[Flac-dev] FLAC 24 bit test results

[Josh Green]

Thanks to those who sent in results for the 24 bit FLAC test I created.
Results were received for Windows, Linux, and Intel Mac.  All results
showed the identical poor compression ratio with the given test file.
This indicates that the issue is likely not related to a certain
platform/compiler.  I also tested FLAC with various other options like
integer only and turning off ASM optimizations (to see if the problem
code path could be narrowed down), which did result in different output
data, but the compression was slightly worse.

At this point I'm assuming that there is certain 24 bit data signals
which are causing poor performance from FLAC.

The next question becomes:

Is there a compression performance issue with the reference encoder in
relation to 24 bit or does the FLAC format itself have trouble
compressing these signals?

My current thought (or rather hope) is that its an issue with the
reference encoder.  Below is a set of other tests I performed on various
24 bit stereo audio samples.  As you can see, FLAC often has rather
comparable compression as Wavpack (under 7% or so difference).  There
are some cases where this difference reaches 20% though, which seems
like possible candidates exhibiting this issue.  Analysis of these FLAC
files showed that most chunks were encoded verbatim.

There is of course the possibility that the Wavpack format is able to
handle these problem audio chunks in a way which FLAC cannot.

I'm not knowledgeable enough in the theory, mathematics or code base of
FLAC to debug this issue, and hope that someone else will put some
effort into discovering what the cause of this is.

Best regards,
	Josh Green


Other 24 bit stereo samples tested:

File
Size
Flac Size
Wpack Size
Flac %
Wpack %
Diff %
Bees1_clip01.wav
    257324
    243159
    201094
     94.50
     78.15
     16.35
Bees1_clip02.wav
    408620
    319382
    300742
     78.16
     73.60
      4.56
Bees2_clip01.wav
    531500
    535266
    438290
    100.71
     82.46
     18.25
Bees3_clip01.wav
    669740
    538943
    498508
     80.47
     74.43
      6.04
Bees3_clip02.wav
    411692
    356050
    316122
     86.48
     76.79
      9.70
Bees3_clip03.wav
    267308
    202738
    192450
     75.84
     72.00
      3.85
Bees3_clip04.wav
    233516
    180798
    169204
     77.42
     72.46
      4.96
Bees3_clip05.wav
    445484
    348487
    326398
     78.23
     73.27
      4.96
Bees3_clip06.wav
    296492
    246837
    221402
     83.25
     74.67
      8.58
Bees4_clip01.wav
    396332
    397719
    318000
    100.35
     80.24
     20.11
Bees4_clip02.wav
    264236
    239811
    206392
     90.76
     78.11
     12.65
Bees4_clip03.wav
    365612
    367556
    293120
    100.53
     80.17
     20.36
Cricket01.wav
    239660
    243255
    204224
    101.50
     85.21
     16.29
Cricket02.wav
    192812
    197026
    161418
    102.19
     83.72
     18.47
Crow1.wav
    123900
     86952
     82852
     70.18
     66.87
      3.31
Dog_Howling01.wav
   2085524
   1692320
   1516442
     81.15
     72.71
      8.43
Dog_Howling02.wav
   1352012
    991368
    942016
     73.33
     69.68
      3.65
Dog_Howling03.wav
   2574530
   2001849
   1850906
     77.76
     71.89
      5.86
GrassHopper_clip01.wav
    768044
    768673
    669232
    100.08
     87.13
     12.95
GrassHopper_clip02.wav
    646700
    638097
    564734
     98.67
     87.33
     11.34
GrassHopper_clip03.wav
    559148
    557699
    495702
     99.74
     88.65
     11.09
GrassHopper_clip04.wav
    565292
    568383
    499236
    100.55
     88.31
     12.23
GrassHopper_clip05.wav
   1544876
   1459159
   1283262
     94.45
     83.07
     11.39
Schweine01.wav
    483372
    337195
    333246
     69.76
     68.94
      0.82
Schweine02.wav
   4326476
   2886941
   2877448
     66.73
     66.51
      0.22
SeedPod1.wav
    257744
    259027
    217720
    100.50
     84.47
     16.03
SeedPod2.wav
    289580
    289343
    243170
     99.92
     83.97
     15.94
SeedPod3.wav
    278060
    273867
    234554
     98.49
     84.35
     14.14