Delta Modulation (DM)

Delta Modulation (DM)

A variety of techniques have been used to improve the performance of PCM or
to reduce its complexity. One of the most popular alternatives to PCM is delta
modulation (DM).
With delta modulation, an analog input is approximated by a staircase function
that moves up or down by one quantization level (0) at each sampling interval
(Ts )' An example is shown in Figure 6.18, where the staircase function is overlaid
on the original analog waveform. The important characteristic of this staircase
function is that its behavior is binary: At each sampling time, the function moves up
or down a constant amount o. Thus, the output of the delta modulation process can
be represented as a single binary digit for each sample. In essence, a bit stream is
produced by approximating the derivative of an analog signal rather than its amplitude:
A 1 is generated if the staircase function is to go up during the next interval; a
ois generated otherwise.
The transition (up or down) that occurs at each sampling interval is chosen
so that the staircase function tracks the original analog waveform as closely as

possible. Figure 6.19 illustrates the logic of the process, which is essentially a feedback
mechanism. For transmission, the following occurs: At each sampling time,
the analog input is compared to the most recent value of the approximating staircase
function. If the value of the sampled waveform exceeds that of the staircase
function, a 1 is generated; otherwise, a 0 is generated. Thus, the staircase is always
changed in the direction of the input signal. The output of the DM process is
therefore a binary sequence that can be used at the receiver to reconstruct the
staircase function. The staircase function can then be smoothed by some type of
integration process or by passing it through a low-pass filter to produce an analog
approximation of the analog input signal.
There are two important parameters in a DM scheme: the size of the step
assigned to each binary digit, 8, and the sampling rate. As Figure 6.18 illustrates, 8
must be chosen to produce a balance between two types of errors or noise. When
the analog waveform is changing very slowly, there will be quantizing noise. This
noise increases as 8 is increased. On the other hand, when the analog waveform is
changing more rapidly than the staircase can follow, there is slope overload noise.
This noise increases as 8 is decreased.
It should be clear that the accuracy of the scheme can be improved by
increasing the sampling rate. However, this increases the data rate of the output
signal. The principal advantage ofDMoverPCM is the simplicity of its implementation.
In general, PCM exhibits better SNR characteristics at the same data rate.