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CS601 - Data Communication - Lecture Handout 16

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Conversions

Types of Digital-to-Digital Encoding

Types of Digital-to-Digital Encoding

Example 5.1

  • Using B8ZS, encode the bit stream 10000000000100. Assume that the polarity of the first 1 is positive.

Example 5.1

  • Analog-to-Digital Conversion
    • We sometimes need to digitize an analog signal
    • To send human voice over a long distance, we need to digitize it, since digital signals are less prone to Noise
    • This is called Analog-to-Digital Conversion or Digitizing an Analog Signal
    • This type of conversion requires a reduction of potentially infinite number of values in the analog signal so that it can be converted to digital bit stream with minimum loss of information.

Analog-to-Digital Conversion

  • CODEC
    • Coder Decoder
  • Digital signal signals can take any of the forms discussed previously
  • Problem is how to convert analog signal from infinite number of values to discrete no. of values without scarifying quality

 

  • Pulse Amplitude Modulation
    • First step in Analog-to-Digital Conversion
    • This technique takes an Analog signal, Samples it, and Generates a series series of Pulses based on the results of Sampling
    • Sampling means measuring amplitudes of signal at equal intervals
    • The original signal is sampled at equal intervals
    • PAM uses a technique called Sample & Hold means At a given moment , signal level is read, then held briefly
    • The pulses are of any amplitude (still analog not digital). To make them digital, we need PCM
  • Pulse Code Modulation
    • Modifies pulses created by PAM to a complete digital signal
    • Four Separate Processes:
      • PAM
      • Quantization
      • Binary Encoding
      • Digital/Digital Encoding
      • Quantization
        PCM’s first step is Quantization

“Quantization is a method of assigning integral values in a specific range to sampled instances”

Pulse Code Modulation

Figure shows a simple method of assigning sign and magnitude values to quantized samples

Results of Binary Encoding

Results of Binary Encoding

Each value is translated into its seven bit binary equivalent. The eight bit indicates the sign

Result of PCM

Result of PCM

Figure shows the result of PCM of original signal encoded finally into a unipolar signal Only first 3 values are shown

Full PCM Process

Full PCM Process

  • Sampling Rate
    • The accuracy of any digital reproduction of an analog signal depends upon the o. of samples taken
    • How many samples are sufficient?
    • <Nyquist theorem>
      • The sapling rate must be at least twice the highest frequency

Sampling Rate

  • Bit Rate
    • Sampling Rate given by Nyquist Theorum
    • No. of bits per sample chosen according to the Precision needed at the receiver end.

Bit Rate

Summary

  • Analog-to-Digital Conversion
  • Pulse Code Modulation (PCM)
    • Pulse Amplitude Modulation (PAM)
    • Quantization
    • Binary Encoding
    • Digital-To-Digital Conversion

Reading Sections

  • Section 5.2, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan