Jim Lesurf wrote:
In article , DAB sounds worse
than FM wrote:
Jim Lesurf wrote:
In article , DAB sounds worse
than FM wrote:


Because Radios 1 & 2 and all the pop stations have audio processing
applied then the spectrum tends to be wide and flat, which tends to
result in aa lot of remaining frequency components after the
psychoacoustic model has produced the masking curves to throw away
the inaudible subbands.

Is that the case in the timescales relevant for the data reduction
'frames' (or whatever the correct term is)? I can see that R1/2 tend
to use audio 'compression' (in the old sense) and this may work to
flatten the medium term power spectrum. However that does not in
itself mean the spectrum is 'white' if it has a finite number of
components. Nor does it necessarily mean that each individual
processed time-frame will have a near uniform power spectral
density. Do you have some data on this relevant to R1/2?


No data; I've just looked at a lot of spectra. I know it's not white,
but it's a hell of a lot flatter and broader for R1/2 than R3. R3
tends to tail-off quickly, whereas R1/2 tails-off significantly
slower and for the vast majority of the time it goes right the way
up to the brickwall filter.

The difficulty is that doesn't necessarily lead to your conclusion.


Common sense dictates that it does.


The spectral components present in any time frame may extend across a
wider range, and be more unform in size. But if the *number* of
components that are resolved in the time frame are sigificantly less,
then the 'weeding' process may lose less info. Impossible to assess
this without much more specific info than simply observing a tendency
for the components that are present to have similar levels, etc.

Hence I think the point you make is certainly an important one, but
it may not establish the conclusion you draw without more specific
evidence.


What I've heard over the last 3 years tell me that I'm right.


Not saying you are wrong. Just saying 'dunno', but 'not
proven' simply from what you have said.


I cannot prove this with absolute certainty, but it's beyond reasonable
doubt IMO.


Is it the case that all MP2/3's encode the spectra as floating point
values? If so, what is the precision?


MPEG Layer I/II use 6 exponent bits (referred to as a scale factor)
which covers -118 dB to +6dB in 2dB steps and between 2 and 15 bits
for the mantissa, depending on subband and masking curve level.

OK. The interesting part here seems to me to be that the matissa may
be down to just a few bits.


Yes, less bits are assigned to higher frequencies because we're less
sensitive, apparently. Having said that, it's the top-end that is the
biggest problem, IMO.


The point pun you make here is interesting as I have been
wondering if some of the artefacts I think I've noticed at low
level may be due to rounding or precision/quantisation errors and
have been wondering if this is due to the *receiver* using too low
a level of precision.


I think it's far more likely that you're hearing an MPEG artefact...

That is certainly possible. Also quite possible that I am imagining
it, or it stems from something else in the chain...

IME, the tracks that fair the worst on digital radio are loud
electric guitar tracks. Even within the same track the audio quality
can vary from being very good to absolutely abysmal. This can happen
when the loud electric guitar pauses and you've just got a vocal,
and then the electric guitar starts again and it is simply
attrocious. This is, and always will be, caused simply by
insufficient bit rate. If VBR (variable bit rate) and statistical
multiplexing across the multiplex (as used on digital TV) could be
used then this suituation could be drastically improved, but we
can't use either, so when a track that is difficult to encode is on
then Radio 1 listeners in particular just have to suffer so that the
Radio 3 listeners don't. So, the next time you think you hear a
slight MPEG artefact, just consider that Radio 1 listeners have to
put up with most tracks consist of audio + MPEG artefacts throughout
the track.

If you can justify that to yourself as being fair then the only
conclusion I can come to is that you're extremely selfish.

Can't really comment on what may be 'fair' here. Just have an
interest in when the system may show audible problems. Since I don't
listen much to R1/2 (and never on DAB) I can't pass any comment on
them one way or the other.


Well I can, and they sound ****e, and it is unfair that they sound ****e
while R3 uses a 50% higher bit rate. Only a fascist would disagree.


FWIW in terms of *video* I have certainly seen very 'odd' effects at
times on BBCTV4 via DTTV. e.g. I have DVD+R's of one prom where the
'live' sic broadcast has a picture that 'stutters' throughout a
large part of the broadcast, but where the late-night repeat is fine.
I assume this was variable rate statmux stealing bitrate from BBCTV4


Probably. The BBC never alter the stat-mux bit rate allocations, which
is incredibly lazy, IMO.


during the evening to give to some other station(s), but a higher
rate being available after midnight for the repeat. Would not
surprise me if similar 'audio' crudities turned up on DAB at times.


I thought you were talking about video?



--
Steve - www.digitalradiotech.co.uk - Digital Radio News & Info

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