PONG in AnalogScott MorrisonNicholas HarringtonMay 6, 2009iAbstractOur team (Nick Harrington and Scott Morrison) built an analog system that dis-plays the game of PONG on a oscilloscope screen. Our game models the ball’s pathin two dimensions, including a downward acceleration due to “gravity” and realisticcollisions with the walls and paddles. Other gameplay features will include the abilityto reset the ball position, and sounds that play at each collision.The core of our project is a simple analog computer that calculates the path of theball, including a set of comparators to detect collisions and start a new trajectory. Allof the physical constants and display parameters are adjustable, and since the collisiondetection is tied intimately to the visual output, the size of the paddles or ball can beadjusted with the turn of a knob.iiContents1 Introduction 12 Design Overview 12.1 Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Circuit Design and Analysis 24 Error Analysis 25 Conclusions 411 IntroductionOur team has taken this opportunity to recreate a classic video game: PONG. Originallyreleased in 1972, PONG became the world’s first commercially successful video game. Anearlier game based on a similar concept, “Tennis for Two”, is often described as the first elec-tronic game with a graphical display. This game was invented in 1958 by American physicistWilliam Higinbotham at Brookhaven National Laboratory, where it served as entertainmentfor visitors.Unlike Atari’s game, which was built primarily with transistor-transistor logic and pro-duced images on a pixelized television screen, our game is designed around a simplified analogcomputer. It generates vector graphics which are displayed directly on an oscilloscope screen.As such, it is inspired more directly by the techniques used by “Tennis for T wo”, which wasbased on an analog computer designed for ballistic missile simulations.We chose to recreate a tennis-like game in part because of the historical significance ofgames such as Pong and Tennis for Two–but mostly because of the simplicity of the graphics.Straight lines and circles are among the easiest shapes to draw on an oscilloscope screen.2 Design Overview2.1 Design GoalsIn our design process, we focused on the needs of our target audience: beginner electronicshobbyists on a tight budget. As we will see, our final pro duct is designed as a series ofsimple modules using low-cost parts. Each module can be built and tested separately; asimple oscilloscope is the only equipment you need. Furthermore, each module showcases auseful component or circuit technique—building this project is a rewarding and educational2experience. By the end of the project the hobbyist is rewarded with a fully-functional videogame, built entirely from the ground up.2.2 Block DiagramFigure 1: A block diagram, showing connections between independent modules.[TODO: discuss the interface between modules, division of project into two separatesystems.] (Figure 1)3 Circuit Design and AnalysisTo be done.[?]4 Error AnalysisIn testing, our circuit has several advantages over most analog circuits. Since most theparts of our circuit are centered around op-amps, the high input impedance and low outputimpedance allows for modules to be tested individually without much worry that their func-tionality will change when integrated into the whole circuit. The frequencies we are dealingwith range from 1 kHz to seve ral seconds. This means stray capacitance from leads and the3protoboard can be safely ignored aiding greatly in integration. Because of this, our buildprocess and error testing was very piecewise. A part to be added to the circuit would bebuilt tested and then integrated.The task of finding “errors” in our game is rather nebulous. While insuring the properfunctionality of every part of the circuit is important in creating a usable game of PONG,small errors do not affect the overall functionality of the game—and playability is our primaryconcern. Most of these errors can be traced back to the non-ideal characteristics of op-ampsand variations in resistor and capacitor values. Variations in capacitor and resistor valuesend up changing time constants and expected gains through many op-circuits. Since thecircuit was designed to be easily customized by the use r, these deviations from the idealare quickly compensated for by the potentiometers used to modify the functionality of thegame. This method of compensation is particularly effective in combating deviations fromideal gain in a long string of inverters, mixers, and amplifiers be cause the error introducedby variations in resistor values affects the gain linearly.The long chain of op-amps in the visualization stage means that the signal is particularlysusceptible to input bias currents which may affect its functionality. Any DC offset intro-duced by an op-amp will be amplified as it propagates down the line, eventually creating anoticeable signal. Many of the analog computer components were also relying on having avery low input bias current to the op-amps. Because of this, JFET op-amps were used inthe analog computer portion and in sections of the circuit that are very far upstream in thecommunication line.The peak finder used in collision detection betwee n the ball and paddle would occasionallydrive the op-amp into saturation. The frequency at which this would happen was only 1kHz, implying that the problem is caused by the slow slew rate of the (low power) dual op-4amp we used. Selecting a new component with sufficiently high slew rate would immediatelyminimize this error.5 ConclusionsWe successfully designed, implemented and tested a new version of PONG—one built en-tirely with analog circuita and an oscilloscope for display. The modular design, entertaininggameplay, and retro charm combine to make this an ideal project for a hobbyist to undertake.While somewhat of a niche market, selling the component and schematics for the circuit toa hobbyists may be profitable. Although the game itself is not new, our “do-it-yourselfvideo game” approach will breathe new life into an old classic—providing entertainment forelectronics enthusiasts of all