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Baseband Pulse and Digital Signaling Chap. 3
In the second DPCM configuration, shown in Fig. 3–30, the predictor operates on
quantized values at the transmitter as well as at the receiver in order to minimize the quantization
noise on the recovered analog signal. The analog output at the receiver is the same as the input
analog signal at the transmitter, except for quantizing noise; furthermore, the quantizing noise
does not accumulate, as was the case in the first configuration.
It can be shown that DPCM, like PCM, follows the 6-dB rule [Jayant and Noll, 1984]
a S N b = 6.02n + a
dB
(3–80a)
where
-3 6 a 6 15 for DPCM speech
(3–80b)
and n is the number of quantizing bits (M = 2 n ). Unlike companded PCM, the a for
DPCM varies over a wide range, depending on the properties of the input analog signal.
Equation (3–80b) gives the range of a for voice-frequency (300 to 3,400 Hz) telephonequality
speech. This DPCM performance may be compared with that for PCM. Equation
(3–26b) indicates that a =-10 dB for µ-law companded PCM with µ = 255. Thus, there
may be an SNR improvement as large as 25 dB when DPCM is used instead of µ = 255
PCM. Alternatively, for the same SNR, DPCM could require 3 or 4 fewer bits per sample
than companded PCM. This is why telephone DPCM systems often operate at a bit rate
of R = 32 kbitss or R = 24 kbitss, instead of the standard 64 kbitss needed for
companded PCM.
The CCITT has adopted a 32-kbitss DPCM standard that uses 4-bit quantization at an
8-ksamples rate for encoding 3.2-kHz bandwidth VF signals [Decina and Modena, 1988].
Moreover, a 64-kbitss DPCM CCITT standard (4-bit quantization and 16 ksampless) has
been adopted for encoding audio signals that have a 7-kHz bandwidth. A detailed analysis of
DPCM systems is difficult and depends on the type of input signal present, the sample rate,
the number of quantizing levels used, the number of stages in the prediction filter, and the predictor
gain coefficients. This type of analysis is beyond the scope of this text, but for further
study, the reader is referred to published work on the topic [O’Neal, 1966b; Jayant, 1974;
Flanagan et al., 1979; Jayant and Noll, 1984 ].
3–8 DELTA MODULATION
From a block diagram point of view, delta modulation (DM) is a special case of DPCM in
which there are two quantizing levels. As shown in Fig. 3–30, for the case of M = 2, the quantized
DPAM signal is binary, and the encoder is not needed because the function of the
encoder is to convert the multilevel DPAM signal to binary code words. For the case of M = 2,
the DPAM signal is a DPCM signal where the code words are one bit long. The cost of a DM
system is less than that of a DPCM system (M > 2) because the analog-to-digital converter
(ADC) and digital-to-analog converter (DAC) are not needed. This is the main attraction of
the DM scheme—it is relatively inexpensive. In fact, the cost may be further reduced by
replacing the predictor by a low-cost integration circuit (such as an RC low-pass filter), as
shown in Fig. 3–31.