Pulse Code Modulation (PCM) Fundamentals

Pulse Code Modulation (PCM)

• A digital encoding modulation technique that converts an analog signal into a digital signal via sampling.
• PCM signal consists of a group of pulses that represent the inputted analog data.

History of PCM

• Invented by Alec H. Reeves in 1937 while working for AT&T.

Advantages of PCM

• Most preferred method in tele-communications for digitizing voice and digital data.
• Offers ease of digitizing voice and digital data with high-speed signal transmission through metallic and optic-fiber cables.

Limitations of Other Modulation Techniques

• PPM and PWM have bandwidth limitations due to resolution requirements in transmitting 16-bits and beyond.
• PAM is susceptible to noise, affecting the recovered signal.

PCM Transmitter and Receiver

• Function of a sampling circuit in a PCM transmitter is to periodically sample the continually changing analog input voltage and convert those samples to a series of constant amplitude pulses.
• Sampling circuit allows for accurate conversion of voltage to a binary code by the ADC.

Sampling Techniques

• Natural Sampling

  • Retains natural shape of sample pulses during the sample interval.
  • Makes it difficult for an ADC to convert the sample to a PCM code.
  • Alters the frequency spectrum of the sampled output.

• Flat-top Sampling

  • Most common method used for sampling voice signals in PCM systems.
  • Accomplished in a sample-and-hold circuit.
  • Introduces less aperture distortion than natural sampling.
  • Can operate with a slower analog-to-digital converter.

Sample-and-Hold Circuit

• Periodically samples the continually changing analog input voltage and converts those samples to a series of constant-amplitude PAM voltage-levels.