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Rockbox > Main > MasLimitations
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MAS Limitations

About

An extended test showed that the MAS MPEG audio decoders seems to not correctly decode all valid MPEG version/ layer/ bitrate combinations. This page is about the details. Problems do exist with MPEG2.5 layer 3.

These limitations usually don't affect music playback at all. Usually music does not use MPEG2.5, these low sample frequencies are aimed at speech compression.

However, there is one area where the MPEG2.5 limitations may hit - creation of voice files. The voice file size is limited to ~1.5 MB. As there are more and more entries added to voicing, there are 2 options to achieve higher compression:

  1. Lower the VBR quality setting (lame: -V <n> - increase n). With too low quality settings, this produces clearly noticeable compression artifacts.
  2. Lower the sample frequency (lame: --resample <n> ).

As soon as the sample frequency is lowered to 12 kHz, the MPEG2.5 playback problems may hit. When frame sizes of >64 kbit/s are used, dropouts occur. To avoid this, use the lame option -B 64 as well to limit the frame size. Avoid a sample rate of 11.025 kHz.

Detailed results

Legend

3507
MAS 3507D
other
MAS 3587F / MAS 3539F
S
Silent play
T
loud twittering noise

MPEG2.5 layer 3 Stereo

Bitrate 8 kHz 11.025 kHz 12 kHz
kbit/s 3507 other 3507 other 3507 other
8 DONE DONE DONE DONE DONE DONE
16 DONE DONE DONE DONE DONE DONE
24 DONE DONE DONE DONE DONE DONE
32 DONE DONE DONE DONE DONE DONE
40 DONE DONE S DONE DONE DONE
48 DONE DONE S DONE DONE DONE
56 DONE DONE S DONE DONE DONE
64 DONE DONE S DONE DONE DONE
80 S S S S S S
96 S S S S S S
112 S S S S S S
128 S S S S S S
144 S S S S S S
160 S S S S S S

MPEG2 layer 3 Stereo

Bitrate 16 kHz 22.05 kHz 24 kHz
kbit/s 3507 other 3507 other 3507 other
8..160 DONE DONE DONE DONE DONE DONE

MPEG1 layer 3 Stereo

Bitrate 32 kHz 44.1 kHz 48 kHz
kbit/s 3507 other 3507 other 3507 other
32..320 DONE DONE DONE DONE DONE DONE

MPEG2 layer 2 Stereo

Bitrate 16 kHz 22.05 kHz 24 kHz
kbit/s 3507 other 3507 other 3507 other
8..160 DONE DONE DONE DONE DONE DONE

MPEG1 layer 2 Stereo

Bitrate 32 kHz 44.1 kHz 48 kHz
kbit/s 3507 other 3507 other 3507 other
32* T T T T T T
48* T T T T T T
56* T T T T T T
64 DONE DONE DONE DONE DONE DONE
80* DONE DONE DONE DONE T T
96 DONE DONE DONE DONE DONE DONE
112 DONE DONE DONE DONE DONE DONE
128 DONE DONE DONE DONE DONE DONE
160 DONE DONE DONE DONE DONE DONE
192 DONE DONE DONE DONE DONE DONE
224 DONE DONE DONE DONE DONE DONE
256 DONE DONE DONE DONE DONE DONE
320 DONE DONE DONE DONE DONE DONE
384 DONE DONE DONE DONE DONE DONE

* These bitrates are forbidden for layer 2 stereo.

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Revision r1.2 - 13 Sep 2005 - 05:13 GMT - JensArnold Copyright © 1999-2005 by the contributing authors.