Flac - Nov 2009 - Questions: FLAC performance, compression ratio and extra documentation

Dear list,

I' m studying FLAC performance, and I'd like to know how much
compression can be achieved for different audio files. 


1) It seems that
for nontonal sound (wideband noise), the compression factor is better
than for compound sound (tones + nontonal components),
which is typically 2. The reason for this result could be the following: the
LPC filter is more suitable for estimating the amplitude spectrum
of non tonal information, and do not take into account the phase contribution
for each spectral component. Do you agree with me, or do you suggest other
reasons for this?


2) As
far as I know, the compression ratio is defined as 

output file size / input
file size, 

where the output file size includes frame header and other information which do
not represent the input samples.

My questions are:

a) What percentage of the output sile size is exclusively representing the input
data? As this performance may depend on how many tones are present in the input,
I'd like to know some average results if possible.

b) Is redundancy added to the compressed data in order to make FLAC more robust?
If this is the case, what's the percentage of these data in the output file?


3) Where can I find more documentation about FLAC format and design? Are there
any more documents than those in the FLAC web page? It' d be extremely
useful not only for me, but also for many people investigating about this
interesting open-source format.


Thanks in advance.

Best regards,

Dr. Fernando A. Marengo
 Rodriguez
Acoustics and Electroacoustics Laboratory
School of Electronics Engineering
Faculty of Sciencies, Engineering and Surveying
National University of Rosario
Rosario, Argentina


      Yahoo! Cocina

Encontra las mejores recetas con Yahoo! Cocina.


http://ar.mujer.yahoo.com/cocina/
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On Nov 12, 2009, at 16:32, Fernando Alberto Marengo Rodriguez wrote:
> I' m studying FLAC performance, and I'd like to know how much  
> compression can be achieved for different audio files.
>
> 1) It seems that for nontonal sound (wideband noise), the  
> compression factor is better than for compound sound (tones +  
> nontonal components), which is typically 2. The reason for this  
> result could be the following: the LPC filter is more suitable for  
> estimating the amplitude spectrum of non tonal information, and do  
> not take into account the phase contribution for each spectral  
> component. Do you agree with me, or do you suggest other reasons  
> for this?
You should also find that lower amplitudes compress better than  
higher amplitudes.  For example, if "live" material with normal  
dynamic range is compressed by FLAC, the ratio will be better than  
the same audio which has been compressed for mastering, which raises  
the levels of all the material in addition to reducing the range  
between quiet and loud.

Your "wideband noise" may be gaussian or uniform distribution, and  
this could greatly affect the compression.  I.e. do not assume that  
all noise is the same.

There are probably other correlations besides just noise vs. tones  
and loud vs. quiet.  It seems like you may want to do additional  
research.

> 2) As far as I know, the compression ratio is defined as
>
> output file size / input file size,
>
> where the output file size includes frame header and other  
> information which do not represent the input samples.
Don't forget that the input file size also includes frame header and  
other information which does not represent the input samples.  This  
input file overhead can be negligible, but is not without some  
consequence.

> My questions are:
>
> a) What percentage of the output sile size is exclusively  
> representing the input data? As this performance may depend on how  
> many tones are present in the input, I'd like to know some average  
> results if possible.

I think you're phrasing the question wrong.  The output size devoted  
to audio data content is variable, so it will not be accurately  
expressed as a percentage.  Instead, there is a fixed amount of  
overhead for a given amount of timeline, which slightly varies  
according to the format chosen (16-bit, 24-bit, etc).  The amount of  
space taken by the audio content is quite variable, and is not  
strictly specified other than algorithmically.  I think you'll have  
to gather statistics rather than look for "specifications," since the
percentage is a side-effect rather than a precise aspect of the design.

> b) Is redundancy added to the compressed data in order to make FLAC  
> more robust? If this is the case, what's the percentage of these  
> data in the output file?
I believe that you can answer this for yourself by carefully  
examining the details on the FLAC web pages.  The entire format is  
described, down to the bit level.

> 3) Where can I find more documentation about FLAC format and  
> design? Are there any more documents than those in the FLAC web  
> page? It' d be extremely useful not only for me, but also for many  
> people investigating about this interesting open-source format.
I think what you are looking for is really a set of benchmarks that  
report on the effectiveness of the design in reaching its goal.   
There is no more documentation about the FLAC format and design than  
on the web site, because the web site includes everything related to  
format and design.  The numerical factors that you are requesting are  
more about analysis than design, and they are fuzzy values rather  
than fixed aspects of format or design.

Perhaps what you are really looking for is a White Paper which  
describes some of the research which went into FLAC, what sorts of  
sounds were analyzed when developing the algorithm, and what range of  
results were achieved by the resulting software.  I do not recall  
seeing such published information, but perhaps the author, Josh  
Coalson, can elaborate.  I'm sure that he had to test a wide variety  
of sounds, and there are certainly a number of test files in the open  
source suite.  I just don't know how much public documentation there  
is at this high level of analysis.

Brian Willoughby
Sound Consulting

Dear list,
>> I' m studying FLAC performance, and I'd like to know how much
compression can be achieved for different audio files.
>> 
>>
1) It seems that for nontonal sound (wideband noise), the compression
factor is better than for compound sound (tones + nontonal components),
which is typically 2. The reason for this result could be the
following: the LPC filter is more suitable for estimating the amplitude
spectrum of non tonal information, and do not take into account the
phase contribution for each spectral component. Do you agree with me,
or do you suggest other reasons for this?
>You should also find
that lower amplitudes compress better than higher amplitudes. For
example, if "live" material with normal dynamic range is compressed by
FLAC, the ratio will be better than the same audio which has been
compressed for mastering, which raises the levels of all the material
in addition to reducing the range between quiet and loud.

If you ask why "live" material can be better compressed may be because
it contains more wideband noise, which is better represented by the LPC
estimator. Is this your point?

>Your
"wideband noise" may be gaussian or uniform distribution, and this
could greatly affect the compression.? I.e. do not assume that all
noise is the same.

I didn't know that different types of noise can produce different
compression ratios. Did you quantify this difference?

>There are probably other correlations besides
just noise vs. tones and loud vs. quiet.? It seems like you may want to
do additional research.

I haven't studied about correlations, but I have decomposed sounds into
tones + wideband noise. Tones were detected from the input spectrogram, and the
background noise was described as white noise filtered by an LCP filter. More
details can be found on this link:

http://www.cimec.org.ar/ojs/index.php/mc/article/viewFile/2711/2653


Best regards,

Dr. Fernando A. Marengo Rodriguez
Acoustics and Electroacoustics Laboratory
National University of Rosario
Rosario, Argentina


      Yahoo! Cocina

Encontra las mejores recetas con Yahoo! Cocina.


http://ar.mujer.yahoo.com/cocina/
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Dear list,

I'm looking for information about flac performance (compression time,
compression ratio, etc.) for different types of sounds. Does anyone has this
information?

Best regards,

Dr. Fernando A. Marengo Rodriguez
Acoustics and Electroacoustics Laboratory
National University of Rosario



      Yahoo! Cocina

Encontra las mejores recetas con Yahoo! Cocina.


http://ar.mujer.yahoo.com/cocina/
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