Indian Head Test Card - As Television System Tool

As Television System Tool

The primary and critical Indian Head Test Pattern was not itself a card. Rather, it was generated directly as a monochrome video signal by means of a monoscope camera.

An RCA TK-1 Monoscope Camera is a 19-inch rack-mounted chassis, which contains electronic circuits needed to operate a glass cathode ray tube housed inside an anti-magnetic steel shield. The cathode ray component is a TV-camera vacuum tube known as a monoscope, because it videographs only one still image, the test pattern. The tube has a perfectly proportioned copy of the test pattern master art inside, permanently deposited as a carbon image on an aluminum target plate or slide. This perfect copy allowed all of the television studio and production control room video monitors, and home television sets, to be identically adjusted for minimum distortions such as ovals instead of circles. When the monitor or TV set was correctly adjusted to show test pattern circles, the received picture's aspect ratio was exactly four units wide by three units high. The 4 by 3 standard was chosen by the National Television System Committee (NTSC) for analog television, so that film movies would be compatible with TV broadcasting. 4:3 is the same aspect ratio used by 16mm and classic 35mm motion picture film frames.

The graphic of the Indian and all of the patterns on the chart served specific purposes. With the chart many typical daily (sometimes hourly) adjustments on cameras, home, and studio monitors could be made. An experienced broadcast engineer could glance at the drawing of the Indian Chief and quickly know if everything was OK or if more careful adjustment was needed. Within the chart the tools necessary to adjust perspective, framing, linearity, frequency response, differential gain, contrast and white level (brightness) are all provided. The grid and circles were used for perspective, framing and linearity. The tapered lines (marked with 20, 25, 30, and 35) were used for resolution and frequency response. The thin lines marked from 575 to 325 on one side and 300 to 50 on the other side referred to lines of resolution. The gray bands emerging from the center off to the lower right and upper left were for differential gain, contrast, and white level.

Only after the monitors were adjusted was an actual Indian Head Test Card used. A cardboard mounted lithograph of the test pattern was typically attached to a rolling vertical easel in each TV studio, to be videographed by each studio camera during test time. Then the cameras were adjusted to appear identical on picture monitors, by alternately switching between and comparing the monoscope image and the test card image. Such adjustments were made on a regular basis because television system electronics then used hot vacuum tubes, the operating characteristics of which drifted throughout each broadcast day.

Test patterns were also broadcast to the public daily to allow regular adjustments by home television set owners and TV shop repair technicians. In this regard, various features in the pattern were included to facilitate focus and contrast settings, and the measurement of resolution. The circular "bulls-eyes" in the centre and the four corners permitted uniform deflection yoke and oscillator amplitude adjustments for centering, pincushioning, and image size.

The test pattern was usually accompanied by a 1,000 or 400 hertz sine wave test tone, which demonstrated that the TV aural receiver was working. If the tone was pure-sounding rather than a buzz or rattle, then transmitted speech and music would not be distorted. 400 Hz is somewhat less annoying for technicians to hear for extended work periods.