Patent GB-1481096-A (GB)
Title: Data transmission system using frequency-differential phase modulation
IPC: H04L27/18
Filed: 1974-09-10
Abstract: 1481096 Data transmission SIEMENS AG 10 Sept 1974 [19 Nov 1973] 39619/74 Heading H4M In a data transmission system using frequency-differential phase modulation, in which a number of different phase locked carriers are phase modulated during successive modulation intervals and transmitted commonly, a first number of periods of a fundamental oscillation and a second different number of modulation intervals occur within the same time and establish a so called superordinate period. In order to ensure that the fundamental periods at transmitter and receiver in the system correspond in time with the same parts of the modulation intervals, a reference signal is generated at the transmitter characterizing the starts of the modulation intervals and the superordinate periods and is used to modulate one of the carriers, so that the particular modulated carrier contains not only data to be transmitted but also information indicating the starts of modulation intervals and information relating to the starts of superordinate periods. This modulated carrier is used to synchronize the phase of the reference carriers at the receiver with the phase of the carriers at the transmitter. In the system shown in Fig. 1, binary data from a source DQ is received serially by a series parallel converter and passed parallel fashion to a coder KD. The parallel bits are applied to phase modulators MD1-MD4 each of which receives a different carrier F1-F4 from a generator GEN. The modulated carriers are applied commonly to a transmitter SE and hence to a receiver EM. The received signal mixture is demodulated by phase demodulators DM1- DM4, which are provided with reference carriers by a device SYN to which the received signal mixture is applied. The demodulator outputs are applied to decoders DK1-DK3 whose outputs are applied via parallel to serial connector PS to a data terminal (teleprinter or display) DS. The modulation interval, during which a given phase change representing a data bit occurs, does not correspond with the basic period of the fundamental oscillator to which the carriers are harmonically related, but there is a period-the superordinate period-where the beginning of a modulation interval coincides with zero transits in the fundamental oscillation. Because in a frequency differential phase modulation system information is transmitted as phase differences between different carriers and the modulation interval is not the same as the period of the fundamental (at the end of which all carriers are brought into the same phase relation) it is essential that at the receiver that corresponding periods of the fundamental are allocated the same parts of the modulation intervals. In order to define the superordinate period a reference signal E consisting of rectangular pulses is provided by the generator GEN, most of the pulses having a width of two modulation intervals, but those corresponding with the start of a superordinate period having a duration of three modulation intervals. This reference signal additionally modulates one of the signpl carriers F4. In the synchronizing device SYN at the receiver, a timing signal of relatively high frequency is produced by an oscillator OS (Fig. 4) and applied to a frequency changer FS followed by a frequency divider FT which provides a master reference C and the various reference frequencies R1-R4 required by the phase demodulators. R4 is also applied to phase modulators MD5 and MD6 which also receive signals from a further frequency divider, so that the phase modulators produce 1, 0, or - 1 outputs depending on the relative phases. The outputs from the modulators are applied to four correlators KR1-KR4 which receive also the received signal G prior to demodulation and which contains synchronizing information, and the master reference C. C defines the correlator integration period. The Specification explains in detail the behaviour of the correlators depending on the extent to which the receiver is synchronized with the transmitted reference. The correlator outputs are applied to a course synchronizing stage GSYN and this provides a signal controlling the frequency changer stage FS. A signal is also supplied to the analyser circuit BW to tell it when coarse sync has been established. The correlator outputs are also used to phase-in the superordinate periods and to do this are applied to an AD converter which provides binary signals to a signal converter SU. This provides an output Q by comparing the various correlator outputs, indicating whether or not there is a dead time stage. This signal is applied to the analyser BW to cause it to shift the receiver time base by one modulation interval, if dead time is indicated. This shifting continues until Q=0, when both course synchronization and the phasing in of the receiver reference carriers R1-R4 has been achieved.