GB1039822A - Signal transmission system - Google Patents

Abstract

1,039,822. Transmission systems; pulse modulation systems. INTERNATIONAL BUSINESS MACHINES CORPORATION. March 5, 1964 [March 18, 1963], No. 9285/64. Headings H4L and H4R. A complex wave, e.g. speech, is transmitted by adding to the wave a constant amplitude reference signal of a frequency outside the frequency band of the complex wave and limiting the resulting signal to obtain a rectangular waveform of constant amplitude and having zero crossings corresponding to the zero crossings of the sum signal. The resulting rectangular wave has the same power-frequency distribution as the original waves but, due to the limiting, the total power has been made constant. At the receiver the complex wave band and the reference signal band are separated and the level of the complex waveband adjusted in accordance with the change in level of the reference signal band so as to restore the original power level to the complex waveband and thus restore the complex wave to its original form. Fig. 1 shows a block diagram of the system in which a signal from the transducer 10 is fed via a pre-emphasis network 12 and a band-pass filter 14 to a summing amplifier 18 where it is added to a reference signal from generator 16. The sum signal may then be modulated to a higher frequency band in device 20 in order that after clipping in limiter 22 the modulation products, which might cause distortion in the speech, lie outside the speech frequency band. The resulting clipped waveform is transmitted to the receiver by means 24 which may include a demodulator if the signal was modulated before clipping. The transmission may be by amplitude modulation of a carrier or the signal may be transmitted as a pulse train. At the receiver the original speech signal is recovered by decoder 26 and de-emphasis network 28. Three forms of decoder are described; in one, Fig. 2, the speech components are separated from the sum signal by band-pass filter 34 and applied to the dividend input of a divider circuit 38, the reference signal is separated by the band-pass filter 32, its envelope detected, and the resulting signal applied to the divisor input of the dividing circuit the output from which, after de-emphasis, is a replica of the original speech. In a second embodiment, Fig. 3, the speech components from filter 40 are applied to a variablegain amplifier, 42, the gain of which is controlled in accordance with the deviations of the level of the received reference signal components from envelope detector 46 as compared with a comparison reference level from generator 48 as determined in difference amplifier 50. In an alternative form, Fig. 4 (not shown), the variable gain amplifier acts on the sum signal which is then applied to the speech component and reference component band-pass filters, the output from the reference component filter being fed back to control the amplifier gain to tend to maintain the reference component level constant, the signal component then being restored to its original form.