For the second factor we assume that once the value of is known, knowledge of the previous states and observations adds no new information. The phoneme dictionary may differ from language to language but for any one language, all sounds are represented. In a channel vocoder, the spectrum is represented by the magnitude signals from a bank of bandpass filters covering the speech spectrum of interest. Since this representation varies slowly with frequency, it can be defined by as few as 15 spectral samples. In LPC, the synthesizer can be modelled as an all-pole digital filter of approximately 10th order.
Empirical results yield a bit rate comparable to that of a channel vocoder.
In all three cases, the excitation parameters and the vocal tract parameters are coded separately. For the low rate versions, slowly varying pitch and voicing parameters are detected and coded. The derived phoneme sequence is transformed into a varying set of parameters to control a specific speech synthesizer. The prototype index is used to look up a discrete density function, i.e., the probability of any state given the vector prototype. Briefly compare this to their use in discrete density HMM-based speech recognizers.