Test cards including large circles were used to confirm the linearity of the set's deflection systems. As solid-state components replaced vacuum tubes in receiver deflection circuits, linearity adjustments were less frequently required (few newer sets have user-adjustable "VERT SIZE" and "VERT LIN" controls, for example). In LCD and other deflectionless displays, the linearity is a function of the display panel's manufacturing quality; for the display to work, the tolerances will already be far tighter than human perception. ... more »

Towards the end of the Indian Head TV era (around the late 1970s), there was no nightly test pattern on some stations, typically when automatic logging and remote transmitter controls allowed shutdown of power immediately after the formal sign-off. After an immediate transmitter power off, in lieu of the Indian Head Test Card and its sine wave tone, a TV viewer heard a loud audio hiss like FM radio interstation noise and saw the video noise colloquially called snow (but resembling "bugs" following a TV-system technical improvement). Audio and video noise received on Indian Head era TV sets respectively indicated the absence of analog aural and visual broadcast carriers. Consumer TVs typically did not have a no-signal noise muting and blanking feature until the late analog TV period. ... more »

I like the BBC test card, because it's a wonderful example of how practical industrial design can develop into an enigmatic work of art. But even better is knowing that the image isn't just art, it has an objective purpose worked into it, a secret meaning that reveals itself under scrutiny. The architecture of industrial design is filled with these subtle codes, and together they create a world around us filled with hidden meaning. ... more »