What is Slope Overload distortion and Granular Noise in Delta Modulation and how it is removed in ADM
The delta modulation has two major drawbacks as under :
- Slope overload distortion
- Granular or idle noise
Now, we will discuss these two drawbacks in detail.
-
Slope Overload Distortion
This distortion arises because of large dynamic range of the input signal.
Fig.1: Quantization Errors in Delta Modulation
We can observe from fig.1 , the rate of rise of input signal x(t) is so high that the staircase signal can not approximate it, the step size ‘Δ’ becomes too small for staircase signal u(t) to follow the step segment of x(t).
Hence, there is a large error between the staircase approximated signal and the original input signal x(t).
This error or noise is known as slope overload distortion .
To reduce this error, the step size must be increased when slope of signal x(t) is high.
2. Granular or Idle Noise
Granular or Idle noise occurs when the step size is too large compared to small variation in the input signal.
This means that for very small variations in the input signal, the staircase signal is changed by large amount (Δ) because of large step size.
Fig.1 shows that when the input signal is almost flat , the staircase signal u(t) keeps on oscillating by ±Δ around the signal.
The error between the input and approximated signal is called granular noise.
The solution to this problem is to make the step size small .
Solution
In order to overcome the quantization errors due to slope overload and granular noise, the step size (Δ) is made adaptive to variations in the input signal x(t).
Particularly in the steep segment of the signal x(t), the step size is increased. And the step is decreased when the input is varying slowly.
This method is known as Adaptive Delta Modulation (ADM).
The adaptive delta modulators can take continuous changes in step size or discrete changes in step size.
