[vorbis] Bit pealing and streaming

[Geoff Shang]

Hi:

Since I'm not a programmer and just want to get stuff worked out properly
in my head, I'm posting this here.

I just want to be sure I have all the concepts regarding vorbis and high
sampling rates at low bit rates all sorted out.  Note that this will
involve questions, some of which I've been looking for answers to for some
time.

The way I understand things is that vorbis is meant to be able to do
44.1khz audio at bit rates as low as 16 kbps per channel, at least in
theory (see vorbis.com & vorbis.org).  However, I am led to understand that
encoding streams below a certain data rate at 44.1khz will involve incoding
at a higher rate and then pealing off bits in order to obtain the desired
data size.  Is this correct?  If so, is this size point yet known or will
this have to be determined as development proceeds?

OK, now suppose you run a server like icecast 2.0, for example.  Will it be
possible for the server to receive a suitably encoded vorbis stream at a
high rate, say 128kbps, and then serve up streams at the right rate for
various connections?  (e.g. pass the full stream on to people using high
bandwidth connections, strip it down for ISDN and strip it down still
further for modem users, all on the one server?)  If this were possible,
this would make life much easier for content providers, being able to make
one version of the content and letting the server slice and dice it to suit
the listener.  This would mean the need for some smarts in the server and
client to determine the optimal data rate.

This of course raises the issue of the processing power needed to do the
pealing.  Is this very resource intensive?  Note that a person wanting to
stream to a server via modem would have to be able to cope with the need to
encode at a higher bit rate *and* peal the data, both in realtime, before
uploading it to the server.  Is this asking too much?  You could have more
smarts here to allow the server and sourcer to determin the highest
sustainable bitrate, with perhaps some fascility for manual over-ride.

Another question.  Seeing you can peal down 44.1khz, can you do the same
to, say, 22.05khz and get, say, 8kbps per channel?  I kinda like the idea
of being able to broadcast at 22.05khz in 16kbps stereo, so I'd like to
know if I should stop dreaming now before I get too set on the idea.

As you've probably guessed, I have a use in mind.  I am involved with a
project called ACB radio (http://www.acbradio.org), and particularly the
ACB radio interactive channel.  This is an interesting channel as it
involves a host of broadcasters from around the world sharing the one
server with scheduled hand-over points.  Some of us are on cable/DSL and
some of us are not.  Likewise our listeners.  If vorbis will be able to do
all that I think it can (above), this would overcome pretty much all that
MP3 does not let us do.

The only thing then that I'd want to lobby for is for all the players to
cope with a change in data rate, that being where one broadcaster switches
over to another.  This might not affect people listening at low bit rates,
as the server would downscale anyway, but say a modem broadcaster handed
over to a broadband user,.  This would result in a change in data rate for
any other cable listeners.  MP3 does not cope well with this at all, and
I'd like to see vorbis address this issue if this is at all possible.

Keep up the great work!

Geoff.

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