A.1 Codec parameter description

3GPP46.020Half rate speechHalf rate speech transcodingRelease 17TS

The following is a list of all the parameters which are coded for each 20 ms speech frame. The basic data rate of the speech coder is 5,6 kbps. Therefore each 20 ms speech frame consists of 112 bits. These bits are given in table A.1.

Table A.1: Codec parameter description

Parameter	No. of bits	Description
Frame bits:
MODE	2	voicing mode
R0	5	frame energy
LPC1	11	reflection coefficient vector r1-r3
LPC2	9	reflection coefficient vector r4-r6
LPC3	8	reflection coefficient vector r7-r10
INT_LPC	1	the soft interpolation bit for the frame
Subframe bits (MODE = 1,2 or 3):
LAG_1	8	lag for first subframe
LAG_2	4	lag delta code for second subframe
LAG_3	4	lag delta code for third subframe
LAG_4	4	lag delta code for fourth subframe
CODE_1	9	codebook, I, for first subframe
CODE_2	9	codebook, I, for second subframe
CODE_3	9	codebook, I, for third subframe
CODE_4	9	codebook, I, for fourth subframe
GSP0_1	5	{P0,GS} code for first subframe
GSP0_2	5	{P0,GS} code for second subframe
GSP0_3	5	{P0,GS} code for third subframe
GSP0_4	5	{P0,GS} code for fourth subframe
Subframe bits (MODE=0):
CODE1_1	7	codebook code, I, for first subframe
CODE2_1	7	codebook code, H, for first subframe
CODE1_2	7	codebook code, I, for second subframe
CODE2_2	7	codebook code, H, for second subframe
CODE1_3	7	codebook code, I, for third subframe
CODE2_3	7	codebook code, H for third subframe
CODE1_4	7	codebook code, I, for fourth subframe
CODE2_4	7	codebook code, H, for fourth subframe
GSP0_1	5	{P0,GS} code for first subframe
GSP0_2	5	{P0,GS} code for second subframe
GSP0_3	5	{P0,GS} code for third subframe
GSP0_4	5	{P0,GS} code for fourth subframe

A.1.1 MODE

The speech coder is defined by 4 voicing modes. MODE is a two bit code which specifies which of the four voicing modes is used at the current frame. The MODE indicates which definition of the frame bits to apply to the current frame.

A.1.2 R0

R0 is a code which represents the average signal power of the input speech for the frame. The average signal power is computed using an analysis window which is centered over the last 100 samples of the frame.

A.1.3 LPC1 – LPC3

The 10 reflection coefficients are vector quantized in three vector segments. The first vector segment codes reflection coefficients r1 – r3, the second vector segment codes coefficients r4-r6, the third vector segment codes coefficients r7 – r10.

A.1.4 LAG_1 – LAG_4

LAG_1, the lag for the first subframe, can take on the value in the range of 21 to 142. Eight bits are used to encode the lag which may be fractional in value. Each of the remaining lag values ( LAG_2 through LAG_4) is delta coded relative to the preceding subframe’s coded value of the lag, with a deviation of ‑8 to +7 allowable lag value levels specified by a four bit code.

A.1.5 CODEx_1 – CODEx_4

If MODE ¹ 0, the code value for the VSELP codebook is the codeword I as derived by the codebook search procedure. If MODE=0, two VSELP codebooks are sequentially searched, with codeword I, specifying the codevector from the first VSELP codebook, assigned onto CODE1_x, and codeword H, specifying the codeword selected from the second VSELP codebook, assigned onto CODE2_x, where x is the subframe number.

A.1.6 GSP0_1 – GSP0_4

The {P0,GS} codebook contains the values needed to determine the gain factors for the excitation vectors of a given subframe. The index of the corresponding codebook entry is assigned to GSP0_x.

The speech coder is a multimode speech coder, defined by four voicing modes:

MODE = 0	unvoiced
MODE = 1	slightly voiced
MODE = 2	moderately voiced
MODE = 3	strongly voiced

If MODE=0, the adaptive codebook (long-term predictor) and the VSELP codebook are replaced by two other VSELP codebooks.