15 Half Rate codec complexity
3GPP46.008Half rate speechPerformance Characterization of the GSM Half Rate speech codecRelease 17TS
The complexity of the half rate codec is characterized by the 3 following items:
– the number of cycles;
– the data memory size;
– the program memory size.
The values of these different figures depend on a specific DSP implementation. Nevertheless, the results obtained by the C description analysis can be used as references.
The speech transcoding functions are specified using a set of basic arithmetic operations. The wMOPs figure quoted is a weighted sum of the operations required to perform transcoding. The weight assigned to each operation is representative of the number of instruction cycles required to perform that operation on a typical DSP device.
The complexity range of the half rate codec is approximately 4,5 times that of the full rate codec.
The number of cycles required by the half rate algorithm is highly dependent on the values of input samples. The execution time of an average and an extreme input case may differ by up to 20 %.
That is why, to evaluate the complexity, it is necessary to compute the theoretical worst case, i.e. the maximum possible number of cycles, and not just observe the results of a simulation.
The principal figures of this evaluation are the following:
Table 9: Principal figures of evaluation
|
Theoretical worst case wMOPs |
Data RAM (note) |
Data ROM (constants) |
Program ROM |
|
|
Speech and channel half rate codec (excluding DTX functions) |
21,2 |
5 002 |
8 781 |
8 000-12 000 |
|
Ratio half rate vers. full rate |
4,5 |
2,4 |
9,7 |
4 |
NOTE: The Data RAM figure can be split in 2 parts: the static variables: 2 100 words; and the dynamic variables (i.e. local to a procedure): 2 900 words.