[Flac-dev] Re: nice idea

[Miroslav Lichvar]

On Thu, Oct 17, 2002 at 09:51:02AM -0500, Brady Patterson wrote:
> 
> Okay, I deleted most of this thread, so I was waiting for another message to
> respond to, so unfortunately this will be out of place in the thread.
> 
> This is in response to Miroslav's idea about variable block sizes.  I may be a
> bit out of my league here as I'm just starting to look at how the actual
> encoding gets done.  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.
> 
> Hope this was interesting and/or useful :) .

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


-- 
Miroslav Lichvar