Linear Predictive Coders (LPC)
LPC are time domain vecoders.Vecoders are attempt to extract the significant features of the speech signal from the time waveform. By using LPC it become possible to transmit the good quality voice at 4.8kbps and poorer quality voice at lower bit rate.
Block Diagram of LPC:
- The linear predictive coding system models the vocal tract as an all pole linear filter with a transfer function described by … (1)
G=Gain of the filter Z-1=A unit delay operation
- The prediction principles used are similar to those in ADPCM coder.
- LPC system transmits only selected characteristics of the error signal,instead of transmitting quantized values of the error signal.
- At the receiver, the received information about the error signal is used to determine the appropriate excitation for the synthesis filter.
- The error signal is the excitation to the decoder.(See Figure 5.2)
- The synthesis filter is designed at the receiver using the received predictor coefficients
Determination of Predictor coefficients:
The linear predictive coder uses a weighted sum of p past samples to estimate the present sample, where p is typically in the range of 10-15. Using this technique, the current sample Sncan be written as a linear sum of the immediately preceding samples Sn-k
en is the prediction error (residual).