ENVS 1200 Chapter Notes - Chapter 14: Pulse-Amplitude Modulation, Discrete-Time Signal, Error Detection And Correction
13 May 2018
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ENVS 1200 Chapter 14 Notes – Summary
Introduction
• The digital signals that represent video in a direct satellite TV system and the digitization
of sound that can be used to store telephone voice mail in a computer.
• One way of converting analog data into digital form is shown
• This method is called pulse code modulation (PCM). There are three steps in the
process.
• In step 1, the analog waveform is sampled at regular time interval, as shown
• The maximum possible amplitude of the waveform is divided into intervals
corresponding to a range of binary numbers.
• This example uses 256 levels, which will result in 8 bits per sample.
• This intermediate step is called pulse amplitude modulation (PAM).
• PAM can be used directly, to provide a discrete signal, if desired.
• More commonly, the sampled values are each converted into their corresponding
number value.
• The result is PCM.
• Incidentally, the information lost in converting data from analog to digital can be seen in
this step
• It consists of the difference between the actual value of the sample and the value
corresponding to the nearest available number.
• Finally, the number is reduced to its binary equivalent.
• The device that performs this conversion is called an A-to-D converter.
• Digital signals are susceptible to noise, attenuation, and distortion, just as analog signals
are.
• However, it is only necessary to distinguish between two levels, so much more
distortion and noise in the channel can be tolerated.
• It is also possible to recreate the original signal at intervals along the channel, since the
original shape is limited to ones and zeros.
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