10 DTX System
3GPP46.008Half rate speechPerformance Characterization of the GSM Half Rate speech codecRelease 17TS
10.1 Assessment of DTX algorithm, Experiment 9
The four laboratories who performed the subjective evaluation of DTX functions concentrated their expert listening on the following effects, using conversational speech;
– Voice Activity Detection (VAD); and
– Comfort Noise Insertion (CNI).
For this, the speech material available was monitored for the following effects:
– speech clipping;
– noise quality;
– noise contrast.
The tests showed that malfunctions of the VAD and the CNI were only predominant with low SNRs. The VAD functions appeared to work well in most situations (i.e. rather little clipping). In many situations, the Comfort Noise Insertion did not operate properly, being poorly matched in terms of quality and/or level. The DTX performed better with hand-held terminals relative to its performance with hands-free.
10.2 Channel activity in DTX mode
10.2.1 Test procedure
Speech material recorded during testing of the full rate DTX system was processed through the codec/DTX hardware. This material comprised real conversations in the English, French, German and Italian languages. The activity of the VAD algorithm was measured for all 480 conversations. The mean channel activity was then calculated by means of a software simulation of the TX DTX handler.
10.2.2 Speech channel activity
The percentage of speech frames scheduled for transmission by the radio sub-system (subsequently referred to as the speech channel activity) varied significantly between conversations. Speech channel activities ranged from 35 % to 85 % for individual sides of a conversation. For this reason, it was not possible to identify any significant trends in the results with regard to terminal type and environmental conditions. The mean speech channel activity, measured over all 480 conversations, was approximately 55 %.
10.2.3 Level compensation
During the expert listening, it was found that the speech material had been processed at a level 6,5 dB below the original recorded level. However, the activity of the basic VAD algorithm rises approximately 0,5 % per dB increase in input level. To compensate for this, a factor of 3 % must be added to the speech channel activity estimate.
10.2.4 SID update rate
The DTX handler simulation used a SID update period of 480 ms. The SID update rate has subsequently been reduced to 240 ms. This modification will raise the speech channel activity by approximately 2 %.
10.2.5 Interleaving compensation
The channel measurements were calculated on a signal frame basis. However, the use of interleaving (depth 4) implies that the TDMA activity will be approximately 2 % higher than the signal frame activity.
10.2.6 Estimated mean TDMA channel activity
The estimated mean TDMA channel activity is shown in table 6.
Table 6: Calculation of mean TDMA channel activity
speech channel activity |
55 % |
level compensation |
3 % |
240 ms SID update period |
2 % |
interleaving compensation |
2 % |
total TDMA channel activity |
62 % |