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Draw the block diagram of Adaptive delta modulation system and explain its operation. What are the advantages of this over delta modulation.

Subject : Principle of Communication Engineering

Topic : PCM and Multiplexing

Difficulty : High

pce(44) • 8.0k  views
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  • 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 (t) , the step size is increased .When the input is varying slowly , the step size is reduced .Then the method is called Adaptive Delta Modulation(ADM).
  • The adaptive delta modulators can take continuous changes in step size or discrete changes in step size.
  • Fig.1 (a) shows the transmitter and Fig.1 (b) shows receiver of adaptive delta modulator. The logic for step size control is added in the diagram.
  • The step size increases or decreases according to certain rule depending on one bit quantizer output .For example if one bit quantizer output is high (1), then step size may be doubled for next sample.
  • enter image description here

    Fig.1(a) Delta modulation transmitter

    enter image description here

    Fig.1(b) Delta modulation receiver

  • In the receiver of adaptive delta delta modulator shown in Fig.1(b) the first part generates the step size from each incoming bit. Exactly the same process is followed as that in transmitter.

  • The previous input and present input decides the step size. It is then given to an accumulator which builds up staircase waveform.

  • The lowpass filter then smoothens out the staircase waveform to reconstruct the smooth signal.

  • If one bit quantizer output is low, then the step size may be reduced by one step. Fig.2 shows the waveforms of adaptive delta modulator and sequence of bits transmitted.

    enter image description here

Fig.2 Waveform of adaptive delta modulation

Advantages of ADM over DM

  • Delta modulation provides a staircase approximation of the input sampled signal where only one bit per sample is transmitted.
  • This one bit is sent by comparing the present sample value with the previous sample value and the result whether the amplitude is to be increased or decreased is transmitted.
  • If the step is reduced, 0 is transmitted and if the step is increased then 1 is transmitted.
  • Delta modulation has a disadvantage of the presence of slope overload distortion and granular noise.
  • Slope overload distortion arises due to large dynamic range of the input signal which results in large error between the original input signal and the staircase approximated signal.
  • When the slope of the signal is high, the step size must be increased to reduce slope overload distortion.
  • Granular noise arises when the step size is too large compared to the small variations in the input signal.
  • To overcome these quantization errors due to slope overload and granular noise, the step size is made adaptive to the variations in the input signal i.e. the step size is not fixed and can be increased or decreased depending on the variations of the input signal. Step size is determined by the previous and the present input samples.
  • If the input is varied slowly then the step size is decreased. These is then applied to the accumulator where staircase waveform is built at the transmitter end and at the receiver low pass filter passes out staircase waveform to reconstruct the original signal.
  • Reduction in the slope overload distortion and granular noise in Adaptive delta modulation induces improved signal to noise ratio as compared to the Delta modulation.
  • Since the step size is variable, the dynamic range of Adaptive delta modulation is wider than Delta modulation.
  • Bandwidth utilization is better in Adaptive delta modulation as compared to Delta modulation.
  • Thus Adaptive delta modulation superior to Delta modulation.
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  • Delta modulation is a Differential Pulse Code modulation (DPCM) technique in which the difference signal is encoded into a single bit.
  • Delta modulation provides a staircase approximation of the input sampled signal where only one bit per sample is transmitted.
  • This one bit is sent by comparing the present sample value with the previous sample value and the result whether the amplitude is to be increased or decreased is transmitted.
  • If the step is reduced, 0 is transmitted and if the step is increased then 1 is transmitted. The Fig1 illustrates the block diagram of Delta modulation transmitter.

    enter image description here

    Fig.1 Block diagram of Delta modulation transmitter.

  • Sample and hold circuit will sample the analog input signal into Pulse amplitude modulated (PAM) signal.

  • The generated PAM signal is given as one of the input to the comparator and the other input is a signal from DAC output.
  • The Up-down counter stores the magnitude of the previous sample in the binary value.
  • This binary number is converted into equivalent voltage in the Digital-to-analog converter (DAC).
  • The PAM signal and the DAC output are compared in the comparator, which implies that the sampled signal is compared against the previous sample to increase or decrease the amplitude of the DM signal.
  • The Up-down counter is incremented or decremented depending on whether the previous sample is larger or smaller than the current sample.
  • This counter is clocked at a rate equal to the sample rate, which is updated after each comparison.
  • Depending on the results of comparison, the output of the comparator generates the Delta pulse code modulated signal.
  • The Fig2 illustrates the block diagram of Delta modulation receiver.

    enter image description here

    Fig.2 Block diagram of Delta modulation receiver.

  • The receiver of the delta modulator consists of DAC, up/down counter and LPF. It does not contain the comparator.

  • The Delta PCM signal is fed to the up/down counter which works at the same sample rate as transmitter.

  • Depending on the binary input received the value in the up/down counter is accordingly incremented or decremented.

  • Based on the input received from the up/down counter, DAC will generate the output PAM signal.

  • The output signal of DAC in the transmitter and receiver is identical to reconstruct the signal.

  • This signal is then allowed to pass through a low pass filter which will filter out the high frequency components from the signal and thus produce the original analog signal.

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