Patent GB-1483274-A (GB)
Title: System for automatic equalization
IPC: H04B3/14
Filed: 1975-04-22
Abstract: 1483274 Automatic equalizers PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 22 April 1975 [25 April 1974] 16557/75 Addition to 1411235 Heading H4R In an automatic equalizer of the type described in parent Specification, in which the transmission band is split into a number of frequency bands by a frequency analyser 35, Fig. 3, having a multiple tap delay circuit 36 with a number of weighting networks connecting the delay circuit taps to output channels 37 forming the sub-band outputs, each of the output channels having an in phase and a quadrature section with respective gain controlled amplifiers 47 ,47<SP>1</SP> by which the amplitude and phase of the sub-band outputs can be controlled, and in which the gain of the amplifiers 47, 47<SP>1</SP>, is controlled by comparison of the outputs of a further frequency analyser, 232, connected to the combined output of the first frequency analyser 35, with the outputs of a third frequency analyser 233, connected to a reference source, the reference source is formed by a pulse regenerator 18 fed with the combined output of the first frequency analyser 35. In the prior equalizer of parent Specification the reference source was constituted by a locally generated reference signal which was compared with a test signal transmitted before or with the information signal, whereas in the present system the equalizer is adjusted using the information signal itself as the test signal and the output of the regenerator 18 as the reference signal. The control voltages for the gain controlled amplifiers in the output channels 37 are developed by the comparators 231 and the mean value calculator 237 which are shown more fully in Fig. 4. The comparator is fed with the complex components of the equalizer output analysis channels, i.e. Yc and Ys representing the "in phase" and "in quadrature" components respectively, and the complex components Zc and Zs of the regenerated pulse stream and the complex control signal is derived from the relation from which are obtained the relations and the circuit 231 calculating these values. The mean of these control coefficients is derived in a sample and hold circuit 237 according to the relations where Kc(t + 1) and Ks(t + 1) are the components of the complex control voltage at sampling instant (t+ 1), and Kc(t) and Ks(t) are the components of the complex control voltage at sampling instant (t), Kc and Ks being the comparator, 231, output signals after low pass filtering in filters 251 and 252. In the system of Fig. 3, at the commencement of transmission the gain control amplifiers 173, 174, are fed with reference control voltages via switches 260 and 260<SP>1</SP> until the normal control voltages have had time to build up in the calculators 231 and 237. In an alternative embodiment, Fig. 5 (not shown) a reference pulse signal is transmitted for a short time prior to information transmission, a similar reference pulse signal is generated at the receiver and a comparator (43) generates gain controlled amplifier control signals from a comparison of the phase and amplitude characteristics of the locally generated reference pulse signal with the signals derived by the analysis channels (37) operating on the received pulse signals, these control signals being used to preset the equalizer before it acts adaptively on the information signal.