Patent GB-1178769-A (GB)
Title: Improvements in or relating to the Transmission of Data
IPC: H04L25/03
Filed: 1967-08-15
Abstract: 1,178,769. Self adaptive transversal equalizer; radio receiving systems. CODEX CORP. 15 Aug., 1967 [19 Aug., 1966], No. 37489/67. Headings H4L and H4R. In an adaptive transversal equalizer including a tapped delay line with adjustable multipliers or the digital equivalent, in which the multipliers, are adjusted in response to the determination of errors between the equalized signal and the transmitted signal, the error signal for adjusting any one multiplier is determined by producing a set of error signals relating to sampling times over a predetermined range about the sampling instant appropriate to the multiplier being adjusted, weighting these error signals in a predetermined fashion dependent on their time of occurrence relative that sampling instant, and summing the weighted sample to produce the multiplier adjusting error signal. The error signal is derived using a reference pulse stream which is added to the data stream at the transmitter and a replica of which is available at the receiver so that it may be correlated with the incoming pulse stream so as to eliminate the effects of the data stream and noise in the error signal derivation, and which may be subtracted from the equalized signal in order to recover the data stream for use. In the example described data pulses at 9600 pps are fed, in groups of three, to a digital to analogue converter to produce for each group of 3 pulses a single pulse having one of eight possible amplitude valves. The resulting 3200 pps data stream has added to it a reference two level pulse stream of predetermined sequence and a 1À6 kc. pilot tone which is used to control synchronization between the transmitter and the receiver. At the receiver, shown in basic form in Fig. 3, the pulse stream is sampled and the resulting pulse stream applied via line 253, Fig. 3, to an analogue to digital converter 254 and a digital transversal filter 300, in which the incoming digital signals from converter 254 are fed into a recirculating memory unit with 32 storage positions, only 29 of which are used making the device equivalent to a delay line filter with 29 taps. The signals in the store are multiplied by various coefficients, stored in a second recirculating memory unit, and summed to produce the output signal Zk. The digital filter output is fed to a further recirculating memory 266 one output from which is applied to a subtractor 280 in which the reference pulse train, identical to that added at the transmitter, from generator 264 is subtracted from the pulse stream Zk in order to recover the data pulse stream at 282. The other output from the memory unit 266, which is a pulse stream at the internal circulating pulse frequency of the memory, which is the same as that used in the digital filter, i.e. 32 times the input p.r.f. to the memory, is fed to a correlator 305, which is also fed with the reference pulse stream at the same p.r.f., i.e. 32 times the p.r.f. of the pulse stream from generator 264, so that the equalized pulse stream and the reference pulse stream are correlated to produce an output Si dependent on the error between the equalized input pulse stream and the reference pulse stream, with the effects due to the data pulse stream and noise in the transmission channel eliminated by the integration in the correlation process. The errors are fed to an error weighting unit 310 where a range of error valves, arranged about an error appropriate to a particular multiplier value in the digital filter 300, are multiplied by a set of weighting values stored in store 278 and summed in order to derive a single value Ci which is multiplied by a factor "a", e.g. 0À1, to provide a correcting value which is used to adjust the appropriate coefficient in the digital filter 300. Derivation of weighting values.-Preferably the weighting values are derived by cross correlating the input pulse stream with the reference pulse stream, the input pulse stream being obtained by setting the multiplying factors in the digital filter all to "zero" except for the central factor which is set to "one". This derivation requires a recirculating memory of greater size than the memories used during normal operation, a saving in equipment is obtained by utilizing portions of the memories used during normal operation which are not required during weighting value derivation, e.g. those in units 305 for deriving the errors and in unit 300 for storing modifying values for the multiplier functions. In an alternative system the weights are made equal to the multiplying coefficients of the digital filter and are updated together with these coefficients. This system is said to be particularly applicable to radio transmission due to its rapid convergence to fully equalized condition. Application to telephone line.-The system requires low pass characteristic in the transmission medium, for line transmission the transmitted signals are modulated, e.g. on a 2À4 kc. carrier, and the lower sideband selected and transmitted with corresponding demodulation at the receiver.