-)

On Mon, 12 Jan 2004 09:24:53 -0000, "Bob Mannix"
wrote:

My Freeview set up also works perfectly. I didn't expect it to, as the
aerial (roof) cable is rather manky with some holes in it (!). I was quite
happy that it might not work but knew I had a strong signal, so I tried it.
It has never failed or dropped signal and the quality is always excellent.
We also have a TV up in the loft conversion, which I use with a cheap multi
element aerial in the loft. The signal here is not very good. Here the
Freeview box struggles and the signal drops out occasionally.

I would agree with Niall that signal strength is the most important factor
and that, although one should not expect a Freeview box to work with the
existing aerial/cable set up, if a strong signal exists, it's worth a try.

With digital TV, the error rate is the most important thing. If the
signal is strong, so can locally reflected signals and these, along
with interference pickup, all result in data errors seen at the
receiver. Discontinuities in cables and poor connectors can lead
to signal reflections in the cable and that will also lead to errors.

Signal strength is also important so that the receiver can
differentiate the data correctly, but once the required threshold is
exceeded, unlike analogue this does not necessarily result in good
reception.

To a point, the transmission system and the receiver can handle and
correct the errors, but they can lead to unpredictable results.

This is not to say that trying an existing arrangement is a bad thing
for the DIYer, but clearly for a professional it would be stupid to
use anything other than good quality cables and antennas. For the
DIYer installing a new reception system it also makes sense to use a
good quality antenna (not a contract grade one) and good CT100 cable.
Like other things, you can either do a job properly or you can bodge
it. Typically if you bodge it, you will have to do it again.


..andy

To email, substitute .nospam with .gl

On Mon, 12 Jan 2004 10:20:43 +0000, The Natural Philosopher
wrote:

Bob Mannix wrote:


I would agree with Niall that signal strength is the most important factor
and that, although one should not expect a Freeview box to work with the
existing aerial/cable set up, if a strong signal exists, it's worth a try.




Signal strength relative to interference is the ONLY important factor
actually.

All the aerial does is collect and selectively amplify the wanted signals

whilst not selectively amplifying unwanted signals.



If you have enough of the wnated signals, you don't need an aerial at
all :-)


That's true if you count reflected signals as interference.....





..andy

To email, substitute .nospam with .gl

In article , Niall
writes
On Thu, 8 Jan 2004 08:53:51 +0000, "Andy Luckman (AJL Electronics)"
wrote:

In article , Niall
wrote:


So if it happens, change the cable. But freeview *can* work perfectly
well without an enormous aerial array and CT100 all over the place.
If you have a reasonable analogue picture, it will probably work fine.

Believe whatever makes you happy. I can only give you the benefit of 16
years in the trade and thousands of digital installations carried out.

I believe that my Freeview installation works *perfectly*. I believe
this because I can see it with my own eyes.

You are telling people that they need to replace their aerial and coax
before it will work. This is not neccesarily true. It is not true in
my case.


Well is this how you look at the rest of your DIY projects?...

I am saying why not try it first? If results are disappointing, yes,
you need CT100, gold plated connectors and an enormous outside aerial.
But you may not.

I also understand why you will always quote for this when asked for a
freeview installation. This is not a criticism. But it's not
applicable for advice to this group.

How much do you spend on your TV?, isn't it worthwhile spending a bit
more to ensure it works properly?...
--
Tony Sayer

Andy Hall wrote:

On Mon, 12 Jan 2004 10:20:43 +0000, The Natural Philosopher
wrote:


Bob Mannix wrote:



I would agree with Niall that signal strength is the most important factor
and that, although one should not expect a Freeview box to work with the
existing aerial/cable set up, if a strong signal exists, it's worth a try.




Signal strength relative to interference is the ONLY important factor
actually.

All the aerial does is collect and selectively amplify the wanted signals

whilst not selectively amplifying unwanted signals.


If you have enough of the wnated signals, you don't need an aerial at
all :-)



That's true if you count reflected signals as interference.....


Thats why I mentioned unwanted signals. Those are intereference.







