In article , tony sayer

wrote:
In article , Jim Lesurf jcgl@st-
and.demon.co.uk writes



The problem isn't the generator, it is the effect of a finite
transmission bandwidth on an FM signal. This means that no matter how
good the generator or RX, there will be a lower limit to the distortion
for high levels of distortion which will be way above the values you
quote.

My recollection of this may be hazy as it is some years since I did
this, however it is that the levels of nonlinearity due to the finite
transmission bandwidth rise, and are particularly a problem for the L-R
component due to the subcarrier components at HF.

Yes so it is, but its to a degree.

Well, the "degree" can mean levels of distortion well above 1 percent. My
point is that I am talking about the nonlinearity produced by the finite
transmission bandwidth for the FM stereo system. Hence this is nothing to
do with tuner design as such.

No of course FM isn't perfect by any means but I think it puts up a very
good performance against the current implementation of DAB in the UK..

I agree that it does/can - but on the basis I explained. i.e. that the
modilation level is generally kept well below the permitted peak deviation.
The problem is that this tends to eat away some of the dynamic range
available to FM.



*300Hz 30% mono THD. However if you do this, you find you have
increased the
distortion for some other form of input modulation pattern. My perhaps
unreliable recollection is that trying to get a genuine (i.e. with a
band limited input) result much below 0.1 to 0.2 percent for mono was
misleading as real stereo TX modulations would end up being worse than
this, and you might make the stereo performance worse by tweaking to
get
an apparent low distortion for a mono test signal.

I think receivers may have come along a bit since that!..


My point was that I am not referring to the levels of nonlinearity which
are determined by tuner design. Thus this isn't a matter of having a 'more
linear' tuner. It is that some designers may have made the tuner
nonlinearity to null a specific signal's distortion, but this would be at
the expense of other signal waveforms becoming more distorted.


[snip]


The problem here is that the R3 engineers have a more limited dyamamic
range for FM in reality than is available for CD. Thus the tendency to
keep the modulation well below peak to avoid noticable peak/HF
distortion, and then the temptation to level-compress to avoid noise or
loss of audience.

I don't think its done for that reason, its done for people in cars and
other compromised listening environments..

To some extent, yes. But the problems remain even for those who do not.

[snip]


What depth do you mean in an FM system?..

Modulation depth. The tradition was to use 30% in magazine reviews
(when they bothered to actually measure tuners). Full mod (100%) would
correspond to 75kHz, nominally.

It have a measure up with a good exciter and couple or three tuners just
to see what is the current standard.

If you are going to do some measurements then I'd suggest:

1) Ensuring the output from the generator is band-limited to give no
sidebands outwith the 200kHz nominal band permitted.

2) Try L-R and Ronly and Lonly for high modulation depths, preferrably as
HF intermod.



I suspect that. like myself, you have measured the distortion of
more than one FM RX whilst trying to align or tweak it, or just to
see if it was working as it should. it is easy enough to get THDs of
the order of 0.2 percent for 300Hz 30 percent mod mono. But when you
then measure higher (signal) levels, etc, the results can be
somewhat different. Ditto for HF intermod or L+R and L-R intermod.

Haven't done that for quite somewhile but a Denon we're using as an
RBR receiver was down to .06% at 10 K at 50 K dev a while ago....

Erm... The second harmonic of 10k is 20k. What components were you
expecting to get through the MPX filtering? Or were you including
intermod with the pilot tone, etc?

Well the residual was in the noise in that instance..

But for even a tuner whose front end was dreadful, the harmonics of 10k
would have been removed by a good MPX filter. If you want to problem the HF
performance then you would need to use intermod, or the methods I outline
above.



Thus even though such tuners are indeed excellent ones, the performance
in real use is perhaps not as impressive as the bench measurements may
indicate.

I think that tuner design has come some what I recent years.

They probably have. :-) However the Bessel functions remain the same, as
does the sideband pattern created by a given FM modulation... ;-


One does wonder about the sideband issue and the practical effect it has
compared to the encoding of DAB...

