The release of the Magnavox Odyssey and Atari's Pong, the former to the home market, the latter into arcades, kicked off the Pong video game craze in the early 1970's. Various hobbyist electronics magazines joined in by publishing construction articles for Pong-style video games. Some of these projects were relatively complex for magazine construction articles. How many were actually built would be a question.

One of the magazine versions of the game was "Pongtronics", presented in the April and May 1976 issues of Popular Electronics. The unit shown and described here is an implementation of Pongtronics, constructed in 1976.

A schematic diagram of Pongtronics is included here. This is a reorganised version of the schematic presented in the Popular Electronics articles, and includes some minor improvements to the design.

Contents (this page): Game Play Technical Overview

Sub-pages: Schematic (this window) Schematic (new window) Unit Log

External Links: The Pong-Story site presents lots of information about Pong, as well as scans of the Popular Electronics Pongtronics articles. N3IC's page with another home-built Pongtronics. (It's well down the page - search for "Pong").

Game Play

Pongtronics is based on 'analog-pulse' circuitry (more about this below) which permits several features to be incorporated into the game with relative simplicity as compared to a pure digital design:

• A continuously-variable ball rebound angle according to where the ball hits on the paddle. This is in contrast to digital Pong games which typically have a small number of discrete rebound angles.

  The continuously-variable rebound angle helps make the game more fluid and lively. It also makes it difficult or impossible to position the paddles such that the ball ends up in a fixed loop.
  
  

• A switch-selected gravity mode puts a downward acceleration curve to the ball path, accomplished merely by adding a couple of resistors to change the charge rate of the ball vertical-speed capacitor.
  
  
• Machine control of either or both paddles is also accomplished with the addition of just a couple of switches and resistors, so in addition to the standard two-player game one can play against the machine or the machine can play against itself. The machine control is nice also in that the paddle does not track the ball precisely so there is some variation in the point on the paddle where the ball hits. The consequent variation in the rebound angle makes the machine response less predictable, it can even be adjusted to occasionally miss the ball.

Another switch-selected option turns the game into single-player hand-ball, by simply lengthening one of the paddles so it spans the vertical height of the screen.

Each player control includes a "SLAM" button which increases the ball speed when pressed.

In the photo showing the screen, one can observe another novel aspect of the design: scoring is implemented as two horizontal bars below the bottom border which progress from left to right as the score increases from 0 to 12. In the photo, the right player (bottom bar) is a couple of points ahead of the left player (top bar). This was a considerably simpler and more economic method of providing scoring than an on-sreen numeric display.

Technical Overview

Broadly speaking, there were two design approaches in the early-mid-1970's to implementing a Pong-style video game:

• A digital approach in which information in the system is represented in discrete form, stored in digital counters or registers.
  
  
• What may be called an 'analog-pulse' approach, involving a hybrid of analog and digital techniques. Information may be represented with both variable-level analog signals and digital pulses. Even in pulse form though, the signals are still essentially analog in that the information content of the signal is a continuously variable value, represented in the time domain by the width of a pulse or time between pulses. Integrators, comparators or switching amplifiers, and numerous R-C components manipulate variable-level signals while analog pulse signals may nonetheless be conveniently manipulated with digital circuitry. This design approach has some origins in the early (1930s) development of TV sync circuitry and radar.

Atari's arcade Pong was a digital design. An analog-pulse design, however, could be more economic in an era when digital counters and integrated circuits were expensive compared to R-C components. The Magnavox Odyssey was an analog-pulse design implemented with discrete transistors. Pongtronics is also an analog-pulse design but uses 4000-series CMOS ICs and a few Norton op-amps. The full game requires 20 ICs, a reduced game without scoring or sound can be accomplished with just 12 ICs.

The Pongtronics design starts with the generation of vertical and horizontal scan ramps. Each screen element (borders, paddles, ball) has one or two comparators fed by the appropriate horizontal or vertical ramp and another signal level representing the element position. The latter level determines where on the scan ramp the comparator trips, translating an analog voltage level into a pulse of varying width or position. Once in pulse form, digital circuitry is used to further process the signals. Integrators are used to generate the ball motion from signals representing the ball horizontal and vertical velocity components.

The 'comparators' in the Pongtronics design are actually CMOS NOR gates, relying on the sharp transfer curve of the gates for the comparator trip action. The Popular Electronics article specifies that only Fairchild 34001 ICs are to be used for these functions. The 34001 gates were buffered CMOS gates and I suspect the specification was to distinguish the type used from the non-buffered CMOS gates being produced in the early 1970's, rather than something more subtle or particular to Fairchild production. I have since replaced a couple of the 34001 ICs with, for example, a later Motorola MC4001B, with success.

The down-side of the analog-pulse design with all the R-C components (see photo of boards) is it leaves something to be desired in terms of stability. There are a dozen internal variable resistors to be aligned. Separate R-C oscillators are used for the horizontal and vertical sync generation, these can drift in phase relative to each other, resulting in some minor jitter of screen elements.

A few improvements to the original design have been incorporated to improve the display stability. These are elaborated in the schematic and unit log.