GB1355648A - Timing or retiming in electrical pulse systems - Google Patents

Abstract

1355648 Television; synchronizing BRITISH BROADCASTING CORP 27 July 1971 [15 June 1970] 28952/70 Headings H4F and H4P [Also in Division G4] An apparatus for timing or retiming electrical pulse signals includes an input selectively connectible to the input of one of three clocked stores the outputs of which are selectively connectible to an output and a clocking means for clocking the stores at an input rate when they are connected to the input and at an output rate when they are connected to the output, the arrangement being such that each store may not be connected to the input and output simultaneously. The apparatus may be used to correct skew on parallel data tracks on a magnetic tape or may be used to retime a number of signals in parallel in the same way. The signals may be digital or sampled analogue, e.g. video signals, the stores being clocked digital or analogue devices with an A/D converter as appropriate. The received data may include a framing sequence which is inserted in the data train as follows. Two or more shift registers A and B are provided and are filled alternately with input data. A further shift register 13 holds the framing sequence and the three registers are connected to an output in the sequence ACBCAC &c. the data being clocked in and out using different frequency clocks to compensate for the added framing sequence. Each data channel has a similar apparatus. The received data is fed into a shift register 21 which is connected to a decoder adapted to recognize the framing sequence. When this sequence is detected the input is switched to one of the stores 22, say 22X. The store has a capacity to the number of inter framing sequence bits and the data is clocked in under the control of a clock signal derived from the data until the data bits fill the store; the framing sequence being lost. At this time a further framing sequence is detected in register 21 and a new store say 22Y is selected. The data in store X22 is then clocked out under the control of clock signals derived at the remote station. The provision of a similar device for each tape channel ensures synchronism of the output. Some bits in the framing sequence may be used to identify other parameters, e.g. elapsed time. If only errors common to all channels are to be corrected the framing sequence is unnecessary and store 21 may be omitted. For use with a video tape on A-D converter is inserted between the recorder and the retiming apparatus and the input clock pulses are derived from a pilot tone recorded on the tape. Alternatively clocked analogue stores may be used together with a sampled analogue input. The apparatus may also be used to delay an incoming TV signal. In the case where there are significant differences between the input and output TV frequencies storage capacity greater than one "field" is provided so that a field may be omitted or repeated if writing "catches up" with reading or vice versa respectively.