.andy

To email, substitute .nospam with .gl

wrote in message
...
Andy Wade wrote:

signal strength alone is not the only issue. Every UHF channel is
re-used hundreds of times over and coverage is mostly limited by
co-channel and adjacent channel interference from other TV
transmitters, rather than by thermal noise. Now, as many will be
aware, UHF propagation is seriously affected by certain weather
conditions which give rise to 'tropospheric ducting', increasing
the signal levels from remote transmitters by large amounts and
causing severe interference[1]. The statistics of this effect
are well-studied and DTT coverage is planned to work for 99% of
the time (c.f. 95% of time for analogue TV). Some of those
receiving in areas which are not officially classed as 'covered'
can expect to experience tropospheric interference for more than
1% of the time.

Yes, so how does that affect my suggestion/comment that 'seeing is
believing'? All I was saying was that for someone who is installing
FreeView (and maybe an aerial) themselves observation of the picture
is a good enough indicator of whether it's working.

A few day's observation under normal conditions isn't necessarily a good
indicator of whether it will continue to work 'as planned' all year round...

If it becomes marginal occasionally due to atmospheric/tropospheric
conditions then one either accepts that it is 'as designed' or one
tries some improvements.

.... OK, but how do you know that you're not suffering more loss of service
than necessary? Most people won't have a clue what's happening when the
screen goes blank; at least with analogue you see what's happening and most
can recognise "interference" in a general sort of way.

Secondly, in the absence of any test equipment, a simple test of
signal margin is to insert a 3 dB attenuator pad at the receiver
input. If this leads to reception failure then your signal is
somewhat marginal.

A good idea. However the on screen diagnostics available with our
Daewoo box are quite good anyway.

Yes, nice features in the SetPal design.

snip
And has concentrated on population served: thus Crystal Palace,
Sutton Coldfield and Winter Hill get more-or-less in-group DTT
channel allocations, but for the rest of the country it's much
more patchy.

Which sort of confirms what I said, for most people a wide band (or
more than one) aerial probably isnn't needed.

That may be true, numerically, but could be construed as misleading to those
who do need a wide(er) bandwidth array. IOW you must look at the data for
_your_ local TX; what a majority do or don't need, nation-wide, is
irrelevant. But having said that, note that the government (in the form of
the DTI Digital Action Plan) trying to encourage the widespread adoption of
wideband (Group W) aerials in order that people don't have to replace them
/again/ when the time for 'digital switchover' (a euphemism for analogue
switch-off) comes. Switchover will bring a big re-shuffle of channel
allocations as parts of the band are re-assigned. (The most likely
scenario, at the moment is that channels 31-40 and 63-68 will be 'lost' -
but this is definitely subject to change.)

As it happens, where we are, served by Sudbury you need a wide[ish]
band aerial to get Channel 5 anyway. I think it's Channel 5, I know
one of the five analogue frequencies is a fair way from the others.
It's not fully 'wide band' either but the frequencies spread over
more than one letter band.

C1-C4 analogue from Sudbury are on ch. 41-51; C5 is on ch. 35 at -7dB. The
existing Group B aerial should fare reasonably well on ch. 35, but the lower
ERP will mean noisy pictures for many. The DTT muxes span ch. 39-68 and I
think you'd struggle to get the ITV/C4 mux on ch. 68 with a Group B aerial
unless you're in a very strong signal area. Remember that the performance
of Yagi-type aerials falls off much more rapidly on the HF side of the
design bandwidth than on the LF side.

snip
I'm expecting people to do a little research and check to see what
type of aerial they have and will need. The information is all
available on the 'net and is pretty easy to find and understand.

I had problems at first finding the former ITC DTT TX pages on the new Ofcom
site. The new URL is
http://www.ofcom.org.uk/static/recep..._guide/index.a
sp-region=show_all.html.

snip
Using well screened coax is necessary, but not sufficient. Any gaps
in the screening will allow the interference to get in, and poorly
screened outlet plates and flyleads are the other betes noir. At
the other end of the coax, the lack of a balun on the aerial will
also provide a coupling mechnanism.

I've never seen a balanced feeder aerial input on any domestic TV
(or set top box) here in the UK, are you saying that the internal
bodge (which is probably what it is) can pick up impulse
interference?

Uh? Where did I mention needing a balanced i/p on the receiver? Some
pre-war Band-I 405-line TVs had balanced-twin inputs, OOI, but coax has been
univseral since the late 40's (except for VHF TV in the US).

This is true enough but it's not the sort of thing that most people
are willing to do anything about as it would involve
digging around inside the FreeView box. Or are you saying something
else and I've misundestood?

