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Listening or Measurement
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understanding of audio compression would be able to say that DAB sounds
poor without even having to listen to it...".
Can someone explain to me how this can be the case?
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This is fairly simple. Mp2 was designed to work at 256k, but it is
actually being used at 1/2 that rate.
There is no way in which the efficiency of mp2 could have been doubled,
as it is a very simple codec, with very little scope for improvements in
encoder technology. Small improvements may well have been possible, but
not huge improvements, and certainly no where near a doubling of encoder
efficiency.
Hence an mp2 encoder working at 128k, can not possibly work the way it
was intended. A lossy encoder works by masking artefacts, and it needs
to b able to use a sufficient number of bits to be able to do this,
otherwise artefacts remain audible. To try ans make mp2 work at 128k
they have to make compromises. Cutting back on the high frequencies, and
making extensive use of intensity stereo (which causes a horrible flat
stereo image), but even with all these compromises it is impossible to
hie all the artefacts properly, unless the listener is using a low
quality receiver, which can not reproduce so much of the detail in the
sound, and hence helps to hide the artefacts that were not hidden by the
encoder.
Basically knowing that the mp2 codec is being used in a way so different
from the way it was designed to be used, it is not hard to work out that
there will be problems.
If you knew a bit about how mp2 actually works then you would understand
this. I posted something about this a year or so ago. People were trying
to say that 128k could work OK with the latest encoders, and I wanted to
explain how this simply isn't possible. Hence I described a few of the
basics of how mp2 actually works. If you want to search back for it, I
titled it Limitations of mp2.
Richard E.
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Ask them. It might be better than posting here unless they use this group.
Make some effort.
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I would, if I knew who they were.
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Easy. DAB uses mp2. Most stations use 128kbps or lower. Some people
familiar with audio compression will already have an idea of what that
sounds like, either by having tried it for themselves, or by knowing
that mp2 is less efficient than mp3 and knowing what mp3 sounds like
at those bitrates.
Mind you, people who haven't actually heard DAB would find it hard to
imagine how bad some of the 128kbps stations sound simply because, if
they're basing their opinion on things they've encoded themselves from
their own CDs, it's unlikely that they've transcoded the audio,
bounced it from digital to analogue to digital a few times, changed
the sample rate, pushed it through an aggressive dynamic range
compressor, and then forced the final encoder to use sub-optimal
encoding parameters. You need to do all of this and more to re-create
the classic 128kbps UK DAB sound as heard on several commercial
stations.
Cheers,
David.
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