Subject : Principle of Communication Engineering

Topic : PCM and Multiplexing

Difficulty : Low

Delta Modulation

• In PCM system, N number of binary digits are transmitted per quantized sample. Hence the signalling rate and transmission channel bandwidth of the PCM system are very large.
• These disadvantages can be overcome by using the delta modulation.
• Delta modulation transmits only one bit per sample instead of N bits transmitted in PCM. This reduces its signalling rate and bandwidth requirement to a greater extent. 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.

  

  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.

  Advantages

• Low signalling rate and low transmission channel bandwidth because in DM only one bit is transmitted per sample

• The delta modulator transmitter and receiver are less complicated to implement as compared to the PCM

  Disadvantages

• The two distortions slope overhead error and granular noise are present.

• Practically the signalling rate with no slope overhead error will be much higher than that of PCM.

  

Delta Modulation

• In PCM system, N number of binary digits are transmitted per quantized sample. Hence the signalling rate and transmission channel bandwidth of the PCM system are very large.
• These disadvantages can be overcome by using the delta modulation.
• Delta modulation transmits only one bit per sample instead of N bits transmitted in PCM. This reduces its signalling rate and bandwidth requirement to a greater extent.
• In the basic of a linear DM a staircase approximated version of the sampled input signal is produced.
• The original signal and its staircase representation are then compared to produce a difference signal.
• This difference signal is quantized in to two levels namely, corresponding to the positive and negative difference respectively.
• That means if the corresponding signal x’(t) lies below x(t) at the same sampling instant, then the approximated signal is increased by certain value and binary output will be 0 .

• Whereas if x’(t) is greater than x(t) at the sampling instant, then x’(t) is decreased by by certain value and binary output will be 1.

  

  Fig.2 Block digram of Delta-Modulation

  Advantages

• Low signalling rate and low transmission channel bandwidth because in DM only one bit is transmitted per sample

• The delta modulator transmitter and receiver are less complicated to implement as compared to the PCM

  Disadvantages

• The two distortions slope overhead error and granular noise are present.

• Practically the signalling rate with no slope overhead error will be much higher than that of PCM.