A monoscope was a special form of
video camera tube which displayed a single still video image. The image was built into the tube, hence the name. The tube resembled a small
cathode ray tube
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), ...
(CRT). Monoscopes were used beginning in the 1950s to generate TV
test patterns and station logos. This type of test card generation system was
technologically obsolete
Obsolescence is the state of being which occurs when an object, service, or practice is no longer maintained or required even though it may still be in good working order. It usually happens when something that is more efficient or less risky r ...
by the 1980s.
Design
The monoscope was similar in construction to a
CRT, with an electron gun at one end and at the other, a metal target screen with an image formed on it. This was in the position where a CRT would have its
phosphor
A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or v ...
-coated display screen. As the
electron beam
Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to el ...
scanned the target, varying numbers of electrons were reflected from the different areas of the image. The reflected electrons were picked up by an internal electrode ring, producing a varying electrical signal which was amplified to become the
video
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) sy ...
output of the tube.
This signal reproduced an accurate still image of the target, so the monoscope was used to produce still images such as test patterns and station logo cards. For example, the classic
Indian Head test card
The Indian-head test pattern is a test card created by RCA of Harrison, New Jersey, which became the standard image of the RCA TK-1 monoscope. It features a drawing of a Native American wearing a headdress and numerous graphic elements designed ...
as used by many television stations in North America, was often produced using a monoscope.
Usage
Monoscopes were available with a wide variety of standard patterns and messages, and could be ordered with a custom image such as a station logo. Monoscope "cameras" were widely used to produce test cards, station logos, special signals for test purposes and standard announcements like "''Please stand by''" and "''normal service will be resumed''....". They had many advantages over using a live camera pointed at a card; an expensive camera was not tied up, they were always ready, and were never misframed or out of focus. Indeed, monoscopes were often used to calibrate the live cameras, by comparing the monoscope image and the live camera image of the same test pattern.
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
A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or v ...
of a monitor displaying it in extreme cases.
Monoscopes were used as
character generator
A character generator, often abbreviated as CG, is a device or software that produces static or animated text (such as news crawls and credits rolls) for keying into a video stream. Modern character generators are computer-based, and they can ...
s for
text mode video rendering in
computer displays
A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a visual display, support electronics, power supply, housing, electrical connectors, and external user controls.
Th ...
for a short time in the 1960s.
The monoscope declined in popularity after the 1960s due to its inability to generate a colour
test card
A test card, also known as a test pattern or start-up/closedown test, is a television test signal, typically broadcast at times when the transmitter is active but no program is being broadcast (often at sign-on and sign-off).
Used since the ea ...
, and the development of
solid state TV test pattern signal generators.
See also
*
Test card
A test card, also known as a test pattern or start-up/closedown test, is a television test signal, typically broadcast at times when the transmitter is active but no program is being broadcast (often at sign-on and sign-off).
Used since the ea ...
, the updated, coloured version of monoscopes.
References
External links
Monoscope tubes* The Museum of the Broadcast TV Camera, picture and description of th
(UK)
* Picture and description of th
(US)
Indian Head - "as transmitted" picture2F21 data sheet
{{Electronic components
Display technology
Television technology
Test cards
Vacuum tubes