Technology: TV audience answers back

 作者:蒙幽     |      日期:2019-03-02 05:04:08
By ELISABETH GEAKE Trials of interactive television began last week on the cable TV network in Salt Lake City, Utah, using a signalling technique that was originally developed to transmit military secrets. Television viewers can use the system, known as spread spectrum, to answer questions on quiz shows or select a film from the online video library. Unisys, a computer company based in Blue Bell, Pennsylvania, is carrying out the three-month trial with Time Warner Cable of Stamford, Connecticut. The technique enables viewers to send messages by cable to the television stations without disrupting transmission of the programmes. In the trial, a second, digital signal is transmitted alongside the one which carries the normal sound and pictures. The spread spectrum signal is divided into perhaps a thousand smaller signals. These are transmitted at frequencies selected at random at a thousandth of the power of the original signal. A code allows the receiver to identify the frequencies on which signals are being transmitted, and then to reassemble the message. Because the fragments are short, low power and random they look just like the background noise present in any radio signal. This has two advantages. First, the television receiver ignores background noise, so spread spectrum does not affect the quality of sound and pictures. Secondly,