That Pesky Television Test Pattern

Indian head test pattern

What came first, television or the television test pattern? By all accounts, the once ubiquitous, static bullseye that appeared on kinescopes and cathode ray tubes from the 40s through the 70s before stations began airing their scheduled programs (or when malfunctions occured) may not have preceeded the actual invention of television, which surprisingly began during the 1880s, but it was the first real transmission that was seen on TV. Although the earliest dimensional image to appear on the screen in the mid-thirties on NBC’s experimental station W2XBS was a rubberized model of Felix the Cat (the only object that would not melt under intensely hot studio lights), the test pattern was the most consistently broadcast image since the early twenties.

NBC test pattern

The origin of the pattern is a story of form following function. Aesthetics were irrelevant to the primary purpose, and the technical draftsmen who anonymously designed it could have never predicted that decades later it would become a nostalgic icon. The intent was to enable engineers, who in the so-called “pre-television” days were the only persons to actually receive broadcasts, to calibrate the extremely small, very crude black and white scans that became the TV picture. While the circular target may seem odd given the rectangular shape of even the earliest screens, in fact, the initial test patterns conformed to the circular shape of an oscilloscope that showed engineers the electrical equivalent of an image in the form of a wave. But there was an even more deliberate rationale.

In the twenties test patterns (or test charts as they were refered then) were more or less varied, but in the late thirties, when a few hundred receivers became commercially available, a standard was embraced by broadcasters. The chart was designed to both check transmitter performance from the studio to the antenna and allow the audience to determine the degree of performance of their individual receivers. In addition to the advantage of being a static signpost, of sorts, the chart revealled geometrical defects, horizontal and vertical degrees of picture resolution, and a range of shadding gradations.

The archetypical chart used by NBC/RCA, which in the thirties had merged to form the first television “network,” consisted of an outer circle that had a diameter equal to four-thirds the diameter of the inner circle; the former touched the sides of the screen and the latter was cut off on top and bottom. This conformed to the standard aspect ratio of 4:3. If the picture was too narrow (less than the aspect ratio), the circles took on an eliptical shape, with the major axis of the ellipses in the vertical direction; the converse was true if the picture was not high enough. The chart was, therefore, a diagnostic device to determine whether the transmitter scanning was too wide or narrow, too great or too little. The large circles had another important use. The scanning of the beams at transmitter and receiver had to move at a perfectly uniform rate or else the image would be expanded or condensed. If the circles were egg-shaped then the scanning was not uniform. The perceived defects could then be fixed by precision controls on the transmitter. Presumably, home receivers required only a one time setting upon installation, but invariably dials would shift so the test chart would aid the viewer in making the necessary adjustments.

KMOX TV test pattern

The interior of the pattern was divided into sections. The innermost, shaded circles, consisted of three concentric circular areas of differing density: the central area being black; the next an intermediate gray tone and the outer, was white. These were used to measure and set the contrast controls either at the transmitter or receiver. If the contrast control was set too high, the two inner areas turned to black, eliminating any degree of shading. Conversely, if the constrast was low, the picture becomes very flat or gray.

The bars that shot out from the bullseye in four directions, called “definition wedges,” consisted of vertical and horizontal black and white lines, arranged to increase in width as they moved out from the center. The horizontal lines were used to measure vertical resolution, and the vertical lines measured the horizontal, the measurments were based on scan lines (a maxiumum then of 350 lines) of the screen. Like registration marks, the wedges highlighted faulty resolution and electrical focus that could be fixed at the point of origin. If the home users had read the TV manual they would know how to use the test chart, which in the early days of television appeared more frequently than the live programming. But by the fifties the test pattern was shown in the early morning or very late at night and most users randomly fiddled with the knobs and antenna, ignoring its functional benefits.

By the early fifties every TV station in America used a version of the same basic test chart until twenty-four hour broadcasting made it obsolete in the seventies. But one question remains unanswered. What about the most ubiquitious of all the pattern designs — the one with the picture of the indian? Where did it originate? Everyone recalls it, yet no one knows why the indian was used.

For a related article, please see this previouse Design Observer post, by Dmitri Siegel: The Mysterious Disappearance of Carol Hersee

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By Steven Heller

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