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LECTURE
Conversions
Types of Digital-to-Digital Encoding
Example 5.
Digital/Digital Encoding
Unipolar Polar Bipolar
Using B8ZS, encode the bit stream 10000000000100. Assume that the polarity of the first 1 is positive.
Analog-to-Digital Conversion o We sometimes need to digitize an analog signal o To send human voice over a long distance, we need to digitize it, since digital signals are less prone to Noise o This is called Analog-to-Digital Conversion or Digitizing an Analog Signal o 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.
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o CODEC Coder Decoder o Digital signal signals can take any of the forms discussed previously o Problem is how to convert analog signal from infinite number of values to discrete no. of values without scarifying quality
Pulse Amplitude Modulation
o First step in Analog-to-Digital Conversion o This technique takes an Analog signal, Samples it, and Generates a series series of Pulses based on the results of Sampling o Sampling means measuring amplitudes of signal at equal intervals
o The original signal is sampled at equal intervals o PAM uses a technique called Sample & Hold means At a given moment , signal level is read, then held briefly o The pulses are of any amplitude (still analog not digital). To make them digital, we need PCM
Pulse Code Modulation
o Modifies pulses created by PAM to a complete digital signal o 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”
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Sampling Rate o The accuracy of any digital reproduction of an analog signal depends upon the o. of samples taken o How many samples are sufficient?
o < Nyquist theorem > The sapling rate must be at least twice the highest frequency
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Bit Rate o Sampling Rate given by Nyquist Theorum o No. of bits per sample chosen according to the Precision needed at the receiver end.
BitRate SamplingRate No. ofbits / sample
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
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