06.123GPPComfort Noise Aspects for Full Rate Speech Traffic ChannelsTS
The comfort noise evaluation algorithm uses the unquantized block amplitude and Log Area Ratio (LAR) parameters of the full rate speech encoder, defined in clauses 4.2.15 and 4.2.6 of GSM 06.10 . These parameters give information on the level and the spectrum of the background noise, respectively.
The evaluated comfort noise parameters are encoded into a special frame, called a SID (Silence Descriptor) frame, for transmission to the receive side.
The SID frame also serves to initiate the comfort noise generation on the receive side, as a SID frame is always sent at the end of a speech burst, i.e. before the radio transmission is cut.
The scheduling of SID or speech frames on the radio path is described in GSM 06.31 .
5.1 Background acoustic noise evaluation
The comfort noise parameters to be encoded into a SID frame are calculated over N=4 consecutive frames marked with VAD=0, as follows:
The Log Area Ratio parameters shall be averaged according to the equation:
i = 1,2..8
where LAR[j](i) is the i’th Log Area Ratio coefficient of the current frame j and j-n indicates the previous frames.
The block amplitude parameter shall be averaged according to the equation:
where xmax[j](i) is the block amplitude in sub-segment i of the current frame. The SID frame containing these averaged parameters is passed to the Radio Subsystem instead of frame number j.
5.2 SID-frame encoding
The SID-frame encoding algorithm exploits the fact that only some of the 260 bits in a frame are needed to code the comfort noise parameters. The other bits can then be used to mark the SID-frame by means of a fixed bit pattern, called the SID code word.
The log area ratio coefficients are replaced by the mean (LAR(i)) values defined above and encoded as described in GSM 06.10 .
The block amplitude values are replaced by the mean (xmax) value defined above, repeated four times inside the frame and encoded as described in GSM 06.10 .
The SID code word consists of 95 bits which are all zero. The bits of the SID code word are inserted in the SID field defined as the positions of those 95 bits of the encoded RPE-pulses Xmc, which are in the error protection class I (see GSM 05.03 , table 2).
The remaining bits in the SID frame are set to zero. The use of these bits is for further study.