Audio Technology / Analysis and Delivery / LPC in acoustic analysis...

Linear Predictive Coding (LPC) is often used by linguists as a formant extraction tool. There are a few important details about LPC that may help avoid common analysis errors. LPC analysis assumes that a signal is the output of a causal linear system. It also assumes that the vocal-tract system is an all-pole filter and that the input to the system is an impulse train. Because of these assumptions, LPC analysis is usually most appropriate for modeling vowels which are periodic and for which the vocal-tract resonator does not usually include zeroes (e.g., in nasalized vowels). The order of an LPC model is the number of poles in the filter. Usually, two poles are included for each formant + 2-4 additional poles to represent the source characteristics. For adult speakers, average formant spacing is in the 1000 Hz range for males and in the 1150 Hz range for females. The LPC order is related to the sample rate of the audio file: 10000 Hz - LPC order = 12-14 (males) and 8-10 (females); 22050 Hz - - LPC order = 24-26 (males) and 22-24 (females).

LPC usually requires a very good speech sample to work with. Many recordings done with omnidirectional microphones contain too little speech detail and too much noise to ascertain reliable LPC readings ( GRAPH>> ).