[vorbis-dev] Thought for the new year

Gregory Maxwell greg at linuxpower.cx
Tue Dec 26 19:56:05 PST 2000

On Wed, Dec 27, 2000 at 04:41:55AM +0100, Segher Boessenkool wrote:
> 1) MDCT is good for image coding
> 2) image coding and audio coding are two very different things
> 3) combine 1 and 2

It follows that MDCT is good for many differnt tasks. No? :)

> 4) if a psycho model is good, after leaving out what it tells you
>    you can without hurting quality, applying the same model should
>    yield the same results as you got before
> 5) from 4: decode -> encode -> decode should result in (almost) the
>    same stream.

Only if the system is stable. With most MP3 encoders, the stupid subbanded bit
allocation routiene is greatly affected by the quantization noise from prior
encoding runs. Also, things like the mid-side selection may be triggered by
quantization noise.

A while back, I did some testing and found Vorbis to be fairly stable
through multiple encode iterations, more so then the mp3 encoders I tested.
I suspect that current and future fine tuning of things like block switching
might, however, make the encoder less stable in this regard.

Whats more intresting is that the framing used in vorbis will allow many
types of editing without a complete decode/recode cycle.

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