[vorbis-dev] Thought for the new year
segher at wanadoo.nl
Wed Dec 27 16:11:00 PST 2000
Frank Klemm wrote:
> :: 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:
By "good for coding" I don't mean quality, of course; we're talking
about a transform for compression here, so a "good" transform is one
that makes it easier to compress, i.e. it should group the features
of the sound together based on some sensible norm. MDCT uses a window
that doesn't change size for differing frequencies; that's no good for
a perceptual codec.
> sounds_like_bitrate = -------------------------
> sqrt ( number_of_codings)
> while using the lowpass frequency of the used_bitrate, not of
> the sounds_like_bitrate.
Man, you're using a lousy encoder! It should stabilize...
> :: 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
That must have been difficult to type. What language is it? ;-)
> Losses become less for every additional generation, you're right.
That's not what I was saying.
> Because there's nothing which can be lost. You can destroy trash.
> This doesn't help. You got the errors of the adjacent granule. This is
You can compensate for that (see MDCT_CORR in my latest Vorbis patch).
> 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.
So you _do_ agree that MDCT is a lousy transform for audio coding?
--- >8 ----
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