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


Exactly! It is the narrow dynamic range that makes R1 and R2 more
difficult to encode than R3.

I bet that's confused ya!

It has certainly puzzled me. Can you explain your reasoning and
define what you mean by "more difficult"?


The noise to mask ratio (NMR - noise (error) energy to energy under
masking curve for each subband) gives a measure of coding head-room,
and you want it to be as low as possible (i.e. noise as far below the
masking threshold as possible).

OK.

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 same is not true for classical music, because its spectrum isn't
as flat, and on average less frequency components remain after
masking.

As you can see above, I can see your general point and it seems
logical. However I'm not certain of your use of terms like 'flat'
here. A signal might only contain a few components of the same level,
or it might give a spectrum with a uniform spectral density, but
these would be quite different cases. Also a spectrum may be uniform
when averaged over one time interval, but not uniform over another.
(Indeed, for music this seems desirable if we don't just want to
listen to white noise. :-) )


I agree that it's not flat, but it is a hell of a lot flatter than for
R3.


Therefore, for a given bit rate, there are more bits per post-masking
frequency component for Radio 3 than for Radios 1 & 2, thus the NMR
is superior (lower) for Radio 3, because the noise energy is the
quantisation noise, which decreases as the bits per frequency
component encoded increases.


FWIW I have no experience of DAB. But with freeview the times I
(think!) I may have noticed problems with R3 are mostly when the
sound levels are quite low. e.g. Strings playing very quietly. i.e.
at levels well below what I hear on R2.


Dynamic range and sound level for MPEG-encoded audio are irrelevant,
because the MPEG encoder changes the sample values to floating point.

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.


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...

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.


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

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