Patent GB-1116021-A (GB)
Title: Process and device for transmitting information signals over channels of limited bandwidth
IPC: H04L25/04
Filed: 1965-07-22
Abstract: 1,116,021. Data transmission systems; equalizing. GRETAG A.G. 22 July, 1965 [23 July, 1964], No. 31309/65. Headings H4P and H4R. [Also in Division G1] In a system for transmitting pulse signals over a channel of limited bandwidth wherein the information is contained in the pulse amplitude or in the amplitude difference between adjacent pulses, pulse response functions which are characteristic of the distortion introduced by the transmission channel for different amplitude pulses are stored at the receiver and the incoming distorted pulse train, in which individual pulses may overlap in time, is reconstituted into its undistorted form by repeated comparison of the stored reference signals with blocks of the incoming pulse signals until the waveform of each block is correctly synthesized by a combination of the stored reference signals. In one method, Figs. 22 to 53 (not shown), the comparison of each block of pulses takes place in a predetermined sequence of the pulses, the sequence being repeated until approximate equality of the two waveforms is reached. In another arrangement, Figs. 54 to 74 (not shown), each comparison takes place at the pulse position giving the maximum difference between the two signals and is repeated as before, until equality is reached. In both cases the comparison process is arranged to take place during a time of one pulse interval of the transmitted signal. As shown in Fig. 77, an information signal NS at the transmitter comprising pulses I 1  to I 8  of varying amplitude, after passing through the transmission channel NK which is represented as a low-pass filter, appears at the receiver as waveform ENS. The information may be transmitted directly in the form of pulse amplitudes or, Fig. 75 (not shown), may be integrated at the transmitter, the pulses being differentiated at the receiver, in this case the information being contained in the amplitude difference between adjacent pulses. The received signal is stored at unit ENSP which may include a digital device including an analogue to digital converter, the stored values E 1  to Eg corresponding to the pulse position I 1  to I 8 . The waveform ENS is of the form shown because the individual pulse response functions overlap each other and the amplitude values in the respective pulse positions do not directly correspond with those at the transmitting end. The comparison signals are produced by sending a pulse of known amplitude and shape, such as pulse I which in a pulse response storage unit IASP produces the response functions such as IAF which represent the channel distortion on an individual pulse at the various amplitudes involved in the transmission of the intelligence signal. The signals from IASP are repeatedly transferred to a storage unit SSP and compared with approximate points of the incoming signal at SV, the error signal from SV controlling a device SVS which increases or decreases the reference signal IAF accordingly until the synthesized waveform SYS agrees with the stored waveform ENS, pulses W 1  to W 8  corresponding to this waveform being provided by a pulse storage unit IWSP. Synchronization of the transmitter and receiver is effected by conventional means. The maximum values at predetermined time intervals of the reference pulse response signals may be stored at IASP in digital form in a plurality of shift registers in which columns represent the amplitude values in binary form and similar devices may be used for stores ENSP, SSP and IWSP. binary addition and subtraction means being provided for units SSP and IWSP, Figs. 83A, 83B (not shown), and Figs. 93A, 93B (not shown). In a system operating on analogue principles, Fig. 106 (not shown), the received signal and the reference pulse response functions are stored on a magnetic drum.