You've misunderstood. The point was that, ideally, you should have a good
balun on the antenna - otherwise RF interference picked up on the outside of
the coax feeder will find its way into the signal path, even if there are no
'leaks' downstream.

Thanks for the details re. impulse interference, it suggests that it
might be worth improving my downleads though that isn't an easy or
cheap option as they're "in the walls" as it were. It might actually
be cheaper (for me) to improve the distrubution amplifier in the loft
and make very sure that the lead from the aerial to the amplifier is
as near perfect as possible.

Yes, that's a good strategy.

--
Andy

"Andy Hall" wrote in message
...

With digital TV, the error rate is the most important thing. If the
signal is strong, so can locally reflected signals and these, along
with interference pickup, all result in data errors seen at the
receiver. Discontinuities in cables and poor connectors can lead
to signal reflections in the cable and that will also lead to errors.

Andy, you need to read up on the basics of COFDM modulation (as used for
DVB-T and DAB). Its whole raison d'être is to provide a multipath reflection
tolerant system. It works by dividing up a fast data stream into a large
number of much slower ones, so that the likely range of echo delays becomes
less than the symbol period. A guard interval is inserted between symbols,
to allow the reflections time to catch up, so to speak. Short-delayed
echoes, up to the guard interval for the system variant in use, don't lead
to errors. In fact they contribute usefully to the received signal power.
The guard interval for the UK DTT implementation is 7 microseconds.
Longer-delayed echoes will cause intersymbol interference, and degrade the
BER, as you'd expect, but tend to be less common, and weaker.

--
Andy

In uk.d-i-y, Andy Wade wrote:

............................ A guard interval is inserted between symbols,
to allow the reflections time to catch up, so to speak. Short-delayed
echoes, up to the guard interval for the system variant in use, don't lead
to errors. In fact they contribute usefully to the received signal power.
The guard interval for the UK DTT implementation is 7 microseconds.
Longer-delayed echoes will cause intersymbol interference, and degrade the
BER, as you'd expect, but tend to be less common, and weaker.

Cool - Better Living Through Mathematics ;-) At those sorts of times (7us),
presumably the intention is to provide robustness to "near" reflections,
e.g. off big buildings close by - since, as we're taught by the Damestress
Grace Hopper herself, a foot is a nanosecond, a microsec is 1000 feet and
7 of them makes 7000 feet which is NADI a mile-and-half, or a couple of
them newfangled keel-o-meters. So it's good for multipath suppression for
different signal paths from the same transmitter; not designed at all,
presumably, for interference between competing transmitters (whose distance
difference at nearly all receiving locations would be well over this
mile-and-half); and unless you're bizarrely unlucky, the difference in
received power would also be considerable; and you presumably monkey with
the chosen transmitter freqs (both now and when we do the Great Switch Off)
so that you avoid transmitting on the same sort of freq at similar received
power anywhere in the intended reception area.

Is that about right, or should I really be asleep by now? ;-)

G'night all - Stefek

On Tue, 13 Jan 2004 00:55:27 -0000, "Andy Wade"
wrote:

"Andy Hall" wrote in message
.. .

With digital TV, the error rate is the most important thing. If the
signal is strong, so can locally reflected signals and these, along
with interference pickup, all result in data errors seen at the
receiver. Discontinuities in cables and poor connectors can lead
to signal reflections in the cable and that will also lead to errors.

Andy, you need to read up on the basics of COFDM modulation (as used for
DVB-T and DAB). Its whole raison d'être is to provide a multipath reflection
tolerant system. It works by dividing up a fast data stream into a large
number of much slower ones, so that the likely range of echo delays becomes
less than the symbol period. A guard interval is inserted between symbols,
to allow the reflections time to catch up, so to speak. Short-delayed
echoes, up to the guard interval for the system variant in use, don't lead
to errors. In fact they contribute usefully to the received signal power.
The guard interval for the UK DTT implementation is 7 microseconds.
Longer-delayed echoes will cause intersymbol interference, and degrade the
BER, as you'd expect, but tend to be less common, and weaker.

I am familiar with the basics of COFDM as well as the rival U.S.
system and have read the papers by Stott and others.

While COFDM should give good multipath performance, the code rate and
the guard intervals mean a compromise between the data rate and signal
robustness. I understand that there are various issues around the UK
system because of the limitations imposed by the current transmission
arrangements, but I haven't looked at that in depth.

