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Chapter 5. Pulse Modulation. From Analog to Digital. 5.6 Pulse-Code Modulation Pulse-Code Modulation. Pulse-code modulation (PCM) Most basic form of digital pulse modulation

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chapter 5 pulse modulation

Chapter 5. Pulse Modulation

From Analog to Digital

5 6 pulse code modulation pulse code modulation
5.6 Pulse-Code ModulationPulse-Code Modulation
  • Pulse-code modulation (PCM)
    • Most basic form of digital pulse modulation
    • A message signal is converted to discrete form in both time and amplitude, and represented by a sequence of coded pulses
  • The basic operation
    • Transmitter: sampling, quantization, encoding
    • Receiver: regeneration, decoding, reconstruction
5 6 pulse code modulation operations in the transmitter
5.6 Pulse-Code ModulationOperations in the Transmitter
  • Sampling
    • The incoming baseband signal is sampled with a train of rectangular pulses at greater than Nyquist rate
    • Anti-alias (low-pass) filter
  • Nonuniform quantization
    • In real sources such as voice, samples of small amplitude appear frequently whereas large amplitude is rare.
    • Nonuniformquantizer is equivalent to compressor followed by uniform quantizer.
law compressor
–law compressor

Output level


Input level

5 6 pulse code modulation operations in the transmitter3
5.6 Pulse-Code ModulationOperations in the Transmitter
  • Encoding
    • To translate the discrete set of sample values to a more appropriate form of signal
    • A binary code
      • The maximum advantage over the effects of noise because a binary symbol withstands a relatively high level of noise.
      • The binary code is easy to generate and regenerate
5 6 pulse code modulation regeneration along the transmission path
5.6 Pulse-Code ModulationRegeneration along the Transmission Path
  • The ability to control the effects of distortion and noise produced by transmitting a PCM signal over a channel
  • Ideally, except for delay, the regenerated signal is exactly the same as the information-bearing signal
  • Equalizer: Compensate for the effects of amplitude and phase distortions produced by the transmission
  • Timing circuitry: Renewed sampling of the equalized pulses
  • Decision-making device
5 6 pulse code modulation operations in the receiver
5.6 Pulse-Code ModulationOperations in the Receiver
  • Decoding
    • Regenerating a pulse whose amplitude is the linear sum of all the pulses in the codeword
  • Expander
    • A subsystem in the receiver with a characteristic complementary to the compressor
    • The combination of a compressor and an expander is called a compander.
  • Reconstruction
    • Recover the message signal by passing the expander output through a low-pass reconstruction filter
5 7 delta modulation basic considerations
5.7 Delta ModulationBasic Considerations
  • Delta modulation (DM)
    • An incoming message signal is oversampled (at a rate much higher than Nyquist rate).
    • The correlation between adjacent samples is introduced.
    • It permits the use of a simple quantization.
    • Quantization into two levels ()

where is an error signal

5 7 delta modulation system details
5.7 Delta ModulationSystem Details
  • Comparator
    • Computes the difference between its two inputs
  • Quantizer
    • Consists of a hard limiter with an input-output characteristic that is a scaled version of the signum function
  • Accumulator
    • Operates on the quantizer output so as to produce an approximation to the message signal
5 7 delta modulation quantization errors
5.7 Delta ModulationQuantization Errors
  • Slope-overload distortion
    • Occurs when the step size is too small
    • The approximation signal falls behind the message signal
  • Granular noise
    • Occurs when the step size is too large
    • The staircase approximation hunts around a flat segment.
5 7 delta modulation delta sigma modulation
5.7 Delta ModulationDelta-Sigma Modulation
  • Drawback of DM
    • An accumulative error in the demodulated signal
  • Delta-sigma modulation (D-M)
    • Integrates the message signal prior to delta modulation
  • Benefit of the integration
    • The low-frequency content of the input signal is pre-emphasized
    • Correlation between adjacent samples of the delta modulator input is increased
    • Design of the receiver is simplified
5 8 differential pulse code modulation prediction
5.8 Differential Pulse-Code ModulationPrediction
  • Samples at a rate higher than Nyquist rate contain redundant information.
  • By removing this redundancy before encoding, we can obtain more efficient encoded signal.
  • If we know the past behavior, it is possible to make some inference about its future values.
  • Predictor
    • : prediction of
    • : prediction error
    • Implemented by tapped-delay-line filter (discrete-time filter)
    • : prediction order
5 8 differential pulse code modulation differential pulse code modulation1
5.8 Differential Pulse-Code ModulationDifferential Pulse-Code Modulation
  • Differential pulse-code modulation (DPCM)
    • Oversampling + differential quantization + encoding
    • The input of differential quantization is the prediction error.

where is quantization error.

    • (5.38) is regardless of the properties of the prediction filter.
    • If the prediction is good, the average power of will be smaller than  Smaller quantization error
5 9 line codes line codes
5.9 Line CodesLine Codes
  • A line code is needed for electrical representation of a binary sequence.
  • Several line codes
    • On-off signaling
    • Nonreturn-to-zero (NRZ)
    • Return-to-zero
    • Bipolar return-to-zero (BRZ)
    • Split-phase (Manchester code)
    • Differential encoding
5 9 line codes line codes1
5.9 Line CodesLine Codes

Bipolar return to zero(BRZ)

On-off signaling

Split-phase (Manchester code)

Non return-to-zero(NRZ)


Differential encoding

5 9 line codes differential encoding
5.9 Line CodesDifferential Encoding

0: Transition

1: No transition

Reference bit: 1

Differential encoding