Input signals to a transversal filter are made random by the scrambler which is provided at the sender side. The transversal filter includes multipliers. A correlation detector having filters is connected to the transversal filter. A descrambler connected to the transversal filter converts the equalized random signals provided by the transversal filter into signals which are similar to the input signals. A connector connects the filters of the correlation detector to the multipliers of the transversal filter to control the multipliers by control signals produced by the filters.
A recursive automatic equalizer with extremely fast convergence is disclosed. The equalizer includes a plurality of equalizer stages connected in cascade which first reduces impulse response of the communication channel to substantially zero. The front portion or sidelobe is reduced to substantially zero distortion by adjusting the tap settings of successive equalizer stages after successive iterations (the first iteration adjusts the tap settings of the first equalizer stage, the second iteration adjusts the tap settings of the second equalizer stage, and so on) such that after n iterations a given initial distortion D is reduced to substantially zero. The tap setting algorithm involved and the method of operating the cascaded equalizer is disclosed. Finally, after n iterations, the output signal which can be described by the function 1-A.sup.(n) is modified by the reciprocal of the foregoing function and, is this manner, the rear or trailing portion of an input signal which is to be equalized has its rear sidelobe distortion reduced to substantially zero leaving only the desired main pulse. Apparatus for modifying the trailing or rear end of a pulse to be equalized is also disclosed.
In a scrambled-communication system, a transmitting station generates a scrambling noise having a periodically-recurrent series of noise patterns and scrambles an information signal by simply summing the scrambling noise to the information signal, whereby the information signal is totally unintelligible by an interceptor. At a receiving station, a noise-cancelling signal is generated by an adaptive transversal filter and supplied to a subtractor where it is combined with the scrambled signal to recover the information signal. For generating the noise-cancelling signal, a descrambling noise identical to the scrambling noise is generated. The filter has a tapped-delay line for receiving the descrambling noise and a plurality of variable tap weights connected respectively to taps of the tapped-delay line. The outputs of the tap weights are summed to produce the noise-cancelling signal which corresponds to the scrambling noise if the tap weights are adjusted to optimum tap coefficients that are derived from the descrambling noise and from the output of the subtractor. The repetition frequency and noise pattern are the key for descrambling the scrambled signal.
A simultaneous bidirectional transmission apparatus for transmitting and receiving differential signals solves a generation of pulse noise problem in the receiving circuit due to the inversion of the output of the transmission circuit. For a pair of input/output devices connected together by transmission lines, each input/output device has a differential transmitting circuit, a differential receiving circuit and six resistors. The output of the transmitting circuit does not affect an input to the receiving circuit, and the receiving circuit receives only the output of the transmitting circuit of the other input/output device. The resistors, a passive element circuit and the output resistance of the transmitting circuit form, in combination, a waveform shaping filter and a matching terminating circuit.
An adaptive equalizer for a telecommunication system, designed to process high-frequency signals in a band on the order of 50 MHz, comprises several weighting networks with input connections to respective taps of a delay line and with output connections to a summing amplifier, each weighting circuit having a control input to which a selected corrective voltage is applied. Each weighting network includes a bridge with two arms constituted by respective secondaries of an input transformer and two other arms in the form of indirectly variable resistance elements, specifically photoresistors photoelectrically coupled to a pair of light-emitting diodes, capacitively isolated for direct current from other parts of the bridge. Energization of the control input with corrective analog voltage of either polarity unbalances the bridge by differentially exciting the light-conductive diodes through respective operational amplifiers also receiving feedback signals derived from the voltage drops across the associated photoresistors which are traversed by constant currents.
An apparatus for simulating the characteristics of a signal channel which uses two or more cascaded delay lines each of which in one embodiment has a single input and a plurality of tapped outputs. The tapped output signals from each delay line are each multiplied by randomly characterized signals and the multiplied signals are combined to supply an input signal to the next succeeding delay line. The input signal to the first of the cascaded delay lines is the channel input signal and the output of the last of the cascaded delay lines is the simulated channel output signal. The randomly characterized signals associated with all but one of the delay lines may be signals having random polarities or having random phases while the randomly characterized signals associated with the remaining one of the delay lines has Gaussian characteristics. Appropriate control of the amplitude of the Gaussian characterized signals permits an approximation to be made of the energy distribution of the signal channel which is being simulated.
