each of 25 frames per second). A teletext page comprises one or more frames, each containing a screen-full of text. The pages are sent out one after the other in a continual loop. When the user requests a particular page the decoder simply waits for it to be sent, and then captures it for display. In order to keep the delays reasonably short, services typically only transmit a few hundred frames in total. Even with this limited number, waits can be up to 30 seconds, although teletext broadcasters can control the speed and priority with which various pages are broadcast. Modern television sets, however, usually have built-in memory, often for a few thousand different pages. This way, the teletext decoder captures every page sent out and stores it in memory, so when a page is requested by the user it can be loaded directly from memory instead of having to wait for the page to be transmitted. When the page is transmitted again, the decoder updates the page in memory. The text can be displayed instead of the television image, or superimposed on it (a mode commonly called mix). Some pages, such as subtitles (closed captioning), are in-vision, meaning that text is displayed in a block on the screen covering part of the television image. The original standard provides a monospaced 40×24 character grid. Characters are sent using a 7-bit codec, with an 8th bit employed for error detection. The standard was improved in 1976 (World System Teletext Level 1) to allow for improved appearance and the ability to individually select the color of each character from a palette of eight. The proposed higher resolution Level 2 (1981) was not adopted in Britain (in-vision services from Ceefax & ORACLE did use it at various times, however, though even this was ceased by the BBC in 1996), although transmission rates were doubled from two to four lines a frame. Levels In the early 1980s, a number of higher extension levels were envisaged for the specification, based on ideas then being promoted for worldwide videotex standards (telephone dial-up services offering a similar mix of text and graphics). The most common implementation is Level 1.5, which supports languages other than English. Virtually any TV sold in Europe since the 1990s has support for this level. After 1994 some stations adopted Level 2.5 Teletext or Hi-Text, which allows for a larger color palette and