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
16
24
32
40			S
48			S
56			S
64			S
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

MPEG1 layer 3 Stereo

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

MPEG2 layer 2 Stereo

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

MPEG1 layer 2 Stereo

* These bitrates are forbidden for layer 2 stereo.