A Range of Low and High Delay CELP Speech Codecs between 8 and 4 kbits/s

Abstract

In this paper we study the performance and the error sensitivities of six CELP [1] based codecs operating between 8 and 4 kbits/s. Codecs using both forward and backward adaption of the linear prediction coefficients and the long term predictor (LTP) are described. Initially we describe four low delay codecs which all use backward adaption of the LPC coefficients but which differ in their use of LTP. These codecs all have frame-lengths of 3 ms or less, and their performance at various bit rates between 8 and 4 kbits/s is examined. Next the error sensitivity of these codecs, and means of improving it, are described. Then an algebraic CELP (ACELP) [2] codec operating at 6.2 kbits/s with a frame-length of 5 ms is described. Our final codec also uses ACELP and operates between 4.7 and 7.1 kbits/s, but it is forward adaptive and so it has a much longer frame-length of up to 30 ms. After describing this codec we compare the performance of our codecs in both error-free conditions and in the presence of channel errors. Surprisingly the error sensitivity of the low delay backward adaptive codec with no LTP is similar to that of the forward adaptive, high delay, ACELP codec.