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A monoscope was a special form of cathode ray tube that was used to generate, rather than display, a video signal. Each tube was only capable of generating a single video signal, hence the name. This kind of test card generation system was technologically obsolete by the 1980s, but served many private and state broadcasters faithfully since its early use in the 1950s.
Essentially similar in construction to an ordinary CRT, the monoscope contained a formed metal target in place of the phosphor coating at its "screen" end and as the electron beam scanned the target, a varying electrical signal was produced.
This signal reproduced an accurate image of the target and so could be used to produce test patterns and the like. For example, the classic Indian Head test card shown on the right was often produced using a monoscope.
Monoscope "cameras" were widely used to produce test cards, Station Logos, special signals for test purposes and of course announcements, like "normal service will be resumed....". They had many advantages over the use of a more expensive live camera, they were always ready, and were never misframed or out of focus.
Pointing an electronic camera at the same stationary monochrome caption for a long period of time could result in the image becoming burnt onto the camera tube's target — and even onto the phosphor of a monitor displaying it in extreme cases.
The monoscope declined in popularity during the 1960s due to its inability to generate a colour test card.
- Monoscope tubes
- The Museum of the Broadcast TV Camera, picture and description of the Marconi BD617B portable Monoscope camera (UK)
- Picture and description of the RCA TK-1 Monoscope (US)
- Indian Head - "as transmitted" picture