Mullard SAA5050
Character generator chip for Teletext and viewdata systems.
The Mullard SAA5050 is a character generator chip built to produce the Teletext character set. It appeared in television sets with teletext capability, viewdata terminals, Prestel adapters such as the AlphaTantel, and several microcomputers, including the Philips P2000, Acorn System 2, BBC Micro, Malzak, and Poly-1. Mullard manufactured the chip for Philips.
The chip took a 7-bit input character code for the current character on a text line and generated the corresponding video output. It also tracked the effects of any control characters defined by the teletext standard that had occurred earlier in that line, which could alter foreground and background colours, switch to or from the alternate block graphics character set, or trigger other effects. The SAA5050’s full-screen resolution was 480 × 500 pixels, arranged as 40 characters per line and 25 lines, with each character occupying a 12 × 20 pixel space. Internally, each character shape was defined on a 5 × 9 pixel grid, loosely based on the Signetics 2513 character ROM chip. This grid was then interpolated by smoothing diagonals to produce a 10 × 18 pixel character with a characteristically angular look, surrounded by two blank pixels above and to the left. This design gave a stable, flicker-free display on interlaced screens. The alternate set of 2 × 3 block graphics characters used the same 12 × 20 pixel grid: the top two blocks were each 6 × 6 pixels, the middle two blocks 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). The pixels were typically displayed with a 1.33:1 or 1.2:1 aspect ratio, yielding a full display close to the standard 4:3 TV ratio—effectively 400 × 300 or 480 × 400 pixels.
The SAA5050 implemented the original World System Teletext standard (Level 1), which did not allow setting black as a foreground text colour. Some alternative chips at the time did permit this, a feature later formalized in the 1981 CEPT videotex standard. Several variants of the chip existed for different localizations, each with a slightly different character set: SAA5051 (German), SAA5052 (Swedish), SAA5053 (Italian), SAA5054 (Belgian), SAA5055 (U.S. ASCII), SAA5056 (Hebrew), and SAA5057 (Cyrillic).
- Full screen resolution
- 480 × 500 pixels (40 × 25 characters)
- Character cell size
- 12 × 20 pixels
- Internal character grid
- 5 × 9 pixels
- Displayed character size
- 10 × 18 pixels
- Block graphics grid
- 2 × 3 blocks per character cell
- Aspect ratios
- 1.33:1 or 1.2:1
- Variants
- SAA5051 (German), SAA5052 (Swedish), SAA5053 (Italian), SAA5054 (Belgian), SAA5055 (U.S. ASCII), SAA5056 (Hebrew), SAA5057 (Cyrillic)
Lore & Background
The SAA5050 generated video output from a 7-bit input character code, tracking control characters that changed foreground/background color, switched to block graphics, or applied other effects. Its full-screen resolution was 480 × 500 pixels, with each character occupying a 12 × 20 pixel space. Internally, character shapes were defined on a 5 × 9 pixel grid, loosely based on the Signetics 2513 ROM, then interpolated to 10 × 18 pixels with smoothing of diagonals, giving a characteristically angular shape surrounded by two pixels of blank space above and to the left. This arrangement provided stable, flicker-free display on interlaced screens.
The alternate set of 2 × 3 block graphic characters used the same 12 × 20 grid, with top and bottom blocks each 6 × 6 pixels and middle blocks 6 × 8 pixels; if the 'separated graphics' control character was sent, each block was reduced by two pixels in each direction. Pixels were typically displayed at 1.33:1 or 1.2:1 aspect ratio to approximate the standard 4:3 TV ratio, yielding effective displays of 400 × 300 or 480 × 400 pixels.
The SAA5050 implemented the original World System Teletext standard (Level 1), which had no provision to set black for foreground text color, unlike some alternative chips that allowed this as formalized in the 1981 CEPT videotex standard. Several localized variants existed with different character sets: SAA5051 (German), SAA5052 (Swedish), SAA5053 (Italian), SAA5054 (Belgian), SAA5055 (U.S. ASCII), SAA5056 (Hebrew), and SAA5057 (Cyrillic).
Reader's Guide
The SAA5050's significance lies in its role as a dedicated Teletext character generator that enabled text and graphics overlay on television displays, a key feature of early information systems like Prestel and viewdata. Its use in microcomputers such as the BBC Micro and Acorn System 2 helped popularize teletext-style graphics in home computing. The chip's character rendering, based on a 5 × 9 grid interpolated to 10 × 18 pixels with diagonal smoothing, produced a distinctive angular look that became iconic. Its block graphics mode allowed simple mosaic-style images, and the stable interlaced display was a practical advantage for TV-based systems. The SAA5050 was later superseded by the SAA5243 CCT chip, which integrated teletext character generation with decoding, timing, and video generation functions, controlled via I2C. The chip's legacy includes inspiring a font called 'Bedstead' based on its character shapes.
Did You Know?
- The SAA5050's internal character grid was 5 × 9 pixels, loosely based on the Signetics 2513 character ROM chip.
- The chip generated a full-screen resolution of 480 × 500 pixels, corresponding to 40 × 25 characters.
- Localized variants included the SAA5055 for U.S. ASCII and the SAA5057 for Cyrillic.
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