Performance and error sensitivity comparison of low and high delay CELP codecs between 8 and 4 kbits/s

In this paper we study the performance and error sensitivities of several CELP based codecs operating between 8 and 4 kbits/s. Both forward and backward adaption techniques are used for the short and the long term predictors, and both trained and algebraic excitation codebooks are used. Three codecs which employ backward adaption of their short term predictors and operate with frame-lengths of 3 ms or less are described. These codecs operate between 8 and 4 kbits/s and their performance at these bit rates is compared to a traditional forward adaptive algebraic CELP codec operating at 4.7, 6.5 and 7.1 kbits/s. Furthermore, the error sensitivity of the backward adaptive codecs, and means of improving this error sensitivity, are investigated. Finally, we compare the error sensitivity of the low delay, backward adaptive codecs to the high delay, forward adaptive codecs. Surprisingly, we found that it is possible to achieve good error resilience, comparable to that of the forward adaptive codec, using low delay backward adaptive codecs.