GB1170432A - Improvements in or relating to Electrical Phase Modulation Transmission Systems - Google Patents

Abstract

1,170,432. Companders. NIPPON ELECTRIC CO. Ltd. 10 Nov., 1966, No. 50528/66. Heading H4R. In a frequency, or phase, modulation transmission system compression is provided at the transmitter and expansion at the receiver with the degree of compression and expansion being dependent on frequency content of the signal to be transmitted. The compression and expansion are increased with increase in frequency of the input signal. Fig. 9A shows a transmitter in which signals from a source 15 are applied via a branching network 17 to a phase modulator 16 and to a modulation sensitivity control circuit, or compressor, 22 which, by rectifying the input signal in rectifier 20, reduces the sensitivity of modulation as the input level is increased. A frequency dependent network 18 increases the level of higher frequency components relative the lower frequency components of the input signal so that the modulation sensitivity is reduced more for H.F. input signals than for L.F. input signals. Frequency converter 5 and H.F. amplifier 6 may be provided if required. Fig. 9B shows a receiver which, conventionally, includes a frequency changer 10, I.F. amp. 11, demodulator 23, and deemphasis circuit 24. The signal from 24 passes via a branching network 25 to a variable gain arrangement, or expander, 30, where the amplitude of the signal is modified by a control signal derived from the signal via a frequency dependent network 26, which emphasises high frequency components, amplifier 27, rectifier 28, and low-pass filter 29, so as to increase the gain of high level signals, the increase being greater with high-frequency input signals than with low. The variable gain arrangement may be a variable gain amplifier, or amplitude modulator, or a pulse width modulator followed by a smoothing circuit.