The width of each subband’s allocation field and what its values mean depend on the sample rate and bit rate. There are four tables for MPEG-1 (ISO/IEC 11172-3, tables B.2a to B.2d) and one for MPEG-2 and 2.5 (13818-3).
Which table applies
The test uses the bit rate per channel in kilobits a second: the frame’s bit rate divided by 2 unless the mode is single channel.
| Table | Used when | Subbands in use |
|---|---|---|
| a (B.2a) | sample rate is 48,000 Hz and bit rate per channel is at least 56; or the rate per channel is 56 to 80 | 27 |
| b (B.2b) | not table a, sample rate is not 48,000 Hz, and bit rate per channel is at least 96 | 30 |
| c (B.2c) | not the above, sample rate is not 32,000 Hz, and bit rate per channel is at most 48 | 8 |
| d (B.2d) | otherwise | 12 |
| MPEG-2, 2.5 | always for those versions | 30 |
Subbands beyond the listed number are not coded and are silent.
The tables
For each subband the table gives the field’s width and the number of quantisation levels each allocation value selects. Allocation value 0 always means no samples.
| Tables | Subbands | Width | Levels for allocation 1, 2, 3 … |
|---|---|---|---|
| a, b | 0–2 | 4 | 3, 7, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 16383, 32767, 65535 |
| a, b | 3–10 | 4 | 3, 5, 7, 9, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 65535 |
| a, b | 11–22 | 3 | 3, 5, 7, 9, 15, 31, 65535 |
| a | 23–26 | 2 | 3, 5, 65535 |
| b | 23–29 | 2 | 3, 5, 65535 |
| c, d | 0–1 | 4 | 3, 5, 9, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 16383, 32767 |
| c | 2–7 | 3 | 3, 5, 9, 15, 31, 63, 127 |
| d | 2–11 | 3 | 3, 5, 9, 15, 31, 63, 127 |
| MPEG-2, 2.5 | 0–3 | 4 | 3, 5, 7, 9, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 16383 |
| MPEG-2, 2.5 | 4–10 | 3 | 3, 5, 9, 15, 31, 63, 127 |
| MPEG-2, 2.5 | 11–29 | 2 | 3, 5, 9 |
The bits per sample and coding for each level count are on the audio data page.
An example
A stereo frame at 192 kbit/s and 44,100 Hz has 96 kbit/s per channel, so table b applies, with 30 subbands. Subband 0’s allocation field is 4 bits; a value of 5 selects 63 levels. Subband 23’s field is 2 bits; a value of 2 selects 5 levels.