LPC (Linear Predictive Coder) Implemented in MATLAB

ABSTRACT

An LPC vocoder (Linear-Predictive Coder), is a lossy compression technique

that is highly optimized for low-bitrate speech transmission. The problem here that we

solve is to take a speech waveform with an 8kHz sampling rate, code it to a much lower

bit-rate so that it can be transmitted over a channel, and finally decode it to a signal that is

of the same quality.

To do this we take advantage of the fact that the human vocal tract can be modelled

as a linear system for a period of about 25 ms. We estimate the spectrum of the vocal

tract in frames of 25 ms (see LPC analysis) and estimate the pitch of the speech once

every 10 ms. We then transmit this data, which is much less than transmitting 8000

samples every second. To reproduce the speech we simply excite the estimated spectrum

with an impulse train of the estimated pitch..

A gain estimation method for an LPC vocoder which utilizes shape indexes. The

gain is estimated based on the envelope of the speech waveform. The gain is estimated

such that the maximum amplitude of the synthetic speech just reaches the speech

waveform envelope. The gain during voiced subframes is estimated as the minimum of

the absolute value of ratio of the envelope and the impulse response of the LPC filter.

The gain during unvoiced subframes is estimated as the minimum of the absolute value of

the ratio of the envelope and the noise response of the LPC filter. The method results in a

fast technique for estimating the gain.