Patent GB-1287376-A (GB)
Title: A digital data receiver
Inventor: GIBSON EARL DOYLE
IPC: H04L25/03
Filed: 1970-12-29
Abstract: 1287376 Digital transmission systems; evaluating signals NORTH AMERICAN ROCKWELL CORP 29 Dec 1970 [11 May 1970] 61646/70 Heading H4P A signal evaluating system for identifying signals affected by intersymbolic interference determines the value of a received digit by subtracting weighted components of prior received signals from later received signals to cancel intersymbol &c. interference caused by components of previously evaluated digits summing with components of the later received digits. In the embodiment described previous digit decisions are utilized to correct a substantial part of intersymbol interference of signals received later within a few digit intervals, the apparatus comprising a means for delaying an incoming signal for at least one data bit period, a correction means responsive to the delayed signal, the incoming signal and a corrected receiver output signal, the latter providing signals corresponding to intersymbol interference caused by previously evaluated digits and subtracting these signals from the delayed signal and incoming signal respectively to eliminate a portion of the intersymbol interference and provide a partially corrected incoming and delayed signals, the latter being amplified to maximize the linear separation between different levels between incoming and delayed signals, the amplifier output being fed to a summing means, the output from which is sampled at bit rate and applied to a threshold detector which determines whether the sampled signal is above or below a determined level and feeds back the result to the correcting means. Received signals which may be distorted are passed through a bandpass filter 10, attenuating noise components outside signal bandwidth, directly to and through a delay 20 of 1 bit period to a correction circuit 30 which passes them to a summing circuit 40. The output from this is sampled at 50 at bit rate preferably just before peak values. The summing means may be a circulating memory. Sampler 50 is controlled by clock pulses delayed at 52 to compensate for delays in the circuitry, the phase of clock pulses relative to incoming signals may be adjusted automatically. A determined voltage from the narrow sampled pulses is subtracted at 60 and the resultant passed to a zero level threshold circuit 61 which gives a positive output indicating "1" or a negative output for a "0". Other voltage levels V1, V2 may be used to represent the binary digits. The subtraction circuit may be eliminated by setting the threshold circuit to a corresponding level. The signals are then passed to the output also returned to the correction circuit, illustrated in Fig. 5 (not shown) which comprises a three bit shift register having weighting resistors at its outputs controlling the gains of summing amplifiers. Incoming signals and delayed signals contain parts of pulse responses not only from the signals being evaluated but from intersymbol interference. Correction circuit 30 converts the feedback signals into signals with the interference simulated and in particular intersymbol interference from previously evaluated signals. Simulated terms are subtracted from incoming and delayed signals to provide corrected signals. The system so far described may be designed to have a characteristic (A), Fig. 3 (not shown), with a slope # to separate the line of division between 0's and 1's, but a more accurate distinction may be obtained by the provision of two circuits operating in parallel, see Fig. 8 (not shown), having differing characteristics (A), (B), Fig. 7 (not shown). In a further arrangement, Fig. 11 (not shown), a delay line (100) provides a succession of n signals each delayed by 1, 2, 3 &c. bit periods which is considered to establish a hyperplane in (n+ 1) dimensional space.