Pulse Code Modulation (PCM) Fundamentals

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Who invented Pulse Code Modulation (PCM) in 1937?

Alec H. Reeves

What is a limitation of PPM and PWM that PCM can overcome?

Bandwidth limitations due to resolution requirements

Why is it necessary for the voltage to be relatively constant during ADC conversion in a PCM transmitter?

So that the ADC can complete the conversion before the voltage level changes

What is the main function of a sampling circuit in a PCM transmitter?

To periodically sample the analog input voltage and convert the samples to pulses

What is a characteristic of Pulse Code Modulation (PCM) signals?

A group of pulses that represents the inputted analog data

Why is PCM the most preferred method in telecommunications?

Because it offers ease of digitizing voice and digital data with high-speed signal transmission

What is the main limitation of natural sampling?

It alters the frequency spectrum and introduces an error called aperture error.

What is the primary function of a sample-and-hold circuit?

To periodically sample the continually changing analog input voltage.

What is the advantage of flat-top sampling over natural sampling?

It introduces less aperture distortion.

What is the primary method used for sampling voice signals in PCM systems?

Flat-top sampling.

What is the result of the sampling process in natural sampling?

The frequency spectrum of the sampled output is different from that of an ideal sample.

What is the purpose of the sample-and-hold circuit in PCM systems?

To periodically sample the continually changing analog input voltage.

Study Notes

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.

Learn about the basics of Pulse Code Modulation, a digital encoding technique used to convert analog signals into digital signals. This quiz covers the concept of PCM, its history, and its applications in tele-communications.

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