[vorbis] reencoding OGG Vorbis

[Kristoff Bonne]

Greetings,

On Wed, 21 Feb 2001, Gregory Maxwell wrote:
>> Inspired by the discussion about mixing Vorbis streams and so on I came
>> to the question whether Vorbisstreams can be, just like MP3, reencoded
>> without further loss.

>> As I heard, and I am pretty sure that it's true, MP3 files can be
>> decoded and reencoded unlimited times without further quality loss (of
>> course the same encoder with the same preferences has to be used all the
>> time). Is this also possible with .OGG?

> I'd like to see the mp3 encoder which does this. In prior expirments I found
> that all the mp3 encoders I could find would eventually reach a almost
> steady-state output after many encode-decode cycles, but the output sounded
> much worse then the first generation.
> In a simmlar test, Vorbis faired much better. However, there are many
> decisions made based on the content of the audio (window switching, floor
> shape, etc) so it's unlikely that 'lossless' reencoding is possible with the
> current encoder.
> However, in actual tests, vorbis fairs well.. I ran audio through a number
> of encode/decode cycles (I believe 50) and ended up with something that
> sounded just like the orignal on a quick surface listening.

Question:
I guess this is done without changing the 'data' in between.
So:
1/ Encode PCM to ogg a first time.
2/ Decode ogg to PCM.
3/ Re-encode the PCM into ogg.
4/ Decode the ogg into PCM for playback.

So, you do not change the PCM-data in-between steps 2 and 3.

Just wondering, would this be any different if you would (actually) change
the PCM-data after step 2. (like mixing a second PCM -pure PCM or decoded
from a ogg- into it).

(I know I can just test this, but I was wondering what the mathymatical
model of ogg says about this).

Cheerio! Kr. Bonne.

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