Patent GB-1307403-A (GB)
Title: Digital synchronization system
IPC: H04L7/02
Filed: 1970-03-26
Abstract: 1307403 Digital transmission systems RCA CORPORATION 26 March 1970 [1 April 1969] 14827/70 Heading H4P In a system wherein a number of transmitters developing characters of differing format i.e. differing bit durations and number N of bits per character, are connected through a common channel to a receiver which does not know in advance which format will be presented, a system is provided to adjust the receiving equipment in correspondence with the presented signals.  The receiver is disposed to receive from any transmitter first timing signals of duration Bo#t where Bo is known and representative of the minimum number of bits from any connected transmitter, and #t the duration of each bit, and second timing signals of duration B#t where B is the number of bits from a transmitting source. Preferably the receiver has means responsive to the first signals Bo#t to generate clock pulses spaced at intervals #t, and means responsive to the second timing signals B#t, #t having been ascertained for generating B clock pulses with each pulse concurrent with a respective character bit interval. The receiver may include a circuit for producing clock pulses at a frequency a multiple of a known number of bit intervals which includes a fixed frequency oscillator feeding a plurality of counters and a group of detectors and comparators operative for detecting particular counts. Preferably the frequency of the oscillator pulses is substantially higher than the repetition rate of the bit intervals. A first of the counters is responsive to receive the timing pulses to count the oscillator signals during the interval of each timing pulse, the first comparator being operative each time the first counter reaches a given count for resetting the first counter to zero by producing an output signal. The second counter records the number of output pulses from the first counter and this again is reset at another determined count and a third counter is responsive to the second counter which again is reset at a third determined count, this being increased by one at each operation of the third counter. A detector is provided for preventing the third counter from further changing the count to which the first comparator is responsive in the absence of an output signal from the third counter during the interval of a timing pulse. The bits comprising a character may be prefixed by a start bit and end with a stop bit. In Fig. 3, a fixed frequency oscillator 10 supplies pulses to counter 12 which is compared with that in a second counter 14 by a comparator 16 which produces output pulses X fed through a third counter 18 to a frequency divider 20 and gate 50. The count in 18 is compared with that set by a manually set switch bank 22 by comparator 24 which produces pulses to a flip-flop 44 resetting counter 18 and advancing counter 14 by one. The system of Fig. 3 receives first timing pulses (37), Fig. 2 (not shown), each of which has a duration [No + 1]#t where No represents the minimum number of bits per character any transmitter can send; it is assumed for this circuit that No = for all transmitters. Each interval between the pulses equals [N#t] and as transmitted may have differing intervals, hence the system adjusts its parameters so that clock pulses are separated by interval #t = say 0À001 sec. so that the timing period [N + 1]#t equals 0À009. This system only adjusts the receiver for differing values of #t i.e. for frequency of transmission. Where values of N differ as well the circuit shown above the dashed line 100 is replaced by the system illustrated in Fig. 4 (not shown) which includes a frequency divider 102, e.g. a decade counter whose output is applied through a decoder (104) and inverters (102, 106 &c.) to gates of which (116, 118) are connected to respective counters (124, 126) which are compared for equality at (128) when a signal is applied to flip-flop (130). The circuit of Fig. 3 first adjusts the system for a bit duration #t which applies a signal, e.g. from flip-flop 58 to a flip-flop (136); a signal is also sent to the transmitter to repeat the timing signals which are applied to terminal (30<SP>1</SP>) to set the circuit to the appropriate number of characters.