Subject : Principle of Communication Engineering

Topic : PCM and Multiplexing

Difficulty : High

• 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.

  

  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.

  

  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.

• 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.

  

  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.

  

  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.