Provided the modulation level (primarily for the L-R) is kept well below
75kHz then the distortion can be expected to be too low to be audible.
However it may well affect musical peaks. The problem is that if you then
use modulation levels well below 75kHz and want the quiet levels to be well
above noise you start having to avoid a large part of the 70dB-ish range
available for FM.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

In article , hwh
wrote:

"Jim Lesurf" schreef in bericht
...
In article , hwh Rather
depends on the extent to which your last statement is a sweeping
generalisation. I don't find that the sound of R3 or BBC4TV on DTTV
generally have a "metallic sound". Nor do I generally find this to be
the case for R3 on DAB.

Well, at 192 kbps (including overhead) MP2 does not really sound very
natural to me.

That may well be so. :-)

However it remains the case that both DTTV and DAB R3 generally sound quite
good to me, and given repeated comparisons and free choice, I've ended up
feeling that DTTV/DAB for R3 tend to deliver better results for my ears.

I agree that this result is perhaps surprising. However people do keep
saying we should "trust our ears" and "rely upon the evidence", so I'm just
basing my views on that - plus perhaps being aware that FM has limitations
that people generally seem not to be aware of. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

In article ,
hwh wrote:

"Dave Plowman (News)" schreef in bericht
...
Few units had been sold because those who wanted 'quality' sound didn't
buy it - although at the high bit rates then on offer it was fine.

But very expensive.

No more so than an upmarket FM tuner. We're talking about Hi-Fi use rather
than portable radios, etc.

Now it has taken off due to offering a wide choice of stations at low
quality on most it's rather too late to complain.

It is. More capacity is the only thing that helps.

It certainly seems to appeal to lots of the public. Easy selection of
stations and freedom from multipath that effects so many portable FM
radios round the house.

--
*The closest I ever got to a 4.0 in school was my blood alcohol content*

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote


It certainly seems to appeal to lots of the public. Easy selection of
stations and freedom from multipath that effects so many portable FM
radios round the house.


Hmm, I wonder how long before some 'drop in' nobody swings into action and
posts that you have an error in the above?

In article .com,
writes
Opinions please.

First, I should mention that I am a bit of a vinyl junkie: my prefered
music source is my Linn LP12, although I also enjoy CDs played on my
Linn Ikemi.

My options for getting Radio 3:
1. I could have a VHF aerial placed on the roof of my house, and
probably upgrade my tuner;
2. The digi-box that supplies cable TV also gives radio: connecting it
to my amplifier gives music that is at least not unpleasant. At the
moment the signal has a long and indirect route between digi-box and
amplifier, but this could be changed;
3. I could buy a new DAB tuner.

So I would welcome your opinions and experiences about which option is
likely to prove most satisfying for me.


Pending some other topics that Jim Lesurf bought up which I'm awaiting
the time and equipment to all be in the same place, you ought to sub to
alt.radio.digital where there is a growing disquiet re FM and DAB for
Radio 3 that is currently being debated.

Herewith is one posting;(

-----------------------

LongJohn wrote:
I must admit I am relatively spoilt when it comes to DAB, because I
only record stuff from Radio 3.

I find 192kbps acceptable (just) compared to FM, but absolutely loathe
the recording quality one gets when they run at 160kbps in the middle
of the day. Last Sunday I conducted a trial in which I recorded a Radio
3 program at 160kbps, while at the same time recording the same using
my crap PC FM radio through a software capture package called 'Total
Recorder' (very good actually) in MP3 at an average bit rate of
224kbps.

The 160kbps BBC DAB rendering was so poor as to be virtually
unlistenable to compared to my digital recording of the (rather) poor
FM broadcast.

Well most of the stations I would like to be able to listen to broadcast
at 128kbps.
So you can probably understand whey I don't listen to them on DAB.


How can the BBC continue with such a shoddy service?

The BBC reduced bit rates to fit in more services.

I have read some suggestions on the NG, that the BBC thought they would
eventually get more capacity, to restore their bit rates back to the way
they used to be.

