[MPlayer-users] audiodump: -vc dummy versus -vc null

[Joe Friedrichsen]

A recent post 
(http://article.gmane.org/gmane.comp.video.mplayer.user/39677) piqued my 
curiosity about -vc dummy when dumping audio from videos. Incidentally, 
"dummy" is not listed for $mplayer -vc help.

Reimar Döffinger wrote that people weren't supposed to use -vc dummy for 
dumping audio, but should only use -vo null. A counter point was made by 
Neil Sleightholm saying that with only -vo null, mplayer suggests "[AO 
PCM] Info: fastest dumping is achieved with -vc dummy -vo null". After 
refreshing my CVS and recompiling, I found that mplayer now suggests 
"[AO PCM] Info: Faster dumping is achieved with -vc null -vo null".

I chose a widely available test video: Pure Pwnage 101 
(http://www.purepwnage.com) and timed those different video codec 
options. Using the combination -vc null -vo null is indeed faster than 
-vo null by itself. Episode 101 runs 11m36s, and using -vc null -vo null 
gave a time improvement over -vo null alone: from 2m38s to 2m20s.  Out 
of curiosity, I tried -vc dummy -vo null anyway and noticed a 
_significant_ time improvement (0m8s). Console output at the end.

These results raise a few questions for me:

(1) Why is -vc null recommended over -vc dummy for audio dumping even 
though -vc dummy is faster?
(2) What function does -vc dummy have then?

Thanks,
Joe

######################################################
Test 1: $time mplayer -quiet -vo null -ao 
pcm:waveheader:file=audiodump.wav Pure_Pwnage_101.avi

MPlayer dev-CVS-051026-18:00-3.3.5 (C) 2000-2005 MPlayer Team
CPU: Intel Pentium 4/Celeron 4 Northwood; Pentium 4 EE/Xeon 
Prestonia,Gallatin (Family: 15, Stepping: 5)
Detected cache-line size is 64 bytes
CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled for x86 CPU with extensions: MMX MMX2 SSE SSE2


Playing Pure_Pwnage_101.avi.
Cache fill:  0.00% (0 bytes)   AVI file format detected.
VIDEO:  [DX50]  384x256  24bpp  29.970 fps  617.5 kbps (75.4 kbyte/s)
==========================================================================
Opening audio decoder: [mp3lib] MPEG layer-2, layer-3
AUDIO: 48000 Hz, 2 ch, s16le, 128.0 kbit/8.33% (ratio: 16000->192000)
Selected audio codec: [mp3] afm: mp3lib (mp3lib MPEG layer-2, layer-3)
==========================================================================
==========================================================================
Opening video decoder: [ffmpeg] FFmpeg's libavcodec codec family
Selected video codec: [ffodivx] vfm: ffmpeg (FFmpeg MPEG-4)
==========================================================================
Building audio filter chain for 48000Hz/2ch/s16le -> 0Hz/0ch/??...
[AO PCM] File: audiodump.wav (WAVE)
PCM: Samplerate: 48000Hz Channels: Stereo Format s16le
[AO PCM] Info: Faster dumping is achieved with -vc null -vo null
PCM: Info: to write WAVE files use -ao pcm:waveheader (default).
AO: [pcm] 48000Hz 2ch s16le (2 bytes per sample)
Building audio filter chain for 48000Hz/2ch/s16le -> 48000Hz/2ch/s16le...
Starting playback...
VDec: vo config request - 384 x 256 (preferred colorspace: Planar YV12)
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is 1.50:1 - prescaling to correct movie aspect.
VO: [null] 384x256 => 384x256 Planar YV12


Exiting... (End of file)

real    2m38.290s
user    0m27.438s
sys     0m1.807s


######################################################
Test 2: $time mplayer -quiet -vc null -vo null -ao 
pcm:waveheader:file=audiodump.wav Pure_Pwnage_101.avi

