Mullard SAA5050

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File:Mullard SAA5050.jpg
A 1979 SAA5050 in a VDU card for Acorn Eurocard systems

The Mullard SAA5050 was a character generator chip for implementing the Teletext character set.[1]

File:Teletext PCB.jpg
Printed circuit board used in a Philips Viewdata unit, featuring a SAA5050 character generator.

The SAA5050 was used in teletext-equipped television sets, viewdata terminals, and microcomputers, most notably on computers like the Philips P2000 (1980), Acorn System 2 (1980), BBC Micro (1982), Malzak and the Poly-1,[2] and Prestel adapters like the AlphaTantel.[3][4][5] This chip was also manufactured by Mullard for Philips.

Operation

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 characters 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.

File:Level 1 teletext test.png
Level 1 teletext block characters and colours

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 that was loosely based on the Signetics 2513 character ROM chip. 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).

0 1 2 3 4 5 6 7 8 9 A B C D E F
2 NBSP File:TRS-80 character 0x81.png File:TRS-80 character 0x82.png File:TRS-80 character 0x83.png File:TRS-80 character 0x84.png File:TRS-80 character 0x85.png File:TRS-80 character 0x86.png File:TRS-80 character 0x87.png File:TRS-80 character 0x88.png File:TRS-80 character 0x89.png File:TRS-80 character 0x8A.png File:TRS-80 character 0x8B.png File:TRS-80 character 0x8C.png File:TRS-80 character 0x8D.png File:TRS-80 character 0x8E.png File:TRS-80 character 0x8F.png
3 File:TRS-80 character 0x90.png File:TRS-80 character 0x91.png File:TRS-80 character 0x92.png File:TRS-80 character 0x93.png File:TRS-80 character 0x94.png File:TRS-80 character 0x95.png File:TRS-80 character 0x96.png File:TRS-80 character 0x97.png File:TRS-80 character 0x98.png File:TRS-80 character 0x99.png File:TRS-80 character 0x9A.png File:TRS-80 character 0x9B.png File:TRS-80 character 0x9C.png File:TRS-80 character 0x9D.png File:TRS-80 character 0x9E.png File:TRS-80 character 0x9F.png
6 File:TRS-80 character 0xA0.png File:TRS-80 character 0xA1.png File:TRS-80 character 0xA2.png File:TRS-80 character 0xA3.png File:TRS-80 character 0xA4.png File:TRS-80 character 0xA5.png File:TRS-80 character 0xA6.png File:TRS-80 character 0xA7.png File:TRS-80 character 0xA8.png File:TRS-80 character 0xA9.png File:TRS-80 character 0xAA.png File:TRS-80 character 0xAB.png File:TRS-80 character 0xAC.png File:TRS-80 character 0xAD.png File:TRS-80 character 0xAE.png File:TRS-80 character 0xAF.png
7 File:TRS-80 character 0xB0.png File:TRS-80 character 0xB1.png File:TRS-80 character 0xB2.png File:TRS-80 character 0xB3.png File:TRS-80 character 0xB4.png File:TRS-80 character 0xB5.png File:TRS-80 character 0xB6.png File:TRS-80 character 0xB7.png File:TRS-80 character 0xB8.png File:TRS-80 character 0xB9.png File:TRS-80 character 0xBA.png File:TRS-80 character 0xBB.png File:TRS-80 character 0xBC.png File:TRS-80 character 0xBD.png File:TRS-80 character 0xBE.png File:TRS-80 character 0xBF.png

The pixels were usually displayed with a 1.33:1 or 1.2:1 aspect ratio to give a full display close to the standard 4:3 TV aspect ratio, effectively a 400 × 300 or 480 × 400 display.

Versions

Compared to other alternative chips, the SAA5050 implemented the original World System Teletext teletext standard (Level 1), which had no provision to set black for the foreground text colour. Some alternative chips at the time did allow this, as became formalized 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),[6] SAA5052 (Swedish),[7] SAA5053 (Italian), SAA5054 (Belgian), SAA5055 (U.S. ASCII), SAA5056 (Hebrew) and SAA5057 (Cyrillic). The SAA5050 was later superseded by the SAA5243 CCT chip, integrating a similar teletext character generator with all previously separately implemented functions such as decoding, timing and video generation. It was controlled through I2C.

See also

References

  1. ↑ SAA5050 Series Teletext Character Generator (PDF). Mullard. 1982.
  2. ↑ "SAA5050 Teletext Character Generator". Vas the Man’s Arcade.
  3. ↑ Graham, Adrian. "AlphaTantel Viewdata Terminal". Binary Dinosaurs. Retrieved 2023-05-02.
  4. ↑ Graham, Adrian. "AlphaTantel Viewdata Terminal Motherboard". Binary Dinosaurs.
  5. ↑ "Machine: AlphaTantel (alphatan)". Vas the Man’s Arcade.
  6. ↑ Born, Uwe. "IC SAA5051 datasheet". Semiconductor data tables.
  7. ↑ "SAA 5052, Tube SAA5052; Röhre SAA 5052 ID78631, IC - Integra". Radiomuseum.org.

External links

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