| 摘要 |
A signal quality detector for a teletext television receiver determines the quality of a received television signal by counting the number of pulses in each line period in a pulse signal waveform separated from the television signal and containing only line, field and equalising pulses. Normally, a line period contains only one or two pulses, but spurious additional pulses are introduced by noise. As shown in the Figure, a counter EC counts the number of pulses in each line period and a counter BL counts the number of "bad" lines in each field. After a number of fields which are "good" or "bad" depending on how many "bad" lines they contain, an up/down counter FC sets or resets a latch SQL to inititate or terminate a signal quality flag SQF which is used to inhibit teletext data acquisition. Latches DL1 and DL2 provide hysteresis in that when the signal quality is "good", more than 14 "bad" lines in a field are required to declare a field "bad", whereas when signal quality is "bad", less than 3 "bad" lines are required to declare a field "good". |
