Материал: spra118
SPRA118
5.4.37COMPRESS (decoder only)
Function: Convert from uniform PCM to either A-law or m-law PCM. sr(k) → sp(k) Input: A (SR), LAW, LAWBIAS, LAWSEG, LAWMASK
Output: A (SP)
Cycles: Min: 20, Max: 26
NOTE: This generic routine is used for both A-law and m-law, due to the use of the variables, LAWxxxx, which make the discrimination between laws for this quantization. First, the A-law PCM word is re-converted into 13-bit signed word by dividing it by two. It is actually divided by four to perform linear quantization directly, but in the case of logarithmic quantization, this right-shift is then compensated. For negative A-law PCM word, subtract one from it before dividing, to perform a correct truncation. Note that G.726 recommendation indicates adding one to it, but it seems to be a printing error. The principle of the logarithm calculation is the same as in LOG (7). See () for more details about PCM companding.
5.4.38SYNC (decoder only)
Function: Re-encode output PCM sample in decoder for synchronous tandem coding. sp(k) → sd(k)
Input: B (ID), *AR1 (IM), SD (SP), LAWMASK
Output: A (SD)
Cycles: Min: 6 (usual), Med: 21, Max: 25
NOTE: After using the routines EXPAND (), SUBTA (), LOG (), SUBTB () again to perform this synchronous adjustment, the routine QUAN () is also used to re-encode the PCM output word. The new coded word is ID, and is compared with the magnitude (given by *AR1) of the original I ADPCM word. In the case where ID = IM, which is the usual case, the routine takes only six clock cycles.
62 G.726 Adaptive Differential Pulse Code Modulation (ADPCM) on the TMS320C54x DSP
SPRA118
6References
1.40-, 32-, 24-, 16-Kbps Adaptive Differential Pulse Code Modulation (ADPCM) Recommendation G.726. General aspects of digital transmission systems: terminal equipments. ITU (International Telecommunication Union). CCITT (International Telegraph and Telephone Consultative committee). Geneva, 1990.
2.Digital Coding of Speech Waveforms: PCM, DPCM, and DM Quantizers. Jayant, Nuggehally S. Proceedings of the IEE, Vol. 62, No. 5, May 1974, pp 612–633.
3.A Simple Approach to DSP (SPRDE01).
4.TMS320C54x DSP CPU and Peripherals Reference Set Volume 1 (SPRU131).
5.Digital Coding of Waveforms. Jayant, N.S. and Noll, P. Prentice Hall: Englewood Cliffs, NJ, 1984.
6.Digital Signal Processing Design. Bateman, A. and Yates, W. Pitman Publishing, London, U.K., 1988.
7.Comparison of Nth Order DPCM Encoder with Linear Transformations and Block Quantization Techniques. Habibi, A. IEEE Transactions on Communications, December 1971, pp. 948–956.
8.Digital Signal Processing with the TMS320 Family, Volume 1, Lin, Kun-Shan. Prentice Hall, Englewood Cliffs, NJ. 1987.
For more information, go to the Texas Instruments web site at http://www.ti.com/
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