> A 48K dial-up service can completely pass the 5.1 compressed data
> stream.
>...
> With AAC encoding you can compress the 6 channels of CD quality 5.1
> into
> space of an uncompressed stereo stream. I found the term
> "psychoacoustic"
> interesting because it is a concept I was unaware until I started
> the
> research into compressing audio data.
Joel,
Not with 6 discrete channels it can't. The usual 5.1 stream is only a
remix of the original multitrack recording with the reverb content
spread out behind and the bass coming from one woofer from all
sources. It's basically the same amount of data as front 2.0 stereo.
With a multimic nature recording you can't do this trick as you've got
discrete rear sounds as well as the front stereo pair.
There is a fundamental difference with nature recording. For a start
some of the important information is random ambient sounds which don't
compress well. MP3 type compression turns rustling leaves into musical
tinkles which we hear as artifacts if we overdo it. Not a problem with
a band or orchestra. Bird calls being more musical compress much
better than other less musical sounds. If you had these on separate
discrete channels you could do the same ambience trick as with the
music ambience and reverb sounds, but the ambience and physical sounds
are mixed in with natural history sounds like the bird calls which you
want up front placed in the stereo image.
A music recording is recorded in a reverberant acoustic and listened
to in a reverberant and echoic room from resonant rice-kellogg
loudspeakers. Your ears know this already and psychoacoustically
adjust for it. To demonstrate this, a mic recording the loupspeakers
won't adjust and will sound horrible. Hi-fi stereo is a psychoacoustic
phenomenon.
Also your ears accept the single mono ".1" bass channel as they have
lost track of the bass direction anyway in the room echoes. Play it
outside and you will hear the bass coming from the woofer. It's just
plain wrong to say our ears can't hear the direction of low frequency
sounds. Check it out with distant thunder and your ears can locate low
frequency transients quite well.
What 5.1 is doing is overriding the acoustic of the listening room and
substituting its own version of the original studio or auditorium
acoustic or synthetic reverberation. The reverb has similar frequency
components to the front sounds so the total bandwidth is not doubled
or quadrupled.
Nature recording doesn't follow the same rules as multi-mic music
recording with reverberation. That is why it is difficult and
fascinating.
I maintain that the only true stereo which can also provide the
surround sound effect is dummy head recording with a dummy head
matching your own and nobody has yet come up with a convincing
explanation how this works except that it is another psychoacoustic
phenomenon. :-)
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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