Garrett Wollman wollman@bimajority.org
Thu Oct 4 18:16:54 EDT 2007

<<On Thu, 4 Oct 2007 16:45:51 -0400, "Dan.Strassberg" <dan.strassberg@att.net> said:

> I was under the impression that in FM-band HD, the aggregate bit rate
> of the main channel plus all ND-n channels was limited to 96 kbps.

Extended hybrid mode adds 12.5, 25, or 50 kbit/s of additional
capacity over standard hybrid mode.  Thus, a station running full
extended-hybrid could carry their main service at 64 kbit/s, a second
audio service at 48 kbit/s, and a third audio service at 32 kbit/s,
leaving 2 kbit/s for PAD and other signalling.

> bandwidth. I find it hard to believe that a stereo HD stream at the
> same 48-kbps rate could have bandwidth any narrower than what I'm
> listening to. If it did, I think it would sound so bad that nobody
> would be likely to listen. But I suspect that it would be quite a feat
> to squeeze the same audio bandwidth into a STEREO stream at half of
> the bit rate of the mono stream I am listening to.

None of these codecs implement lossless stereo.  The standard
technique (I don't think HE-AAC aka "HDC" actually does this) is to
compute a source-direction vector for each subband in the encoder, and
then quantize it down to just a few bits per block of audio.


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