Posted by Ronan on Nov 27, 2007 in Uncategorized | 1 comment
The four candles/fork handles sketch is up for sale
For those who have never seen it..
Over coffee we were wondering how Sky can control their boxes over the air and how teletext works.. Well here is how teletext works:
Teletext information is broadcast in the vertical blanking interval* between image frames in a broadcast television signal. Teletext uses a numbered page metaphor to present its information, all of which is broadcast in sequence; when a viewer keys in a page number, the receiver waits until that information is broadcast again, typically within a few seconds, and retrieves it for display on-screen. More sophisticated systems use a buffer memory to store some or all of the teletext pages, for instantaneous display.
Sky use the Ka band in the downlink so maybe they are using a separate part of the band for control/configuration. I’ll do a little further digging and post the results.
*The time difference between the last line of one frame or field of a raster display, and the beginning of the next.
According to Wikipedia:
Sky uses VideoGuard and access flags can be downloaded to the subscriber’s card either over the air (via ‘hidden’ channels) or by using the box’s built in modem