[vorbis-dev] Thought for the new year

[Frank Klemm]

::  Gregory Maxwell wrote:
::  > 
::  > 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? :)
::  
::  "good" is subjective ;-) Hey, I said for the new year, you're too
::  early!
::  
A very rough estimation of quality can be get by:

                            used_bitrate
sounds_like_bitrate = -------------------------
                      sqrt ( number_of_codings)

while using the lowpass frequency of the used_bitrate, not of
the sounds_like_bitrate.

Examples: 

  4 times coding and decoding with 320 kbps sounds like coding with 160 kbps
  and disabling the lowpass (320 kbps uses no lowpass).

  4 times coding and decoding with 192 kbps sounds like coding with 96 kbps
  and using a lowpass at 20.8 kHz-21.3 kHz (the 192 kbps lowpass).

  36 times coding and decoding with 192 kbps sounds like coding with 32 kbps
  and using a lowpass at 20.8 kHz-21.3 kHz (the 192 kbps lowpass).

::  Even if unstable: "almost" the same results.
::  
A 100th generation MP3 encoding sounds like
laf"& f&f(tr dq(&/u)ipw(& d-.jwe(;qr d:_&jqrj_:I&oq qej

Losses become less for every additional generation, you're right.
Because there's nothing which can be lost. You can destroy trash.

::  > 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.
::  
::  Don't quantize by rounding to nearest integer; don't do M/S <-> L/R selection
::  based on only high-frequency energy.
::  
This doesn't help. You got the errors of the adjacent granule. This is
enough so that on average 40% of the spectral coefficients are different
from the previous pass. You are smearing the signal into the time and
frequency domain. This is enough so you get different results in every pass.


-- 
Mit freundlichen Grüßen
Frank Klemm
 
eMail | pfk at uni-jena.de       home: pfk at schnecke.offl.uni-jena.de
phone | +49 (3641) 64-2721    home: +49 (3641) 390545
sMail | R.-Breitscheid-Str. 43, 07747 Jena, Germany

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