| 摘要 |
In order to judge whether a received digital signal has a true polarity or an inverted polarity, a polarity judging arrangement comprises first and second synchronization word detecting circuits for detecting, from the received digital signal, first and second predetermined synchronization words for multiframe and frame synchronization signals, respectively. The second predetermined synchronization word is defined by inverting the first predetermined synchronization word. Each multiframe has at least two frame synchronization signals. A counting circuit has an initial value and counts down and up a count in response to first and second detection pulses produced by the first and the second synchronization word detecting circuits, respectively. The counting circuit produces first and second judgement result signals representative of the true and the inverted polarities when the count increases up to an upper threshold value and when the count decreases down to a lower threshold value, respectively. The upper threshold value is higher than the initial value by a preselected value M, M representing an integer greater than one. The lower threshold value is lower than the initial value by the preselected value. Use may be made of another counting circuit which counts up and down a count in response to the first and the second detection pulses, respectively.
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