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load the DPCM files (dpcm.tar).
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here to down load the DPCM files (dpcm.zip).
In PCM, each sample of the waveform is encoded independently of all the other samples. However, most source signals including speech sampled at the Nyquist rate or faster exhibit significant correlation between successive samples. In other words, the average change in amplitude between successive samples is relatively small. Consequently an encoding scheme that exploits the redundancy in the samples will result in a lower bit rate for the source output.
A relatively simple solution is to encode the differences between successive samples rather than the samples themselves. The resulting technique is called differential pulse code modulation (DPCM). Since differences between samples are expected to be smaller than the actual sampled amplitudes, fewer bits are requrired to represent the differences. In this case we quantize and transmit the differenced signal sequence
where s(n) is the sampled sequence of s(t).
A natural refinement of this general approach
is to predict the current sample based on the previous M samples utilizing
linear prediction (LP), where LP parameters are dynamically estimated.
Block diagram of a DPCM encoder and decoder is shown below. Part (a) shows
DPCM encoder and part (b) shows DPCM decoder at the receiver.
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The "dpcm_demo" shows the use of DPCM to approximate a input sine wave signal and a speech signal that were sampled at 2 KHz and 44 KHz, respectively. The source code file of the MATLAB code and the output can be viewed using MATLAB.
To view these you need to download the zipped MATLAB files and sound file into a directory, unzip them (example: "unzip fname.zip" on unix), then run demo file on MATLAB. To run the demo file, type "dpcm_demo" at the MATLAB prompt. (Remember the change directory into the same directory that the files were placed in.)
An alternative solution is to employ logarithmic compressor and a variable step size that adapts itself to the short-term characteristics of the signal, yields adaptive DPCM (ADPCM).