post discussing quality comparison of AAC vs. MP3 iPlayer radio streams (was: RE: BBC Radio music sessions/interview dowload?)

[Christopher Woods (CustomMade)]

This is a crosspost of mine from the BBC Backstage list, felt it was
relevant to the earlier discussion about AAC/AAC+ and World Service material
so posting for people to see what I was on about.

====================

> Chris,
> Can't speak for my colleagues elsewhere in radio, but WS doesn't 
> transcode between codecs anywhere in our longform workflow and never 
> has done.

I did a quick bit of testing and it appears that WS Listen Again material is
only available through /iplayer as 64kbps AAC+, so it all sounds a bit
sub-par compared with domestic channels' 128kbps AAC. Perhaps it's ingested
in a different way or do you encode internally then deliver to the iPlayer
team? I have noticed the PIDs for WS material have a different range (p***
as opposed to b***)...

To show you what I mean about the MP3 vs AAC quality difference, here's a
quick quality comparison (randomly chose an episode of The Archers, from
Radio 4 the other day). The first time is the MP3 encode, the second is the
AAC encode (served by default through the Flash player):

http://bit.ly/bbciprtest1al (~3.8MB)

Even on average speakers you should be able to hear a difference - the MP3
is "rumblier", warbly and speech is distinctly less clear with noticeable
distortion under the main frequency of the speaker's voice. If you use
headphones or good monitors you should be able to clearly hear the inferior
quality of the MP3 version.

Comparing the two clips spectrally also shows a visible difference, there's
less 'cohesion' in the MP3 clip, what appears to be double-encoded noise and
the frequency ranges containing the speech energy are less distinct.

Neither speech nor musical content comes off well in the MP3 versions -
either the iPlayer's using an *AWFUL* MP3 codec (because both the AAC and
MP3 files are 128kbps) or the MP3 version is being transcoded from the
original AAC source, which would explain a lot.