[vorbis-dev] Monty on holiday

[Steve Underwood]

"Willmore, David (VS Central)" wrote:

> There had been some talk on this group about another vocoder for low bit
> rate use in Vorbis.
>
> Yes, you're right that most amateur packet radio is 1200 baud (minus
> headers, protocol, etc.) and isn't much useful for speech.  I'm looking into
> other uses within amateur radio--HF for one is going beyond that data rate.
>
> 2K4 and 3K2 are becoming possible in a single voice sized channel.  The
> benefits of digital coding can make it quite useful even if it doesn't same
> spectrum.
>
> Has anyone looked into a way to stream where some data is better coded--more
> likely to survive sync loss?  What about channel coding?  It is quite
> practical to split a stream into two different priority streams at that
> channel level--encode them differently, transmit them, receive them, and
> then re-interleave them.  In doing so, one of the channels may have errors
> (the primary reason to do this is so that one channel can be better coded
> than the other) while the other may not.
>
> This is done in GSM.  There are A bits, B bits, and C bits.  The A bits
> *must* survive if the frame is to be recovered.  The B bits are very useful,
> but the frame will just be 'noisy'.  The C bits are "yeah, it would be nice
> to have them, but it won't kill us."  So, they are encoded differently in
> the RF channel (it's more complex than that, but that's the short version).
> This allows the A bits to survive when the B and C might not.
>
> Is there some "priority" marker in the streams within Vorbis?  Lower
> (transport) layers could benefit from this. :)
>

This is a strategy common to every digital voice radio system I have seen. I
believe it is also used by the digital stereo broadcast radio, and digital TV
systems. In some systems the important things (e.g the MS bits of the energy
value) have quite massive amounts of error correction assigned to them, while
some other bits have none.

Monty asked a while ago if anyone had experience with fixed length VQ code
words versus variable length. I suspect there has been very little research on
this. Most voice and video coders have been designed either for broadcast (e.g.
GSM) or usuable for broadcast (e.g. MPEG2). The need for rapid recovery from
massive channel errors pretty much says that all blocks must be equal size, and
the nature of most channels says the bit rate should also be constant. That
means people usually only bother to investigate the best fixed length VQ code
book.

Steve

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