Bad timing

 作者:阳盐钚     |      日期:2019-03-08 06:20:07
By Barry Fox A BUG in the BBC’s teletext system has been stopping some newer VCRs from taping TV programmes since the start of British Summer Time on 28 March. And the BBC’s engineers are still not sure what has gone wrong. Modern VCRs use a system called Programme Delivery Control which looks for marker signals at the beginning and end of each programme. This lets the VCR delay taping of a programme if it starts later than the advertised time, so the VCR does not cut out before the end. For PDC to work, the VCR’s clock must match the time used by the broadcaster to add PDC markers. So the teletext time signal is used to adjust the VCR’s clock at regular intervals. But the BBC recently altered its teletext system so that 12 regions round the UK can broadcast local news. Each region was given its own clock generator, which works to Greenwich Mean Time, and moves the time by an hour when BST starts. Unfortunately, some regions made the shift at midnight, and some before or after. So the real time, date and PDC time are no longer synchronised and the VCR either tapes the wrong programme, or nothing at all. The problem persists until the VCR readjusts its time, which may be hours later. Different VCRs behave in different ways and some regions, such as Wales, suffer while others, including London,