MPlayer dev-CVS-051026-18:00-3.3.5 (C) 2000-2005 MPlayer Team
CPU: Intel Pentium 4/Celeron 4 Northwood; Pentium 4 EE/Xeon 
Prestonia,Gallatin (Family: 15, Stepping: 5)
Detected cache-line size is 64 bytes
CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled for x86 CPU with extensions: MMX MMX2 SSE SSE2


Playing Pure_Pwnage_101.avi.
Cache fill:  0.00% (0 bytes)   AVI file format detected.
VIDEO:  [DX50]  384x256  24bpp  29.970 fps  617.5 kbps (75.4 kbyte/s)
==========================================================================
Opening audio decoder: [mp3lib] MPEG layer-2, layer-3
AUDIO: 48000 Hz, 2 ch, s16le, 128.0 kbit/8.33% (ratio: 16000->192000)
Selected audio codec: [mp3] afm: mp3lib (mp3lib MPEG layer-2, layer-3)
==========================================================================
==========================================================================
Forced video codec: null
Opening video decoder: [null] Null video decoder
VDec: vo config request - 384 x 256 (preferred colorspace: BGR 24-bit)
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is undefined - no prescaling applied.
VO: [null] 384x256 => 384x256 Planar YV12
Selected video codec: [null] vfm: null (NULL codec (no decoding!))
==========================================================================
Building audio filter chain for 48000Hz/2ch/s16le -> 0Hz/0ch/??...
[AO PCM] File: audiodump.wav (WAVE)
PCM: Samplerate: 48000Hz Channels: Stereo Format s16le
[AO PCM] Info: Faster dumping is achieved with -vc null -vo null
PCM: Info: to write WAVE files use -ao pcm:waveheader (default).
AO: [pcm] 48000Hz 2ch s16le (2 bytes per sample)
Building audio filter chain for 48000Hz/2ch/s16le -> 48000Hz/2ch/s16le...
Starting playback...


Exiting... (End of file)

real    2m20.160s
user    0m11.226s
sys     0m2.092s


######################################################
Test 3: $time mplayer -quiet -vc dummy -vo null-ao 
pcm:waveheader:file=audiodump.wav Pure_Pwnage_101.avi

MPlayer dev-CVS-051026-18:00-3.3.5 (C) 2000-2005 MPlayer Team
CPU: Intel Pentium 4/Celeron 4 Northwood; Pentium 4 EE/Xeon 
Prestonia,Gallatin (Family: 15, Stepping: 5)
Detected cache-line size is 64 bytes
CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled for x86 CPU with extensions: MMX MMX2 SSE SSE2


Playing Pure_Pwnage_101.avi.
Cache fill:  0.00% (0 bytes)   AVI file format detected.
VIDEO:  [DX50]  384x256  24bpp  29.970 fps  617.5 kbps (75.4 kbyte/s)
==========================================================================
Opening audio decoder: [mp3lib] MPEG layer-2, layer-3
AUDIO: 48000 Hz, 2 ch, s16le, 128.0 kbit/8.33% (ratio: 16000->192000)
Selected audio codec: [mp3] afm: mp3lib (mp3lib MPEG layer-2, layer-3)
==========================================================================
==========================================================================
Forced video codec: dummy
Cannot find codec matching selected -vo and video format 0x30355844.
Read DOCS/HTML/en/codecs.html!
==========================================================================
Building audio filter chain for 48000Hz/2ch/s16le -> 0Hz/0ch/??...
[AO PCM] File: audiodump.wav (WAVE)
PCM: Samplerate: 48000Hz Channels: Stereo Format s16le
[AO PCM] Info: Faster dumping is achieved with -vc null -vo null
PCM: Info: to write WAVE files use -ao pcm:waveheader (default).
AO: [pcm] 48000Hz 2ch s16le (2 bytes per sample)
Building audio filter chain for 48000Hz/2ch/s16le -> 48000Hz/2ch/s16le...
Video: no video
Starting playback...


Exiting... (End of file)

real    0m8.483s
user    0m5.005s
sys     0m0.692s