During five years of activity, the Traffic CHannel Half rate Speech (TCH-HS) Experts Group has produced a number of test plans and experiments to assess the performance of the candidate algorithms submitted for the GSM half rate standardization. An aid in this task was a large knowledge base made available from previous CCITT (now ITU-T) and ETSI activities on codec assessment (see annex A references 1) 2) 3) 4) 5)), plus the use of recommendations in the field (see annex A references 6) 7) 8)). Here are reported 3 different phases of the standardization of the GSM half rate codec: Characterization Phase 1, Characterization Phase 2 and Verification phase. The selection of the codec candidate for the GSM half rate traffic channel was based on the results of the characterization phase 1. Test results reported hereafter are based on version 3.3 of the GSM half rate codec. Characterization Phase 1 (Experiments 1 to 5):

• Experiment 1: Quality under error conditions (A-law, IRS);
• Experiment 2: Quality under error conditions (UPCM, No IRS);
• Experiment 3: Quality under tandeming conditions;
• Experiment 4: Quality under background noise conditions (ACR);
• Experiment 5: Quality under background noise conditions (DCR).

Characterization Phase 2 (Experiments 6 to 9):

• Experiment 6: Assessment of equivalent qdu;
• Experiment 7: Effect of tandeming with other standards;
• Experiment 8: Talker Dependency;
• Experiment 9: Assessment of DTX algorithm.

Verification phase:

• Special background noise;
• Channel activity in DTX mode;
• Performance with DTMF tones;
• Performance with signalling tones;
• Delay;
• Frequency response;
• Complexity.

For the characterization tests, a practical "indirect" method of performance comparison between different codecs was adopted, that utilizes the Modulated Noise Reference Unit (MNRU) (see annex A reference 7)) as a reference degradation in a subjective experiment including the codecs under test. The MNRU provides the additional function of normalization across laboratories carrying out the same experiment, i.e. all MOS are converted to Equivalent Q (dB) and the results can be analysed statistically for differences between laboratories. An appropriate analysis of variance (ANOVA) was identified to evaluate the statistical significance of the experimental factors. The aim was to show that the subjective performance of the GSM half rate algorithm is at least as good as that of the full rate codec over a selected set of conditions. To allow for experimental error, the half rate candidate had to perform better than 1 dB below the performance of the full rate (for the overall figure of merit) and better than 3 dB below the performance of the full rate for individual test conditions. To model its use in a network, the half rate candidate codec had to be placed between either a ITU-T Recommendation G.711 [1] PCM coder and decoder, or a Uniform PCM, which provided the necessary A/D and D/A conversions. Source files of speech, produced either by using an "average" telephone set (called IRS - Intermediate Reference System) or a microphone showing a "flat" sending frequency characteristic (No IRS or "flat"), could then be processed through the different experimental conditions, for presentation to subjects in listening experiments. Among the different experimental conditions were error conditions at different input levels under both IRS A-Law PCM and No-IRS Linear PCM audio parts, tandeming conditions for different error patterns and background noise conditions. During all phases of testing, the host laboratory functions for the processing were provided by Aachen University of Technology (RWTH at Aachen, Germany). The whole set of "individual" and "global" data, collected in Experiment 1 to Experiment 9 were extensively analysed and discussed within TCH-HS expert group; for each condition, the MOS (or DMOS for Experiment 5) were computed, separately for male and female speech, as well as averaged together, and the effects of different factors and their interactions were subject to analysis of variance (ANOVA). Within characterization Phase 1, conversion to Q values and weighted averages were calculated for the whole set of results, in order to assess that the global figure of merit of the GSM half rate algorithm meets the quality requirement.

The present document gives background information on the performance of the GSM half rate speech codec. Experimental results from the characterization and verification tests carried out during the selection process by the Traffic CHannel Half rate Speech (TCH-HS) expert group are reported to give a more detailed picture of the behaviour of the GSM half rate speech codec under different conditions of operation.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

For the purposes of the present document, the following abbreviations apply: A/D ACR ANOVA C/I CEPT CNI D/A DAT DCR DSP DTMF DTX EID ETSI GBER GSM IRS HLCS ITU-T MNRU MOS MS OVL PCM Q qdu RPE-LTP SCD SFC SID SMG SNR TCH-HS TDMA UPCM VAD wMOPs   Four different Error Patterns (EP0, EP1, EP2 and EP3) were used, where:

• EP0 without channel errors;
• EP1 C/I=10 dB; 5 % GBER (well inside a cell);
• EP2 C/I= 7 dB; 8 % GBER (at a cell boundary);
• EP3 C/I= 4 dB; 13 % GBER (outside a cell).