Laser Communication

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  • Topic: Digital, Analog-to-digital converter, Digital signal processing
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  • Published : January 11, 2013
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Display for the Virginia Museum of Science
Digital Communications

Date Submitted: 6 October 2002
Independent Research Project EE 491
Digital Communications
Cadets: Joseph Wunder
Brian Holt

I. Introduction
Many electronic devices today communicate with each other externally and internally. The information in these devices can be numbers, words, sounds, and pictures. The simplest form of this information is digital one’s and zero’s. Sequences of one’s and zero’s are used by electronic devices to pass information back and forth as well as process the information. The concept of representing information with zero’s and one’s was devised in 1940 by Claude Shannon in his master thesis at MIT. He devised theorems that showed how digital one’s and zero’s (or bits) can be used to describe information. He then pioneered ways in which these bits can be manipulated or sent to other devices with little or no error.

To demonstrate, in simple terms, the concept of digital communication using bits, a museum display involving analog to digital conversion and laser bit communication was constructed. The exhibit also demonstrates how more bits can be used to communicate a more accurate signal. For this task, an audio information source (CD player) produces a signal that is converted to digital information. This information will then be sent to a receiver using laser light switching on and off to communicate zero’s and one’s. The receiver will pick up the laser light and convert the digital information back into an analogue signal for output to a speaker. See Figure 1 for a system block diagram. Covering some of the lasers (removing bits) will demonstrate that less bits result in a poorer signal and reduced audio quality.

Information
Source (CD
Player)
Speaker
Output

A/D
Conversion

D/A
Conversi
on

Transmitter
(8-Laser
Array)
Receiver
(8-Phototransistor
Array)

Figure 1: System block diagram for museum display

II. System Functionality
To send digital information through the laser communication device, the input audio signal must be converted into a series of digital bits. Before this can be accomplished, some formatting has to be performed on the analog audio signal coming from the CD player. The analog input is a 100-200mVp-p signal. It is first sent through a filtering circuit to block any DC offset and center the input signal at 0V. The signal is then amplified with a gain of 20 using an opamp. Finally, a filter and clamping circuit are used to attenuate frequencies below 10Hz and flip the negative values positive for A/D conversion. The conversion is completed using an AD7819 chip. This chip samples the analog signal at 125kHz (controlled by a clock signal from 555 timer) and outputs to an 8-bit parallel interface. Sampling is accomplished by reading the voltage level of the analog signal at a certain time interval (for this system, the sample period is 8µs). This voltage is then assigned one of 256 distinct voltage levels and given an 8-bit sequence to describe the voltage level in digital terms.

The 8-bit sequence is then sent in parallel (8 lines sending the bits all at the same time) to an array of transistors. The transistors switch 8 lasers off and on to transmit the digital sequences. A “1” turns a given laser on while a “0” turns it off. The laser light

pulses are sensed by eight phototransistors that are give off a “1” when excited by laser light and a “0” when not excited.
To reverse the process for output to the audio speaker, digital to analog conversion needs to take place. To do this, the digital bits from the laser are fed into a latch (74HC161) that sustains the digital values coming from the lasers until another value overwrites it. After going through an inverter array, the 8-bit signal arrives at the D/A converter chip (the **D/A converter chip name**). This chip takes each 8-bit digital sample and converts it to one of the 256 voltage levels used by the A/D chip. The...
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