Differential Pulse Code Modulation

Section Overview

The quantization in the section Quantizing a Signal requires no a priori knowledge about the transmitted signal. In practice, you can often make educated guesses about the present signal based on past signal transmissions. Using such educated guesses to help quantize a signal is known as predictive quantization. The most common predictive quantization method is differential pulse code modulation (DPCM).

The functions dpcmenco, dpcmdeco, and dpcmopt can help you implement a DPCM predictive quantizer with a linear predictor.

DPCM Terminology

To determine an encoder for such a quantizer, you must supply not only a partition and codebook as described in Represent Partitions and Represent Codebooks, but also a predictor. The predictor is a function that the DPCM encoder uses to produce the educated guess at each step. A linear predictor has the form

y(k) = p(1)x(k-1) + p(2)x(k-2) + ... + p(m-1)x(k-m+1) + p(m)x(k-m)

where x is the original signal, y(k) attempts to predict the value of x(k), and p is an m-tuple of real numbers. Instead of quantizing x itself, the DPCM encoder quantizes the predictive error, x-y. The integer m above is called the predictive order. The special case when m = 1 is called delta modulation.

Represent Predictors

If the guess for the kth value of the signal x, based on earlier values of x, is

y(k) = p(1)x(k-1) + p(2)x(k-2) +...+ p(m-1)x(k-m+1) + p(m)x(k-m)

then the corresponding predictor vector for toolbox functions is

predictor = [0, p(1), p(2), p(3),..., p(m-1), p(m)]

Note

The initial zero in the predictor vector makes sense if you view the vector as the polynomial transfer function of a finite impulse response (FIR) filter.

Example: DPCM Encoding and Decoding

A simple special case of DPCM quantizes the difference between the signal's current value and its value at the previous step. Thus the predictor is just y(k) = x (k - 1). The code below implements this scheme. It encodes a sawtooth signal, decodes it, and plots both the original and decoded signals. The solid line is the original signal, while the dashed line is the recovered signals. The example also computes the mean square error between the original and decoded signals.

predictor = [0 1]; % y(k)=x(k-1)
partition = [-1:.1:.9];
codebook = [-1:.1:1];
t = [0:pi/50:2*pi];
x = sawtooth(3*t); % Original signal
% Quantize x using DPCM.
encodedx = dpcmenco(x,codebook,partition,predictor);
% Try to recover x from the modulated signal.
decodedx = dpcmdeco(encodedx,codebook,predictor);
plot(t,x,t,decodedx,'--')
legend('Original signal','Decoded signal','Location','NorthOutside');
distor = sum((x-decodedx).^2)/length(x) % Mean square error

The output is

distor =

    0.0327

Optimize DPCM Parameters

Section Overview

The section Optimize Quantization Parameters describes how to use training data with the lloyds function to help find quantization parameters that will minimize signal distortion.

This section describes similar procedures for using the dpcmopt function in conjunction with the two functions dpcmenco and dpcmdeco, which first appear in the previous section.

Note

The training data you use with dpcmopt should be typical of the kinds of signals you will actually be quantizing with dpcmenco.

Example: Comparing Optimized and Nonoptimized DPCM Parameters

This example is similar to the one in the last section. However, where the last example created predictor, partition, and codebook in a straightforward but haphazard way, this example uses the same codebook (now called initcodebook) as an initial guess for a new optimized codebook parameter. This example also uses the predictive order, 1, as the desired order of the new optimized predictor. The dpcmopt function creates these optimized parameters, using the sawtooth signal x as training data. The example goes on to quantize the training data itself; in theory, the optimized parameters are suitable for quantizing other data that is similar to x. Notice that the mean square distortion here is much less than the distortion in the previous example.

t = [0:pi/50:2*pi];
x = sawtooth(3*t); % Original signal
initcodebook = [-1:.1:1]; % Initial guess at codebook
% Optimize parameters, using initial codebook and order 1.
[predictor,codebook,partition] = dpcmopt(x,1,initcodebook);
% Quantize x using DPCM.
encodedx = dpcmenco(x,codebook,partition,predictor);
% Try to recover x from the modulated signal.
decodedx = dpcmdeco(encodedx,codebook,predictor);
distor = sum((x-decodedx).^2)/length(x) % Mean square error

The output is

distor =

    0.0063