Patent GB-1411235-A (GB)
Title: Automatic equalization system
IPC: H04L25/04
Filed: 1972-11-28
Abstract: 1411235 Automatic equalizers PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 28 Nov 1972 [1 Dec 1971 4 Oct 1972] 54813/72 Headings H4R and H4P In an automatic equalizer the transmission band is split into a number of sub bands by a frequency analyser 35 comprising a multiple tap delay circuit 36 with a number of weighting networks 46 connecting the taps of the delay circuit to a number of output channels 37, forming the sub band outputs, which together cover the complete transmission band. Phase and amplitude control circuits 41, 42, are incorporated in each output channel, controlled by comparator circuits 43 fed with spectrum components of a received adjusting signal, split into components by the frequency analyser 35, and a local reference signal 48. After phase andamplitude control the various sub bands are combined at 45, to provide the equalized output. The equilizer may be used in a preset or adaptive fashion, or as a pre transmission equalizer controlled by frequency analysis and comparator circuits located at the receiver. As described the frequency analyser comprises the tapped delay circuit 36 the stages of which are connected to the columns of a matrix of weighting elements 38, 40, &c., pairs of rows of which form the output channels e.g. 37. The two rows of weighting elements of each channel provide identical amplitude-frequency response but phase responses differing by 90 degrees. The phase control circuit 41 of the channel differentially varies the contribution of the two rows of weighting elements to provide control of phase of the output. The phase control is derived by comparison of the phase of the received signals from the rows of weighting elements, with the phase of a locally generated signal from generator 48 in the sampling circuits 62, 63. The amplitude control 42 is derived from the phase comparison signals from sampling circuits 62, 63, by squaring and adding the samples, in 66, 67, 70, and is effected by controlling the gain of the amplifier 65 in inverse ratio to this squared sum, after attenuation in accordance with a reference voltage from source 71. Gates 75, 76, 76 are provided to inhibit comparator operation when no setting up signal is being transmitted. In order to simplify the apparatus one sub band channel 177 may be considered as at a reference phase and the local reference pulse generator 48 has its phase controlled with reference to the phase of the signal in that channel. In the normal channels 37 the amplitude control can precede the phase control, Fig. 10 (not shown), or the phase and amplitude can be controlled simultaneously, Figs. 11 and 12 (not shown). The local reference signal may be split into frequency components before feeding to the comparators, using a similar delay device and matrix of weighting elements. On a transmission channel having a relative flat phase and frequency response in the middle of the band a single comparator circuit may provide control signals for a number of standard width sub band channels, Fig. 17 (not shown), or one wide band sub band channel, Fig. 18 (not shown). The system may be implemented digitally, Fig. 21 (not shown), with a delta modulator provided before the delay device, which would be a shift register. The data may be treated before transmission to provide better use of the bandwidth available, for example by using partial response signalling, in which case the local reference signal is converted to the same form, Fig. 23 (not shown). For use as an adaptive equalizer which is continuously reset during transmission a pseudo random pulse sequency is added to the data signal at the transmitter, and a similar pseudo random sequency generated as the local reference signal at the receiver, to compare with the received signal to provide equalizer adjustment and to subtract from the equalized received signal to obtain the data, Figs. 25, 26 (not shown). In such a system the data can be treated to provide partial response signalling which provides gaps in the data signal spectrum at the locations of the frequency components of the pseudo random setting up signal. Also described is a system including an arrangement for synchronizing the pseudo random generator at the receiver by comparison of the received and locally generated sequences and modifying the clock feed to the pseudo random generator until the two signals are synchronous, Figs. 35, 36 (not shown).