[Flac-dev] flac can occasionally be worse than shorten

[Josh Coalson]

well, I took a look at the files.  from my
knowledge of shorten there are two things it does
that flac doesn't do:

1. it estimates the mean of the signal for each
block, subtracts it out and stores it separately.
but this is pretty useless for the predictors that
shorten uses as they are pretty insensitive to the
mean (try different values of -m from 0 to whatever
and note practically no difference in file size).

2. it tests for exploitable bitshift within a block;
e.g. if all the samples in a block have 0 for the
least significant two bits, it stores the bitshift
(2) and shifts the signal down 2 bits.  normally this
is also useless for CD audio since even the LSB is not
stationary and there is no exploitable bit shift.

BUT, you have stumbled on some recordings where the
LSB is 0 for much of the file.  as a matter of fact,
in
the worse track (track 6) almost the entire signal in
both channels is like that.  taking a 15
bit-per-sample
signal and just shifting it left one bit is worse
than just adding 1 bit of noise, and as distortion
goes may actually be more audible, so I'm not sure who
engineered these tracks.

I've never come across a sample like this which is
why I thought it wasn't useful to add that
functionality to FLAC... maybe if this is a common
practice I should put it in.

Josh

--- Mark Powell <M.S.Powell at salford.ac.uk> wrote:
>> yeah, could you put up the FLAC version of the
>> worst track that is less than 20 megs compressed?
>> (I'll have to grab it with a 56k modem).  by worst
>> I mean the one where shorten beats flac by the
most.
>> also:
>> 
>> 1. what version of shorten are you using?
>
> v3.1
>
> # shorten -h | head -1
> shorten: version 3.1: (c) 1992-1999 Tony Robinson
and SoftSound Ltd
>
>> 2. what command-line options for flac and shorten
>> did you use on this track?
>
> Nothing fancy.
>
> $ shorten file.wav
>
> $ flac -V file.wav file.flac


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