6 Quality under tandeming conditions

3GPP46.008Half rate speechPerformance Characterization of the GSM Half Rate speech codecRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.1 Quality under tandeming conditions, Experiment 3

A listening-only test was chosen, adopting the Absolute Category Rating (ACR) method. Subjective tests were carried out by BT (United Kingdom), CSELT (Italy), and DEUTSCHE TELEKOM (Germany). Table 3 reports the results obtained in Experiment 3: each cell shows the difference in terms of equivalent Q values between the candidate and the full rate, negative values meaning worse performance than the full rate.

Table 3: Results from experiment 3 (Tandem Conditions)

	A-Law PCM (with IRS) DQ (dB) = (HR+HR)-(FR+FR)				Linear PCM (No IRS) DQ (dB) = (HR+HR)-(FR+FR)
	Input Level (dB relative to OVL)				Input Level (dB relative to OVL)
Error Pattern		-12	-22	-32	-12	-22	-32
EP0		-0,14	-0,56	-0,03	-5,20	-5,43	-3,89
EP1		-0,46	-0,75	0,49	-4,98	-4,14	-2,79

NOTE: The figures indicate DQ values in dB, where DQ = QHR – QFR.

In general, two candidate codecs in tandem performed equally well or slightly worse than two full rate codecs in tandem for the A-Law IRS audio part, while in most cases exceeded the -3 dB limit for the Uniform PCM No IRS audio part.

In operating networks, A-law coding and decoding is performed between both speech processing steps in both mobile to mobile calls. Therefore, the results of real network configurations are expected to be somewhere in between the figures obtained using the A-law input speech material and those obtained using the linear PCM speech material for each condition.

6.2 Effect of tandeming with other standards, Experiment 7

The experiment was conducted in two different laboratories: BT (UK) and CNET (France).

The following standards were tandemed with the half rate codec in this experiment: half rate, full rate, ITU-T Recommendation G.726 [2] (at 32 kbit/s) and G.728 [3]. Both possible orders of tandeming were tested for each of these cases, in both error free and EP1 conditions. The error pattern EP1 was only applied to the full and half rate codecs.

The main conclusion that can be drawn is that the performance is always better when the half rate codec follows the other codec in the tandeming chain. This effect is most pronounced at the higher speech input level (12 dB below overload point).