UploadFile_6417

618 Chapter 12 APPLICATIONS IN COMMUNICATIONS
decoder, including an inverse adaptive quantizer, is embedded in the
encoder so that all internal variables are updated, based on the same
data. This ensures that the encoder and decoder operate in synchronism
without the need to transmit any information on the values of internal
variables.
The adaptive predictor computes a weighted average of the last six
dequantized difference values and the last two predicted values. Hence this
predictor is basically a 2-pole (p =2)and 6-zero (m =6)filter governed
by the difference equation given by (12.16). The filter coefficients are
updated adaptively for every new input sample.
At the receiving decoder and at the decoder that is embedded in
the encoder, the 4-bit transmitted ADPCM value is used to update the
inverse adaptive quantizer, whose output is a dequantized version of the
difference signal. This dequantized value is added to the value generated
by the adaptive predictor to produce the reconstructed speech sample.
This signal is the output of the decoder, which is converted to compressed
PCM format at the receiver.
12.3.2 PROJECT 12.3: ADPCM
The objective of this project is to gain familiarity with, and understanding
of, ADPCM and its interface with a PCM encoder/decoder (transcoder).
As described, the ADPCM transcoder is inserted between the PCM compressor
and the PCM expander as shown in Figure 12.7. Use the already
developed MATLAB PCM and DPCM modules for this project.
The input to the PCM-ADPCM transcoder system can be supplied
from internally generated waveform data files, just as in the case of the
PCM project. The output of the transcoder can be plotted. Comparisons
should be made between the output signal from the PCM-ADPCM
transcoder with the signal from the PCM transcoder (PCM Project 12.1),
and with the original input signal.
12.4 DELTA MODULATION (DM)
Delta modulation may be viewed as a simplified form of DPCM in which
a 2-level (1-bit) quantizer is used in conjunction with a fixed 1st-order
predictor. The block diagram of a DM encoder-decoder is shown in
Figure 12.8. We note that
̂˜s(n) =˜s (n − 1) = ̂˜s (n − 1) + ẽ (n − 1) (12.21)
Since
[ ]
q(n) =ẽ(n) − e(n) =ẽ(n) − s(n) − ̂˜s(n)
Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.