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?


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. :-) )


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?

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.

Slainte,

Jim

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