In practice I have seen cases where the bit error rate is noticably
worse when cabling is poor or the antenna is not aligned correctly and
there is visual evidence of multipath reception on analogue.

I suspect, therefore, that as Stott admits in at least one of his
papers, while one can model some aspects of COFDM mathematically, some
issues can only be determined experimentally. It would appear that
the practicality may not be quite what the theory predicts.

DAB suffers in practice from limitations arising from the broadcasters
trying to squeeze more into the spectrum than will give the best
results. I haven't looked at the detail, but it would appear that
DVB-T has some issues as well.




..andy

To email, substitute .nospam with .gl

On 13 Jan 2004 01:12:58 GMT, wrote:

In uk.d-i-y, Andy Wade wrote:

............................ A guard interval is inserted between symbols,
to allow the reflections time to catch up, so to speak. Short-delayed
echoes, up to the guard interval for the system variant in use, don't lead
to errors. In fact they contribute usefully to the received signal power.
The guard interval for the UK DTT implementation is 7 microseconds.
Longer-delayed echoes will cause intersymbol interference, and degrade the
BER, as you'd expect, but tend to be less common, and weaker.

Cool - Better Living Through Mathematics ;-)

Only if you use one of those reverse wotsit calculators. Never could
get the hang of them.

At those sorts of times (7us),
presumably the intention is to provide robustness to "near" reflections,
e.g. off big buildings close by - since, as we're taught by the Damestress
Grace Hopper herself, a foot is a nanosecond, a microsec is 1000 feet and
7 of them makes 7000 feet which is NADI a mile-and-half, or a couple of
them newfangled keel-o-meters. So it's good for multipath suppression for
different signal paths from the same transmitter; not designed at all,
presumably, for interference between competing transmitters (whose distance
difference at nearly all receiving locations would be well over this
mile-and-half); and unless you're bizarrely unlucky, the difference in
received power would also be considerable; and you presumably monkey with
the chosen transmitter freqs (both now and when we do the Great Switch Off)
so that you avoid transmitting on the same sort of freq at similar received
power anywhere in the intended reception area.

There's a whole discussion on single frequency networks (or not) and
the implications thereof.

What may be nice to do in theory, may not work out in practice so
easily because of the installed base of reception equipment etc.,
especially when coverage targets (a sensitive issue) are considered.



Is that about right, or should I really be asleep by now? ;-)

G'night all - Stefek

..andy

To email, substitute .nospam with .gl

Andy Wade wrote:
wrote in message
Yes, so how does that affect my suggestion/comment that 'seeing is
believing'? All I was saying was that for someone who is installing
FreeView (and maybe an aerial) themselves observation of the picture
is a good enough indicator of whether it's working.

A few day's observation under normal conditions isn't necessarily a good
indicator of whether it will continue to work 'as planned' all year round...

No, and *if* problems appear after six months then one can make some
small improvements. As I keep saying it's a non-event to have to do
this if it's a DIY installation and I have also said that a commercial
installer *can't* do it.


If it becomes marginal occasionally due to atmospheric/tropospheric
conditions then one either accepts that it is 'as designed' or one
tries some improvements.

... OK, but how do you know that you're not suffering more loss of service
than necessary? Most people won't have a clue what's happening when the
screen goes blank; at least with analogue you see what's happening and most
can recognise "interference" in a general sort of way.

If the screen 'goes blank' then (from my point of view anyway) that's
worse than acceptable. In reality 'interference' on a digital signal
tends to produce artefacts which are recognised by the viewer as
deterioration in the signal.


Using well screened coax is necessary, but not sufficient. Any gaps
in the screening will allow the interference to get in, and poorly
screened outlet plates and flyleads are the other betes noir. At
the other end of the coax, the lack of a balun on the aerial will
also provide a coupling mechnanism.

I've never seen a balanced feeder aerial input on any domestic TV
(or set top box) here in the UK, are you saying that the internal
bodge (which is probably what it is) can pick up impulse
interference?

Uh? Where did I mention needing a balanced i/p on the receiver? Some
pre-war Band-I 405-line TVs had balanced-twin inputs, OOI, but coax has been
univseral since the late 40's (except for VHF TV in the US).

