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# CS601 - Data Communication - Lecture Handout 16

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# Conversions

## Example 5.1

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

• 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.

• 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”

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

## Results of Binary Encoding

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

## 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

• 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

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

## 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