Low bit rate LPC vocoders using vector quantization and interpolation

Results of simulations are presented for LPC (linear predictive coding) vocoders using both scalar and vector quantization as well as interpolation of the quantized parameters. A conventional LPC-8 vocoder was initially carefully designed to be used as a reference system. The use of a pitch detector based on a skeleton of the speech signal and a well-designed quantization table for the log area ratio coefficients provided a high level of speech quality and intelligibility at 2200 b/s. By employing vector quantization for the LPC coefficients and codebooks based on a large database, a similar performance was achieved at a bit rate in the range from 900 to 1150 b/s. Furthermore, with the use of frame interpolation it was possible to reach 450 b/s with a quality suitable for many low bit rate applications.