You said "... end of the coax, the lack of a balun on the aerial ...",
are you then saying that there should be a balun at the aerial end of
the coax? I thought that most aerials have a natural impedance of
around 75 ohms which is why 75 ohm co-ax is used. Are you just saying
that there should be a 75 ohm to 75 ohm (as opposed to imedance
matching) balun at the aerial because the aerial is balanced and the
coax isn't? Will this actually provide much improvement? If so it's
a fairly simple and cheap thing to do.


This is true enough but it's not the sort of thing that most people
are willing to do anything about as it would involve
digging around inside the FreeView box. Or are you saying something
else and I've misundestood?

You've misunderstood. The point was that, ideally, you should have a good
balun on the antenna - otherwise RF interference picked up on the outside of
the coax feeder will find its way into the signal path, even if there are no
'leaks' downstream.

Ah, OK, the above *is* what you mean. Can one get ready made 75ohm to
75ohm baluns?

--
Chris Green

"Dave Plowman" wrote in message ...

[Detecting whether the TV is using an RGB feed]

The easy way to check is by slowly removing/connecting one of the
SCART cables at an angle - you can usually make one of the RGB feeds
break up first.

There's an easier way than that: if the TV is using RGB then its colour
(saturation) control will have no effect. If the picture goes to black and
white when you turn the colour right down, the interface must be either
composite or Y/C, not RGB.

--
Andy

wrote in message
...

If the screen 'goes blank' then (from my point of view anyway) that's
worse than acceptable. In reality 'interference' on a digital signal
tends to produce artefacts which are recognised by the viewer as
deterioration in the signal.

That's true of impulsive interference or other short-term effects, but
tropospheric co-channel interference (which was the context) will tend to
last for minutes or hours and if its strong enough to push you over the
'digital cliff' then you will get a blank screen (or frozen frame, or BSofD,
depending on the decoder).

You said "... end of the coax, the lack of a balun on the
aerial ...",
are you then saying that there should be a balun at the aerial end of
the coax? I thought that most aerials have a natural impedance of
around 75 ohms which is why 75 ohm co-ax is used.

A half-wave dipole has a 'natural impedance' of 75 ohms and that's what gave
rise to 75 ohm coax originally. 75 ohms is also the impedance at which an
air-spaced cable of a given o/d has minimum loss (c.f. 50 ohms for a solid
polyethylene dielectric cable).

A TV Yagi doesn't have a natural impedance of anything in particular since
adding reflector and directors tends to lower the feedpoint impedance; in
particular the spacing between the driven element and the first director can
have a dramatic effect. The design of TV antennas is still a largely
empirical black art (despite some impact from CEM techniques) and involves a
juggling act between impedance, gain and bandwidth. Just optimising for
maximum gain would give narrow bandwidth and an impractically low feedpoint
impedance.

Are you just saying that there should be a 75 ohm to 75 ohm (as
opposed to imedance matching) balun at the aerial because the aerial
is balanced and the coax isn't?

Almost -- I'm saying that there should be a balun at the aerial feedpoint
because the aerial is balanced and the coax isn't. The balun's impedance
ratio could be 1:1, or 4:1 or something else and that's a matter for the
designer that doesn't particulary concern the user.

Will this actually provide much improvement? If so it's
a fairly simple and cheap thing to do.

Yes, it provides a demonstrable improvement in immunity to impulsive
interference, especially if the downlead goes through the loft near to mains
wiring. To pass the CAI & DTG's 'benchmark' tests an antenna has to pass a
'feeder pickup' test, whcich amounts to a test of the effectiveness of the
balun. See
http://www.dtg.org.uk/publications/b...rk_aerials.pdf
for further details.

--
Andy

wrote in message
...

Cool - Better Living Through Mathematics ;-)

I like that. Lots of "throbbing DSPs" in a DTT receiver, to use a phrase
coined by an acquaintance of mine, who posts here occasionally.

IFFT for the COFDM demod, soft-decision Viterbi and Reed-solomon FEC
decoding & then MPEG decoding. Try doing all that with valves (he said,
being half-way through reading Gerogina Ferry's book about LEO).

