Technology
A fundamental difference in technical philosophy between Antiope and the Ceefax teletext system developed by the BBC stemmed from the fact that Antiope was developed by telecommunications engineers, while Ceefax was developed by television engineers. The television engineers created a system which filled the screen with data at a fixed, predictable rate; in contrast Antiope was constructed like a packet-switching system, with variable length packets of data, as might be used on a telephone network.
In both cases, the teletext data was transmitted during the vertical blanking interval, the portion of time allocated for the electron beam to return from the bottom of the screen back up to the top between each frame of the image (which corresponds to a transmission time of 25 lines in each field of PAL or SECAM), and each line used conveyed a fixed number of bytes each cycle: 40 bytes for European systems, 32 in the United States. In the BBC Ceefax system each group of 40 bytes corresponded directly to one row of 40 characters, each byte representing both the character and its place in the row – a videosynchronous system.
In contrast, in the case of Antiope the delivery packet was independent of the screen presentation. One 40-byte packet might contain several rows; another might contain only part of a row, if it contained several colour changes or accented characters (coded with two or three bytes). In return, colour changes, or switches to graphical mode (called "mosaic"), which each had to take up one character space in the row on a Ceefax page, could be made freely in Antiope, allowing the construction of more elaborate pages.
Ceefax pages could be displayed using fairly simple hardware, using a character generator and a simple latch to keep track of the attributes in force at a particular character position as the page is scanned. Antiope was only a little more complicated, the relevant attributes for each character position (about 13 bits, or a few more if further additional fonts were in use) being stored in a separate memory page in the decoder, distinct from the memory page used for the characters.
This flexibility of Antiope provided a foundation on which multilingual and multialphabetic systems were developed, and also systems using dynamically redefinable character sets (DRCS), especially in variants delivered over the telephone (videotex).
Read more about this topic: Antiope (teletext)
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