[FFmpeg-trac] #2768(avcodec:new): ALAC encoder is not lossless with 24-bit samples

[FFmpeg]

#2768: ALAC encoder is not lossless with 24-bit samples
---------------------------------+--------------------------------------
             Reporter:  pneuman  |                     Type:  defect
               Status:  new      |                 Priority:  normal
            Component:  avcodec  |                  Version:  git-master
             Keywords:  alac     |               Blocked By:
             Blocking:           |  Reproduced by developer:  0
Analyzed by developer:  0        |
---------------------------------+--------------------------------------
 Hi,

 When converting 24-bit audio to ALAC using ffmpeg git, the output is not
 lossless. Examining the decoded ALAC alongside the source file in Audacity
 shows that the ALAC is phase inverted; mixing the ALAC and the source
 together reveals some smaller errors, too.

 I've tried with a few 24-bit files, and had the same happen in each case.
 As an example, this file (a 24-bit FLAC of audio from the Musopen project,
 in the public domain) exhibits the problem:

 http://wootangent.net/~lsd/blah/figaro.flac

 Encoder command line, output, and CRC comparisons with the source are as
 follows:


 {{{
 lsd at cletus:~/projects/alac/ffmpeg$ ./ffmpeg -i figaro.flac -acodec alac -f
 ipod figaro.m4a
 ffmpeg version N-54600-g09f59d6 Copyright (c) 2000-2013 the FFmpeg
 developers
   built on Jul 10 2013 10:15:49 with gcc 4.7 (Ubuntu/Linaro
 4.7.2-2ubuntu1)
   configuration:
   libavutil      52. 38.100 / 52. 38.100
   libavcodec     55. 18.102 / 55. 18.102
   libavformat    55. 12.100 / 55. 12.100
   libavdevice    55.  3.100 / 55.  3.100
   libavfilter     3. 79.101 /  3. 79.101
   libswscale      2.  3.100 /  2.  3.100
   libswresample   0. 17.102 /  0. 17.102
 Input #0, flac, from 'figaro.flac':
   Metadata:
     MAJOR_BRAND     : M4A
     MINOR_VERSION   : 0
     COMPATIBLE_BRANDS: M4A mp42isom
     CREATION_TIME   : 1904-01-22 23:03:39
     COMPOSER        : Wolfgang Amadeus Mozart
     GENRE           : Classical
     ARTIST          : Czech National Symphony Orchestra
     album_artist    : Czech National Symphony Orchestra
     ALBUM           : Musopen Kickstarter Project
     ENCODER         : Lavf54.20.0
   Duration: 00:04:01.00, bitrate: 1462 kb/s
     Stream #0:0: Audio: flac, 48000 Hz, stereo, s32
 Output #0, ipod, to 'figaro.m4a':
   Metadata:
     MAJOR_BRAND     : M4A
     MINOR_VERSION   : 0
     COMPATIBLE_BRANDS: M4A mp42isom
     ALBUM           : Musopen Kickstarter Project
     COMPOSER        : Wolfgang Amadeus Mozart
     GENRE           : Classical
     ARTIST          : Czech National Symphony Orchestra
     album_artist    : Czech National Symphony Orchestra
     encoder         : Lavf55.12.100
     Stream #0:0: Audio: alac (alac / 0x63616C61), 48000 Hz, stereo, s32p,
 128 kb/s
 Stream mapping:
   Stream #0:0 -> #0:0 (flac -> alac)
 Press [q] to stop, [?] for help
 size=   44225kB time=00:04:01.06 bitrate=1502.9kbits/s
 video:0kB audio:44213kB subtitle:0 global headers:0kB muxing overhead
 0.028529%
 lsd at cletus:~/projects/alac/ffmpeg$ ./ffmpeg -i figaro.flac -f crc -
 2>/dev/null
 CRC=0xe162a58b
 lsd at cletus:~/projects/alac/ffmpeg$ ./ffmpeg -i figaro.m4a -f crc -
 2>/dev/null
 CRC=0x40b1090b
 }}}

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2768>
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