However instead of that, they just seem to have encouraged nearly all
the other broadcasters to also use 128k, and now it looks like the BBC
will never get the extra capacity they need.


Who can one write and complain to at the BBC?

Well from what I hear, writing to the BBC complaining about DAB sound
quality, will usually result in a reply from reception advise.
Completely in appropriate.

I tried writing to my MP once, and he passed my letter onto OfCom. It
didn't achieve much, as OfCom just wrote back with a load of b*ll sh*t
justifying their use of low bit rates.

One thing you could do is click on the petition at http://www.digitalrad
iotech.co.uk/
It sends an Email to the BBC. Not that one Email would make much
difference, but if a lot of people do it then they might take some
notice.


Sounds like a typical British approach of 'they won't mind the quality
so long they get the quantity' (and we get the money..).

And the same goes for freeview. Loads of stations, but I frequently find
myself distracted by the appearance of pixel squares.



Many German stations seem to broadcast DAB at 192kbps (and one at
224kbps)

Well in the UK OfCom only seem interested in giving the commercial
broadcasters what they want. The commercial broadcasters want more
stations, because that way they can sell more advertising, and hence
make more profits. The fact that the public might not appreciate the
resulting poor quality seems to be something that OfCom go out of their
way to ignore.

The BBC, have indicated that they would like to improve the quality of
their DAB services, but they have backed themselves into a corner. They
can not increase bit rates because their national multiplex is already
full to bursting point, and they can not get any extra capacity.

Since the BBC want to encourage the use of DAB, and they can't improve
DAB, they seem to be deliberately decreasing sound quality on their
other digital radio platforms. This reduces the difference between DAB
and other platforms, so that DAB doesn't seem quite so bad. Trouble is,
many of us still find DAB unlistenable.

Probably our best hope for the future, is the eventual introduction of
better digital broadcasting systems. It might not happen, but we can
still hope. For example DABv2 would allow the BBC to drastically improve
sound quality without needing any extra DAB capacity. If that does
happen however it will not be until several years into the future.

Richard E.
--
Tony Sayer

In article , tony sayer

wrote:


Pending some other topics that Jim Lesurf bought up which I'm awaiting
the time and equipment to all be in the same place,

[snip]

Since you mention the above, the following comments may also be of
interest... :-)

I did a search through the JAES I have and much to my surprise I couldn't
find a computation of the level of nominal distortion for 'Zenith' stereo
as a result of the limited transmission bandwidth. Anyone know if this has
been published? I would have assumed it had, but I suppose that working
this out 20+ years ago would have been a pain due to the computational
requirements.

To save me re-inventing the wheel I've also been looking for a textbook
which I recall had worked out the sidebands for FM modulation with two
components. (Nearly all texts avoid anything more than a simple sinewave
modulation as an example for fear of the maths involved. :-) ) This would
at least be closer to L-R modulation than simply using an HF sinewave as a
test. Alas, not yet found the text I can recall reading - admittedly it was
about 10 years ago! :-)

I've been doing some estimates based on single-frequency modulation, but
not yet satisfied with the results as I'm not happy with the Bessel
appoximations I used - although it behaves in general terms as I'd
expected.

If I get a chance, sometime soon I'll do a better analysis and report the
results. Might make an interesting comparison with your measurements, Tony.
:-)

For simplicity, I'd tend to assume a 'top hat' filter with a 240 kHz
transmission bandwidth, whereas in reality the filtering will have sloped
sides (and an imperfect inband response). Hence I'd expect an analysis and
the measured results not to be identical, but should share similar levels
and trends.

BTW Was looking at the manual for the CT7000 and that shows a plot of TDH
versus modulation depth for a 400Hz mono modulation which goes up to well
over 100%! This is useful as it shows the bandwidth is more than +/-75kHz
as you'd expect, but they give no data for HF L-R modulation.

BTW2 the topic of level compression came up on R4's 'Feeback' last week.
combined with a throw-away comment about DAB... :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html