Real-time implementation and performance of a 16 kb/s low-delay CELP speech coder

A real-time implementation of the low-delay code-excited linear prediction (LD-CELP) speech coder based on the AT&T DSP32C is described. The performance of the coder is described. Recursive windowing is used to calculate the autocorrelation coefficients by dividing the Levinson-Durbin recursion into smaller parts and spreading the computation over several speech vectors. The LD-CELP encoder uses about 90% of the processor time of an 80-ns DSP32C, while the decoder uses about 40%. Hence, the coder is implemented on two DSP32Cs. For a single encoding with clear or noisy channels, the speech quality produced by this coder is either equivalent to or better than the CCITT G.721 standard 32-kb/s ADPCM. However, for multiple asynchronous encodings the ADPCM coders gives slightly better performance. The LD-CELP coder passes dual-tone multifrequency tones and 300-, 1200-, and 2400-b/s modem signals