5 Adaptive multi-rate speech channel transcoding

06.713GPPAdaptive Multi-Rate speech processing functionsGeneral descriptionTS

GSM 06.90 [7] describes the adaptive multi-rate speech codec and GSM 06.73 [8] defines the ANSI-C code, thus enabling the verification of compliance to GSM 06.90 [7] to a high degree of confidence by use of a set of digital test sequences given in GSM 06.74 [6].

5.1 Full rate channel

As shown in , the speech encoder takes its input as a 13‑bit uniform Pulse Code Modulated (PCM) signal either from the audio part of the Mobile Station or on the network side, from the Public Switched Telephone Network (PSTN) via an 8‑bit A‑law or -law to 13-bit uniform PCM conversion. The encoded speech at the output of the speech encoder is delivered to the channel coding function defined in GSM 05.03 [4] to produce an encoded block consisting of 456 bits leading to a gross bit rate of 22.8 kbit/s.

In the receive direction, the inverse operations take place. GSM 06.90 [7] describes the detailed mapping between input blocks of 160 speech samples in 13‑bit uniform PCM format to encoded blocks (in which the number of bits depends on the used codec mode) and from these to output blocks of 160 reconstructed speech samples. The coding scheme is Multi-Rate Algebraic Code Excited Linear Prediction. The bit-rates of the source codec for the adaptive multi-rate full rate channel (TCH/AFS) are listed in Table 1. An adaptive multi-rate compliant MS shall support all full rate source rates listed in Table 1.

5.2 Half rate channel

As shown in , the speech encoder takes its input as a 13‑bit uniform Pulse Code Modulated (PCM) signal either from the audio part of the Mobile Station or on the network side, from the Public Switched Telephone Network (PSTN) via an 8‑bit A‑law or -law to 13‑bit uniform PCM conversion. The encoded speech at the output of the speech encoder is delivered to the channel coding function defined in GSM 05.03 [4] to produce an encoded block consisting of 228 bits leading to a gross bit rate of 11.4 kbit/s.

In the receive direction, the inverse operations take place. GSM 06.90 [7] describes the detailed mapping between input blocks of 160 speech samples in 13‑bit uniform PCM format to encoded blocks (in which the number of bits depends on the used codec mode) and from these to output blocks of 160 reconstructed speech samples. The coding scheme is Multi-Rate Algebraic Code Excited Linear Prediction. The bit-rates of the source codec for the adaptive multi-rate half rate channel (TCH/AHS) are listed in Table 1. An adaptive multi-rate compliant MS supporting the half rate mode of the adaptive multi-rate speech channel shall support all half rate source rates listed in Table 1.

Table 1: Source codec bit-rates for TCH/AFS and TCH/AHS channels

Channel

Source codec bit-rate

12.2 kbit/s (GSM EFR)

10.2 kbit/s

7.95 kbit/s

TCH/AFS

7.40 kbit/s (IS-641)

6.70 kbit/s

5.90 kbit/s

5.15 kbit/s

4.75 kbit/s

7.95 kbit/s

7.40 kbit/s (IS-641)

TCH/AHS

6.70 kbit/s

5.90 kbit/s

5.15 kbit/s

4.75 kbit/s

NOTE: IS-641is the TIA/EIA IS-641 TDMA Enhanced Full Rate SpeechCodec.