At those sorts of times (7us), presumably the intention is to provide
robustness to "near" reflections, e.g. off big buildings close by -

Yes.

since, as we're taught by the Damestress Grace Hopper herself, a foot
is a nanosecond,

Interesting - I think I first heard "one foot per ns" from somebody at
Mullard; I didn't know there was connection with Ms Hopper. Didn't she
invent the concept of the compiler?

them newfangled keel-o-meters. So it's good for multipath suppression
for different signal paths from the same transmitter; not designed at
all, presumably, for interference between competing transmitters

That's how its currently used for UK DTT. Another application is the
'single frequency network' concept used for the national DAB multiplexes.
This depends on having precisely synchronised transmitters (done using GPS
for the timing, I think) all radiating the same stream, and, obviously, a
longer guard interval -- 246us for Mode 1 DAB (the mode used on Band III).
From the receiver's point of view there's no difference between a reflected
signal and one from another synchronous TX. Clever innit?

--
Andy

"Andy Hall" wrote in message
...

I am familiar with the basics of COFDM as well as the rival U.S.
system and have read the papers by Stott and others.

OK, sorry, but that wasn't clear from what you said.

While COFDM should give good multipath performance, the code rate and
the guard intervals mean a compromise between the data rate and
signal robustness.

It should and does give good multipath performance, up to (a little over)
the guard interval. As we're not using SFNs for TV the guard interval
fraction has been set to the lowest DVB option (1/32) to minimise the loss
of capacity.

I understand that there are various issues around the UK
system because of the limitations imposed by the current transmission
arrangements, but I haven't looked at that in depth.

ISTM that you're thinking more here of the choice of modulation
constellation and code rate. In 1998 DTT launched with 64-QAM modulation &
2/3 code rate, giving about 24Mb/s net data rate. Following the demise of
ITV Digital the BBC and Crown Castle decided to opt for 16-QAM to increase
robustness, but to reduce the code rate to 3/4 to claw back some of the lost
capacity. This combination gives about a 4 dB advantage in a gaussian
channel. So we now have a hybrid situation with the BBC & CCI running
16-QAM 3/4 (~18Mb/s) but with the D3&4 mux (ITV, C4, etc.) and SDN (C5, S4C,
etc.) still running 64-QAM 3/4.

In practice I have seen cases where the bit error rate is noticably
worse when cabling is poor or the antenna is not aligned correctly
and there is visual evidence of multipath reception on analogue.

The higher BER there might just be due to the signal being weaker, of
course, or to long-delayed echoes.

I suspect, therefore, that as Stott admits in at least one of his
papers, while one can model some aspects of COFDM mathematically,
some issues can only be determined experimentally. It would appear
that the practicality may not be quite what the theory predicts.

I think most aspects of it are now pretty well understood and measurements
agree well with theory. The performance of different receivers in the
presence of long-delayed echoes does vary though, and depends (IIRC)
somewhat on the Viterbi implementation. Unless you can be more specific
about which of Jonathan Stott's papers you're referring to I can't really
add any more.

DAB suffers in practice from limitations arising from the
broadcasters trying to squeeze more into the spectrum than will give
the best results.

That's a completely separate issue, not COFDM related. There isn't, AFAIK,
the same flexibility as there is in DVB-T to change the transmission
parameters to trade off capacity and signal robustness.

--
Andy

In uk.d-i-y, Andy Wade wrote:

Interesting - I think I first heard "one foot per ns" from somebody at
Mullard; I didn't know there was connection with Ms Hopper. Didn't she
invent the concept of the compiler?

I don't think she's the one we thank for compilers: early FORTRAN beat
her to that punch. But we do have her to thank for COBOL (I presume you
know the one about its Stroustrup-analogue object-oriented follow-on,
ADD ONE TO COBOL. ?) And the story goes she used to enliven her public
talks by handing out 'nanoseconds' - one-foot lengths of copper wire.

That's how its currently used for UK DTT. Another application is the
'single frequency network' concept used for the national DAB multiplexes.
This depends on having precisely synchronised transmitters (done using GPS
for the timing, I think) all radiating the same stream, and, obviously, a
longer guard interval -- 246us for Mode 1 DAB (the mode used on Band III).
From the receiver's point of view there's no difference between a reflected
signal and one from another synchronous TX. Clever innit?

You've answered the follow-up question I stopped myself from asking! Couldn't
immediately see how the DAB network would cope given single-frequency
transmission with the maths working out as for Freeview, but wasn't
curious enough to either think through the obvious answer of "much longer
guard interval", or to ask. And here it comes, unasked for but no less
gratefully received ;-)

Cheers, Stefek

Andy Wade wrote:



Interesting - I think I first heard "one foot per ns" from somebody at
Mullard; I didn't know there was connection with Ms Hopper. Didn't she
invent the concept of the compiler?



