GB1369692A - Method of reducing level fluctuations caused by distortion and apparatus therefor - Google Patents

Abstract

1369692 Radio signalling LICENTIA PATENT-VERWALTUNGS-GmbH 15 Nov 1971 [17 Nov 1970] 52913/71 Heading H4L In a system for reducing level fluctuations in the baseband in a frequency-modulation transmission system caused by echo distortion during transmission, at the transmitter 1a plurality of pilot tones q 1 , q 2 , q 3 phase modulate the F.M. signal s 0 which has been phase shifted at 2 by 90 degrees, the resultant signal being added to s 0 at x. The pilot signals are arranged to be distributed as widely as possible over the baseband. At the receiver 3 the received signal S 1 is supplied via networks N 1 , N 2 , which introduce distortion reducing signals, to a frequency discriminator FD whose output x contains the signal bands and the pilot signals q 1 &c. The output x is supplied to a mixer M receiving sequential frequencies q<SP>1</SP> 1 , q<SP>1</SP> 2 , q<SP>1</SP> 3 under the control of a rotary switch Sch driven by circuit ST and the output of the mixer is passed through a bandpass filter BP1. By this means each pilot signal is transposed successively into an intermediate frequency Z which is amplitude modulated in accordance with the distortion on the pilot signals at the sampling periods. Signal Z is demodulated at G 1 and supplied via bandpass filter BP2 to a rectifier G2, the output of G2 being smoothed via a low pass filter TP to provide a voltage v which is a measure of the distortion. This voltage causes an optimizer 5 automatically to adjust resistors R1, R2 &c. which terminate lines L1, L2 &c. whereby compensating echo signals in phase opposition to the echo distortion are introduced via networks N1 &c. into the signal S1. The output of discriminator FD is supplied via an amplitude and phase correcting filter BP3 to a phase modulator T12 receiving also the signal S 1 phase shifted by 90 degrees, the output of the phase modulator being combined in phase opposition with the signal S 1 to remove the pilot signals therefrom.