Audio Coders for Video
Here I compare a number of audio compressors for video, as currently available for Apple Macintosh OS8.6. I tested them using Premiere 5.1 Macintosh, although I could have used the standard Quicktime software for that as well.
Test sounds:
(1) scale jet taking off, time 12s:11f (=12.44s). All compressors were set to 44khz, 16 bit (unless indicated differently), mono. Stereo seems not very important here, as the plane is at large distance.
(2) male voice, time 11s:15f (=11.60s). All compressed at 22kHz, mono, 16 bit (unless indicated differently).
(3) as (2) but at 11kHz.
For the audio downsampling, enhanced rate conversion was set to best and log audio fades on.
| Compressor, settings | datarate (bytes/s) | Sound quality |
| Uncompressed mono 1, 2, 3 | 87702, 42348,21211 | Very good. In (3) high frequencies clearly lacking. |
| alaw 2:1 1, 2, 3 | 43880,21205,10636 | Very good. |
| ulaw 2:1 1 | 43880 | Very good. |
| ima 4:1 1, 2, 3 | 23342,11258,5683 | Good. Very little added noise, only hearable with good audio installation. |
| ima 4:1 downsampled to 22khz 1 | 11696 | High frequencies clearly missing. |
| mace 3:1, 8 bit 1, 2 | 12669, 7113 | For jet, increased noise level at more silent parts is not disturbing. For voice the noise is more disturbing. |
| mace 6:1, 8 bit 1, 2 | 7366,3578 | High frequencies are filtered off drastically, increased noise level. Not acceptable for voice, dubious for jet sound. |
| quallcom purevoice 'smart rate' 1, 2, 3 | 9653,4496,2385 | Low frequencies are filtered out. There is some 5-Hz flutter added to the sound. In the voice, low and high frequencies sound slightly 'separated' as if two voices were mixed, but not disturbing. Quallcom seems to ignore quality settings completely. |
| qdesign 24 kbps (default) 1, 2 | 2977,2854 | Sounds as played through a pipe. QDesign ignores the kbps settings completely. |
For the jet sound, high frequencies are essential. Mace 3:1 in this case compresses
at a factor 6 without disturbing artefacts, although the noise might become more
disturbing in more silent parts. In that case ima 4:1 seems the next alternative at
a compressor factor of 3.7.
For the voice, the quallcom purevoice 'smart rate' coder sounds very
good at a compression factor of roughly 9.
Both at 22kHz and at 11kHz the best choice. If you want better quality,
IMA4:1 is the next choice but this nearly triples the datarate.
I don't share Tabanger's enthousiasm
for the QDesign. It gives high compression factors, but both the jet and voice sound highly deformed, and the quallcom compressor compresses to even lower bitrates.