Well it was 3ns a meter by the time I got into a proper electronics lab.

We even took a 100meter reel of cable, and shoved a pulse generator on
it to check

The reflection did take about 600ns to come all teh way back..

wrote:

In uk.d-i-y, Andy Wade wrote:

Interesting - I think I first heard "one foot per ns" from somebody at
Mullard; I didn't know there was connection with Ms Hopper. Didn't she
invent the concept of the compiler?


I don't think she's the one we thank for compilers: early FORTRAN beat
her to that punch.


You sure about that?

I assumed COBOL predated Fortan.


But we do have her to thank for COBOL

"The Natural Philosopher" wrote in message
...
wrote:

In uk.d-i-y, Andy Wade wrote:

Interesting - I think I first heard "one foot per ns" from somebody at
Mullard; I didn't know there was connection with Ms Hopper. Didn't she
invent the concept of the compiler?


I don't think she's the one we thank for compilers: early FORTRAN beat
her to that punch.


You sure about that?

I assumed COBOL predated Fortan.

No. Fortran (FORmula TRAnslation) was the first high level language, well 3
GL. COBOL was in 1959. It was a temporary measure that they expected to
last no more than 5 years.



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On Fri, 16 Jan 2004 00:19:33 -0000, "IMM" wrote:

"The Natural Philosopher" wrote in message
...

I assumed COBOL predated Fortan.

No. Fortran (FORmula TRAnslation) was the first high level language, well 3
GL. COBOL was in 1959. It was a temporary measure that they expected to
last no more than 5 years.

Correct. Lisp and Algol predated Cobol, too.

--
To err is human; to blame it on someone else is politics.

Mail john rather than nospam...

"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could understand
why anyone could prefer Fortran...

--
Andy

In uk.d-i-y, Andy Wade wrote:

Nostalgia: I used to be quite proficient in Algol 60; never could understand
why anyone could prefer Fortran...

I think it was most of all availability of libraries/bits of program others
had already written: an early case of first-to-market wins.

I too have a fondness for Algol 60: the first program I ever wrote, doing
self-teaching on the University (of York, if anyone's interested) timesharing
machine was in Algol-60, a few days after I'd got my account and got
fatally bitten by the computing addiction while nominally doing a biochem
degree (swapped to CompSci after a year). A little automated Q&A system
recommended which language you should learn, and though it had been put
together by Simula fanatics - so Simula was always somewhere on the top 3
of its recommendations - Algol was wot floated to the top, so I hied me off
to the university library, got out a book, and started on the slippery slope.
Deepening the addiction, the 6-line program compiled & ran first time...

Ah, TOPS-10: now *there* was an OS. And a 36-bit machine with ability to
address bytes of arbitrary size (from 1 to 36 bits) and step within and
across words as you incremented along... and perform arbitrary levels of
address redirection (briefly opening up the possibility of an uninterruptible
instruction, JRST @. - causing the machine to fetch the address of the
address of the address of the address of [repeat ad inf] the place to jump to.

Stefek 'has this drifted off topic far enough yet'?

On 16 Jan 2004 20:50:44 GMT, wrote:

In uk.d-i-y, Andy Wade wrote:

Nostalgia: I used to be quite proficient in Algol 60; never could understand
why anyone could prefer Fortran...

Another factor was that a lot of UK universities bought Elliott 803s,
mainly in the 60's IIRC, and then followed by buying 4130s.
University of Wales had one of those, and I believe York and
Manchester did as well.

Algol-60 was the preferred compiler on both of those. On the 803 you
had to load the compiler from paper tape (about 4k of it) and then
your own stuff. On the 4130 you would type up cards on a punch and
give a stack to an operator. At some time later, you would get a
stack of cards back and some listing.

Mentioning York, reminded me, since we were discussing mail the other
day, that there used to be a device invented at York called a 'York
box". IIRC, it was some kind of mail gateway or perhaps protocol
translator. They sold units to most of UK academia. I can't find
any references to it now though...... I think it was early 80s?


I think it was most of all availability of libraries/bits of program others
had already written: an early case of first-to-market wins.

