Mullard SAA5050
Encyclopedia
The Mullard
SAA5050 was a character generator chip for implementing the teletext character set. The SAA5050 was used in teletext
-equipped television sets, viewdata
terminals, and microcomputer
s, most notably the Philips P2000T
homecomputer (1980), Acorn
's System 2
(1980), and the BBC micro
's teletext-compatible 'Mode 7' display mode (1982). This chip was also manufactured by Philips
(now NXP Semiconductors) and Signetics
.
The chip generated appropriate video output for a 7-bit input character code representing the current character on the text line, while keeping track of the effect of any of the various control character
s defined by the teletext standard that had previously occurred in that text line, which could be used to change the foreground and background colour, switch to or from the alternate block graphics character set, or various other effects.
Full-screen resolution generated by the SAA5050 was 480×500 pixels, corresponding to 40×25 characters. Each character position therefore corresponded to a 12×20 pixel space. Internally each character shape was defined on a 5×9 pixel grid. This was then interpolated by smoothing diagonals to give a 10×18 pixel character, with a characteristically angular shape, surrounded to the top and to the left by two pixels of blank space. This gave a particularly stable and flicker-free arrangement on interlaced displays. The alternate set of 2×3 block graphic characters were created on the same 12×20 pixel grid, so that the top two blocks were each 6×6 pixels, the middle two blocks each 6×8 pixels, and the bottom two blocks again 6×6 pixels (or two fewer in each direction, if the "separated graphics" control character had been sent).
One curiosity, compared to other alternative chips, was that the SAA5050 implemented the original 1976 teletext standard, which had no provision to set black for the foreground text colour. Some alternative chips at the time did allow this, as became formalised in the 1981 CEPT videotex standard.
In addition to the UK version, several variants of the chip existed with slightly different character sets for particular localizations and/or languages. These had part numbers SAA5051 (German), SAA5052 (Swedish), SAA5053 (Italian), SAA5054 (Belgian), SAA5055 (U.S. ASCII
), SAA5056 (Hebrew
) and SAA5057 (Cyrillic
).
The SAA5050 was later superseded by the SAA5243, a similar teletext video generator chip, not only a character generator but a complete stand alone video generator, controlled through I²C
.
Mullard
Mullard Limited was a British manufacturer of electronic components. The Mullard Radio Valve Co. Ltd. of Southfields, London, was founded in 1920 by Captain Stanley R. Mullard, who had previously designed valves for the Admiralty before becoming managing director of the Z Electric Lamp Co. The...
SAA5050 was a character generator chip for implementing the teletext character set. The SAA5050 was used in teletext
Teletext
Teletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
-equipped television sets, viewdata
Viewdata
Viewdata is a Videotex implementation. It is a type of information retrieval service in which a subscriber can access a remote database via a common carrier channel, request data and receive requested data on a video display over a separate channel. Samuel Fedida was credited as inventor of the...
terminals, and microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
s, most notably the Philips P2000T
Philips P2000
--The Philips P2000T home computer was Philips' first real entry in the home computer market, after the Philips Videopac G7000 game system which they already sold to compete with the Atari 2600 and similar game systems. There was also an P2000M version with an additional 80-column card for use...
homecomputer (1980), Acorn
Acorn Computers
Acorn Computers Ltd. was a British computer company established in Cambridge, England, in 1978. The company produced a number of computers which were especially popular in the UK. These included the Acorn Electron, the BBC Micro, and the Acorn Archimedes...
's System 2
Acorn System 2
The Acorn Eurocard systems were a series of modular microcomputer systems based on rack-mounted Eurocards developed by Acorn Computers from 1979 to 1982, aimed primarily at industrial and laboratory use, but also home enthusiasts....
(1980), and the BBC micro
BBC Micro
The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
's teletext-compatible 'Mode 7' display mode (1982). This chip was also manufactured by Philips
Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
(now NXP Semiconductors) and Signetics
Signetics
Signetics, once a major player in semiconductor manufacturing, made a variety of devices which included integrated circuits, bipolar and MOS, the Dolby circuit, logic, memory and analog circuits. They developed microprocessors like the 2650, the bipolar 8X300 and had licensed Motorola 68000...
.
The chip generated appropriate video output for a 7-bit input character code representing the current character on the text line, while keeping track of the effect of any of the various control character
Control character
In computing and telecommunication, a control character or non-printing character is a code point in a character set, that does not in itself represent a written symbol.It is in-band signaling in the context of character encoding....
s defined by the teletext standard that had previously occurred in that text line, which could be used to change the foreground and background colour, switch to or from the alternate block graphics character set, or various other effects.
Full-screen resolution generated by the SAA5050 was 480×500 pixels, corresponding to 40×25 characters. Each character position therefore corresponded to a 12×20 pixel space. Internally each character shape was defined on a 5×9 pixel grid. This was then interpolated by smoothing diagonals to give a 10×18 pixel character, with a characteristically angular shape, surrounded to the top and to the left by two pixels of blank space. This gave a particularly stable and flicker-free arrangement on interlaced displays. The alternate set of 2×3 block graphic characters were created on the same 12×20 pixel grid, so that the top two blocks were each 6×6 pixels, the middle two blocks each 6×8 pixels, and the bottom two blocks again 6×6 pixels (or two fewer in each direction, if the "separated graphics" control character had been sent).
One curiosity, compared to other alternative chips, was that the SAA5050 implemented the original 1976 teletext standard, which had no provision to set black for the foreground text colour. Some alternative chips at the time did allow this, as became formalised in the 1981 CEPT videotex standard.
In addition to the UK version, several variants of the chip existed with slightly different character sets for particular localizations and/or languages. These had part numbers SAA5051 (German), SAA5052 (Swedish), SAA5053 (Italian), SAA5054 (Belgian), SAA5055 (U.S. ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
), SAA5056 (Hebrew
Hebrew alphabet
The Hebrew alphabet , known variously by scholars as the Jewish script, square script, block script, or more historically, the Assyrian script, is used in the writing of the Hebrew language, as well as other Jewish languages, most notably Yiddish, Ladino, and Judeo-Arabic. There have been two...
) and SAA5057 (Cyrillic
Cyrillic alphabet
The Cyrillic script or azbuka is an alphabetic writing system developed in the First Bulgarian Empire during the 10th century AD at the Preslav Literary School...
).
The SAA5050 was later superseded by the SAA5243, a similar teletext video generator chip, not only a character generator but a complete stand alone video generator, controlled through I²C
I²C
I²C is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device. Since the mid 1990s, several competitors I²C ("i-squared cee" or "i-two cee"; Inter-Integrated Circuit;...
.
External links
- SAA5050 data sheet
- Bedstead, a font inspired by the SAA5050 character shapes