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detector JV over logic circuits L :t and L., direct to the input of the ring counter.The revolution period of the counter is diminished by l /s2 during one cycle.During the retardation period a synchronizing pulse will block the path viaflip-flop F A , differentiation circuit d and logic circuit L, for one of the pulsesproceeding from the quartz oscillator. Thus the revolution period of thering counter is increased by "32 during one cycle. If no zero crossings occurthe revolution period of the counter will not change. In the detector everybit is scanned in the middle by means of the regenerated bit frequency. Arepeating flip-flop will reproduce the data signal in the form of square waves.3. Use of the SystemAs was mentioned in the preceding pages, a data transmission terminalis connected to a transmission system as well as to a data-handling equipment.In order to facilitate the matching of the data signals to the lines their spectracan be shifted by changing the oscillators G L , of the data transmission terminals.A line connection over a loaded cable, for instance, requires a signal spectrumshift to 0.5—2.1 kc/s. This is obtained when /., = 8.7 kc/s. The flexibility inrelation to the data-handling equipment is increased by the possibility ofaltering the bit frequency by means of frequency divider R,. which is variablein steps. To achieve the best possible pulse shaping and interference elimination,the pulse-shaping filter of the sender and the intermediate-frequency filterof the receiver have to be changed when the bit frequency is changed.System using differential codingData transmission systems using differential coding are particularly suitablefor continuous transmission between data-handling equipment providedwith buffer stores.In order to eliminate ambiguity in demodulation a differential codingequipment is inserted between the data-handling equipment and the datatransmission equipment. At the sending end a "0"-voltage at the input ofthe encoder will result in a polarity change in the middle of the bit at theencoder output, whereas a "1 "-voltage will not affect the output voltage ofthe encoder (see fig. 6).With the aid of an external or internal bit-frequency clock-pulse generatora data flow (0, 1) is read out from the data-handling equipment and fedto the encoder, which in its turn produces a data signal (N. E). The outputsignals of the encoder will pass through the data transmission equipmentoutlined in figs. 2 and 4 and. when demodulated in phase position A, willPDBSKoDatasignalClockpulsesDifferential- coding data --un^^^F-Fig. 6Data transmission system usingcodingDBS — Data-handling senderQBM — Data-handling receiverP — Pulse regenerationKo — EncoderDe — DecoderKl — Clock-pulse regenerationKle — External or internal clock-pu se generationDTS — Data transmission senderDTM — Data transmission receiver66X 8368differentialKL 2 DTSKl\Distortion^linetOTM3 De'DBMssDifferentialcoding datasignalClockpulsesDatasignalI E 1 N3E3E3Phase position B 1J__^^^tHzFtftr'pB
esult in signal P A , whereas when demodulated in phase position B a signalPn will be produced. If two pulse elements of different polarity occur insuccession at the input of the receiver decoder a "0"-voltage will be generatedat the output during one bit period.Two pulse elements of the same polarity will produce a "1 "-voltage atthe output. Consequently, the output signal of the decoder is independent ofwhether the modulation is carried out in phase position A or B and correspondswith the input signal of the encoder at the sending end. In step with theregenerated bit frequency the bipolar data signal is fed to the data-handlingterminal at the receiving end.67
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