Speech Codec

It is known that speech codecs give different values of subjective speech quality, depending on the compression rates and the encoding algorithm employed (as discussed in Section 3.5). For no network impairments (LR=0), the subjective quality obtained is 4.6, 4.1 and 3.7 in the Arabic language when using PCM, ADPCM and GSM codecs respectively. In Spanish language, the obtained scores are 4.7, 4.2 and 3.8. (Refer to Figure 7.3.) These results show that the subjective quality tests we have carried out and explained in Chapter 5 is in agreement with what can be found in the literature, as it is reported that the respective scores for these codecs are 4.4, 4.1 and 3.6 for the English language [60,79]. We can also see from Figure 7.2 that the codecs considered do not tolerate the loss equally. Thus, for the same network conditions, by changing the speech codec, we can get better quality. For example, there is an improvement of 0.6 when changing from GSM to PCM at both LR=5 % and 10 %.

Figure 7.1: On the left, we show the impact of LR and CLP on speech quality for the different codecs and PI=20 ms. On the right we show the effect of LR and PI on speech quality for CLP=1.

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Figure 7.2: The impact of CLP and LR on speech quality when LR=5 % (left) and when LR=10 %(right) for PCM, ADPCM and GSM codecs.

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Figure 7.3: The variations of the quality as a function of the LR and the employed speech codec in both languages for PI=20 ms and CLP=2.