Delta Modulation - Principles of Communication

What is Delta Modulation?

The sampling rate of a signal must be higher than the Nyquist rate, to attain better sampling. If this sampling interval in a Differential PCM (DPCM) is condensed significantly, the sample-to-sample amplitude alteration is very small, as if the difference is 1-bit quantization, then the step-size is very small i.e., Δ (delta).

What is Delta Modulation?

The type of modulation, where the sampling rate is much higher and in which the stepsize after quantization is of smaller value Δ, such a modulation is termed as delta modulation.

Features of Delta Modulation

  • An over-sampled input is taken to make full use of a signal correlation.
  • The quantization design is simple.
  • The input sequence is much higher than Nyquist rate.
  • The quality is moderate.
  • The design of the modulator and the demodulator is simple.
  • The stair-case approximation of output waveform.
  • The step-size is very small, i.e., Δ (delta).
  • The bit rate can be decided by the user.
  • It requires simpler implementation.

Delta Modulation is a shortened form of DPCM technique, also viewed as 1-bit DPCM scheme. As the sampling interval is reduced, the signal correlation will be higher.

Delta Modulator

The Delta Modulator includes of a 1-bit quantizer and a delay circuit along with two summer circuits. Resulting is the block diagram of a delta modulator.


delta_modulator

A stair-case estimated waveform will be the output of the delta modulator with the step-size as delta (Δ). The output quality of the waveform is moderate.

Delta Demodulator

The delta demodulator includes of a low pass filter, a summer, and a delay circuit. The predictor circuit is removed here and therefore no expected input is given to the demodulator.

Resulting is the block diagram for delta demodulator.

delta_demodulator

Low pass filter is used for numerous reasons, but the noticeable one is noise elimination for out-of-band signals. The step-size error that may happen at the transmitter is called granular noise, which is removed here. If there is no noise present, then the modulator output equals the demodulator input.

Advantages of DM over DPCM

  • 1-bit quantizer
  • Very easy design of modulator & demodulator

On the other hand, there exists some noise in DM and following are the types of noise.

  • Slope Over load distortion (when Δ is small)
  • Granular noise (when Δ is large)

Adaptive Delta Modulation

In digital modulation, we originate across assured problems in determining the step-size, which effects the quality of the output wave.

The larger step-size is desirable in the steep slope of modulating signal and a smaller stepsize is desirable where the message has a small slope. As a result, the minute details get missed. Therefore, it would be better if we can control the adjustment of step-size, according to our condition in order to obtain the sampling in a desired fashion. This is the concept of Adaptive Delta Modulation (ADM).

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