[Flac-dev] Re: nice idea
Josh Coalson
xflac at yahoo.com
Sun Oct 20 23:56:53 PDT 2002
--- Miroslav Lichvar <lichvarm at phoenix.inf.upol.cz> wrote:
> On Thu, Oct 17, 2002 at 09:51:02AM -0500, Brady Patterson wrote:
> > ... But it seems to me that you could make a decent guess
> > about when something "new" happens based on the second derivative
> of the signal
> > (where the first derivative is the difference between a given
> sample and the
> > previous, and the second is you-get-the-idea).
> >
> > Here's my rationale: high-amplitude, high-frequency sections are
> the hard ones
> > to encode, or at least will work best in their own frame. Those
> > characteristics imply a high first derivative. You want to put
> such sections
> > in their own block, and boundaries of such blocks will be where the
> second
> > derivative is relatively high.
> >
> > Okay, that's not quite right, since the first derivative will be
> negative about
> > half the time, and large negative has the same effect as lange
> positive. So I
> > think what you really want is the first derivative of the absolute
> value of the
> > first derivative.
> >
> > Then there's the question of where to put the boundaries. Some
> trial-and-error
> > is probably the best approach here. For files on which the above
> formula is
> > consistently high, it will probably be desirable to set the limit
> high to avoid
> > too much frame overhead.
>
> Well, i took 10 CD and test my primitive implementations of these
> algos. Here are my results:
>
> size encoding time
> (0) 6401778544 1.0000
> (1) 4193699407 0.6551 1.0000 1.00
> (2) 4180011683 0.6529 0.9967 1.18
> (3) 4186509853 0.6540 0.9983 1.15
>
> "best" CD:
> (0) 503448568 1.0000
> (1) 349525363 0.6942 1.0000
> (2) 347167639 0.6896 0.9933
> (3) 347864119 0.6910 0.9952
>
> "best" track:
> (0) 44111804 1.0000
> (1) 28091683 0.6368 1.0000
> (2) 27769870 0.6295 0.9885
> (3) 27864205 0.6317 0.9919
>
> where:
> (0) wav files
> (1) flac files, fixed blocksize 4608
> (2) flac files, variable blocksize, "lpc idea"
> (3) flac files, variable blocksize, watching average of absolute
> values of first and second derivative
Interesting, looks like the best case is ~ 0.75% increase in
compression for 18% increase in encode time. The compression
increase is similar to my old brute force test but much faster.
The question is, is it worth it from the user's point of view?
Josh
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