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



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.

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.

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 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. But as I say, I have no direct experience of
that so can only surmise. The effect I think I've heard on DTTV R3 seems to
occur on some occasions and not others even when the music is similar.

Slainte,

Jim

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