I too have a fondness for Algol 60: the first program I ever wrote, doing
self-teaching on the University (of York, if anyone's interested) timesharing
machine was in Algol-60, a few days after I'd got my account and got
fatally bitten by the computing addiction while nominally doing a biochem
degree (swapped to CompSci after a year). A little automated Q&A system
recommended which language you should learn, and though it had been put
together by Simula fanatics - so Simula was always somewhere on the top 3
of its recommendations - Algol was wot floated to the top, so I hied me off
to the university library, got out a book, and started on the slippery slope.
Deepening the addiction, the 6-line program compiled & ran first time...

Ah, TOPS-10: now *there* was an OS. And a 36-bit machine with ability to
address bytes of arbitrary size (from 1 to 36 bits) and step within and
across words as you incremented along... and perform arbitrary levels of
address redirection (briefly opening up the possibility of an uninterruptible
instruction, JRST @. - causing the machine to fetch the address of the
address of the address of the address of [repeat ad inf] the place to jump to.

Stefek 'has this drifted off topic far enough yet'?

..andy

To email, substitute .nospam with .gl

"Andy Wade" wrote in message
...
"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could
understand
why anyone could prefer Fortran...

I was taught by one of the men who formed Algol, and another who developed
the LCD display.


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"Andy Wade" wrote in message
...
"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could
understand
why anyone could prefer Fortran...

"C" beats em all. operating systems have been written in C.



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On Fri, 16 Jan 2004 23:02:03 -0000, "IMM" wrote:


"Andy Wade" wrote in message
...
"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could
understand
why anyone could prefer Fortran...

I was taught by one of the men who formed Algol, and another who developed
the LCD display.


... and who were they?

---

..andy

To email, substitute .nospam with .gl

"Andy Hall" wrote in message
...
On Fri, 16 Jan 2004 23:02:03 -0000, "IMM" wrote:


"Andy Wade" wrote in message
...
"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could
understand
why anyone could prefer Fortran...

I was taught by one of the men who formed Algol, and another who
developed
the LCD display.


.. and who were they?

lecturers of course. duh!!


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On Fri, 16 Jan 2004 23:31:41 -0000, "IMM" wrote:


"Andy Hall" wrote in message
.. .
On Fri, 16 Jan 2004 23:02:03 -0000, "IMM" wrote:


"Andy Wade" wrote in message
...
"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could
understand
why anyone could prefer Fortran...

I was taught by one of the men who formed Algol, and another who
developed
the LCD display.


.. and who were they?

lecturers of course. duh!!

In trigonometry, no doubt... obtuse angles.....


---

..andy

To email, substitute .nospam with .gl

"Andy Hall" wrote in message
...
On Fri, 16 Jan 2004 23:31:41 -0000, "IMM" wrote:


"Andy Hall" wrote in message
.. .
On Fri, 16 Jan 2004 23:02:03 -0000, "IMM" wrote:


"Andy Wade" wrote in message
...
"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could
understand
why anyone could prefer Fortran...

I was taught by one of the men who formed Algol, and another who
developed
the LCD display.


.. and who were they?

lecturers of course. duh!!

In trigonometry, no doubt... obtuse angles.....

They have obtuse angles in trig there days? My oh my!


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On Fri, 16 Jan 2004 23:07:24 -0000, "IMM" wrote:

"Andy Wade" wrote in message
...
"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could
understand
why anyone could prefer Fortran...

"C" beats em all. operating systems have been written in C.

That's all that should be written in C, in my book (and there are better
languages for that). And even then only by people with at least 0.9 of a
clue.

--
Marriage: begging for money for upgrades.

Mail john rather than nospam...

"John Laird" wrote in message
...
On Fri, 16 Jan 2004 23:07:24 -0000, "IMM" wrote:

"Andy Wade" wrote in message
...
"John Laird" wrote in message
...

Correct. Lisp and Algol predated Cobol, too.

Nostalgia: I used to be quite proficient in Algol 60; never could
understand
why anyone could prefer Fortran...

"C" beats em all. operating systems have been written in C.

That's all that should be written in C, in my book (and there are better
languages for that). And even then only by people with at least 0.9 of a
clue.

You obviously don't know about C. The problem with C is that if the
programmer is not educated to write structured code it is awful and
spaghetti like. Pascal is the best language to learn from as it forces the
user into a reasonably structured way. Write all languages like in Pascal
and you can't go wrong.

C is a programers language. It is for getting your hands dirty.


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