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Sec. 3–8 Delta Modulation 201
Granular Noise and Slope Overload Noise
From Fig. 3–32a, it is seen that the accumulator output signal does not always track the
analog input signal. The quantizing noise error signal may be classified into two types of
noise: slope overload noise and granular noise. Slope overload noise occurs when the step
size d is too small for the accumulator output to follow quick changes in the input waveform.
Granular noise occurs for any step size, but is smaller for a small step size. Thus, we
would like to have d as small as possible to minimize the granular noise. The granular
noise in a DM system is similar to the granular noise in a PCM system, whereas slope
overload noise is a new phenomenon due to a differential signal (instead of the original
signal itself) being encoded. Both phenomena are also present in the DPCM system discussed
earlier.
It is clear that there should be an optimum value for the step size d, because if d is
increased, the granular noise will increase, but the slope overload noise will decrease. This
relationship is illustrated in Fig. 3–33.
Slope overload
dominates
Granular noise
dominates
Signal-to-noise ratio (dB)
Step size, Î
Optimum value
of Î
Figure 3–33
Signal-to-noise ratio out of a DM system as a function of step size.