A.2 Subjective relevance of the speech coder output bits
06.103GPPFull Rate Speech TranscodingTS
Since no valid objective quality criterion for speech signals is available, the only way to build up such a relevance table is to perform listening tests. The procedure described below was used to obtain the relevance classification given in table A.2.1 of the recommendation.
To classify a single bit, say bit i of parameter k, a short speech signal (2 sec) was encoded, then this bit was inverted in each frame (the other bits were left unchanged) and the resulting bit stream was fed into the speech decoder. The listeners had to compare the quality of the signal with the quality of six reference signals with different levels of distortion. Repeating this procedure for all bits would result in a subdivision of the 260 bits into six relevance classes. It can be observed that many of the bits have the same physical meaning and it can be expected that bits with the same meaning have the same relevance (e.g. the MSB’s of the RPE samples). Relying on this assumption, only one of the equivalent parameters was considered. Since there are 13 parameters with different physical meaning with 56 bits in total, the number of tests is reduced from 260 to 56.
The reference signals were the same speech signal distorted by inverting one of the six bits of LAR coefficient number one. This resulted in an adequate quantization of distortion levels ranging from "not intelligible" (MSB inverted) to "negligible distortion" (LSB inverted).
The test was carried out using three listeners and one female speaker. Since the three listeners came to rather similar results, no more listeners were considered to be required. Averaging the three outcomes led to the relevance table given in table A.2.1, where the order of all bits between two successive bits of the first parameter (LAR 1) are arbitrarily chosen.
Table A.2.1a: Subjective importance of encoded bits
(the parameter and bit numbers refer to table 1.1)
Importance | Parameter | Parameter | Bit number |
class | name | number | |
1 | Log.area ratio 1 | 1 | b6 |
Block amplitude | 12,29,46,63 | b53,b109,b165,b221 | |
Log.area ratio 1 | 1 | b5 | |
2 | Log.area ratio 2 | 2 | b12 |
Log.area ratio 3 | 3 | b17 | |
Log.area ratio 1 | 1 | b4 | |
Log.area ratio 2 | 2 | b11 | |
Log.area ratio 3 | 3 | b16 | |
Log.area ratio 4 | 4 | b22 | |
LTP lag | 9,26,43,60 | b43,b99,b155,b211 | |
3 | Block amplitude | 12,29,46,63 | b52,b108,b164,b220 |
Log.area ratio 2,5,6 | 2,5,6 | b10,b26,b30 | |
LTP lag | 9,26,43,60 | b42,b98,b154,b210 | |
LTP lag | 9,26,43,60 | b41,b97,b153,b209 | |
LTP lag | 9,26,43,60 | b40,b96,b152,b208 | |
LTP lag | 9,26,43,60 | b39,b95,b151,b207 | |
Block amplitude | 12,29,46,63 | b51,b107,b163,b219 | |
Log.area ratio 1 | 1 | b3 | |
Log.area ratio 4 | 4 | b21 | |
Log.area ratio 7 | 7 | b33 | |
4 | LTP lag | 9,26,43,60 | b38,b94,b150,b206 |
Log.area ratio 5,6 | 5,6 | b25,b29 | |
LTP gain | 10,27,44,61 | b45,b101,b157,b213 | |
LTP lag | 9,26,43,60 | b37,b93,b149,b205 | |
Grid position | 11,28,45,62 | b47,b103,b159,b215 |
Table A.2.1b: Subjective importance of encoded bits
(the parameter and bit numbers refer to table 1.1)
Importance | Parameter | Parameter | Bit number |
class | name | number | |
Log.area ratio 1 | 1 | b2 | |
Log.area ratio 2,3,8,4 | 2,3,8,4 | b9,b15,b36,b20 | |
Log.area ratio 5,7 | 5,7 | b24,b32 | |
LTP gain | 10,27,44,61 | b44,b100,b156,b212 | |
Block amplitude | 12,29,46,63 | b50,b106,b162,b218 | |
RPE pulses | 13..25 | b56,b59,..,b92 | |
RPE pulses | 30..42 | b112,b115,..,b148 | |
RPE pulses | 47..59 | b168,b171,..,b204 | |
5 | RPE pulses | 64..76 | b224,b227,..,b260 |
Grid position | 11,28,45,62 | b46,b102,b158,b214 | |
Block amplitude | 12,29,46,63 | b49,b105,b161,b217 | |
RPE pulses | 13..25 | b55,b58,..,b91 | |
RPE pulses | 30..42 | b111,b114,..,b147 | |
RPE pulses | 47..59 | b167,b170,..,b203 | |
RPE pulses | 64..67 | b223,b226,b229,b232 | |
RPE pulses | 68..76 | b235,b238,..,b259 | |
Log.area ratio 1 | 1 | b1 | |
Log.area ratio 2,3,6 | 2,3,6 | b8,b14,b28 | |
Log.area ratio 7 | 7 | b31 | |
Log.area ratio 8 | 8 | b35 | |
Log.area ratio 8,3 | 8,3 | b34,b13 | |
Log.area ratio 4 | 4 | b19 | |
6 | Log.area ratio 4,5 | 4,5 | b18,b23 |
Block amplitude | 12,29,46,63 | b48,b104,b160,b216 | |
RPE pulses | 13..25 | b54,b57,..,b90 | |
RPE pulses | 30..42 | b110,b113,..,b146 | |
RPE pulses | 47..59 | b166,b169,..,b202 | |
RPE pulses | 64..76 | b222,b225,..,b258 | |
Log.area ratio 2,6 | 2,6 | b7,b27 |