Efficient parameter quantisation for 2.4/1.2 kb/s split-band LPC coding

Speech coding at very low bit rates has many applications such as answering machines, IP telephony, mobile communications, military communications etc. Most low bit rate coders operate at around 2.4 kb/s, as the speech quality degrades too much below this bit rate. We describe a frequency domain speech coder capable of operating at both 2.9 and 1.2 kb/s, and produces good quality synthesised speech. Both rates use the same analysis and synthesis building blocks over 20 ms, but the 1.2 kb/s coder jointly quantises three sets of parameters every 60 ms to reduce the bit rate while maintaining speech quality. We also describe the quantisation methods used to lower the bit rate from 2.4 kb/s to 1.2 kb/s while retaining most of the quality of the higher bit rate version.