GB1528460A - Signal transmitter - Google Patents

Abstract

1528460 Compander systems PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 26 Sept 1975 [1 Oct 1974] 39528/75 Heading H4R In a compander for a transmission system in which a pilot signal is added to the information system prior to compression at the transmitter and the pilot signal used to control expansion at the receiver, the information signal is applied to a double sideband suppressed carrier modulator before it is added to the pilot signal, and the pilot signal is formed by the carrier signal used in the modulation stage but at a different phase angle to that which the carrier would have if it were not suppressed. The modulated signal plus pilot is compressed in a conventional manner and at the receiver the expansion can be accomplished conventionally as in Fig. 2 (not shown) or preferably the received signal can be expanded by applying the composite signal to a filter which converts the double sideband signal to a frequency modulated signal which, after limiting and frequency demodulating provides a signal with the dynamic range restored to the original value. As described the transmitter Fig. 1, has a band limiting filter 2 and amplitude limiter 3 from which the level control signal is derived by rectifier filter 7 and comparator 10 with reference 11. The control signal controls the impedance of a diode connected across the signal path in the control circuit 26, Fig. 3, to which is applied the information signal from limiter 3 after it has been double sideband suppressed carrier modulated in modulator 23 and combined with the carrier signal from source 12 after the carrier has been phase shifted 90 degrees in circuit 27 so that the pilot signal is in quadrature with the carrier supplied to modulator 23, the pilot signal being at a level equal to the level at which compression commences to become effective. At the receiver the amplified received signal is applied to a circuit 55 tuned to the pilot, or carrier, frequency and to the limiter 56. The filter has a response which falls at 6 dB/octave above 300 Hz spacing from the carrier which converts the signal from double sideband amplitude modulation to frequency modulation, while the limiter modifies the frequency spectrum such that when demodulated in discriminator 57 the original amplitude range is restored. A double sideband suppressed carrier modulator and compressor circuit is described with respect to Fig. 4 (not shown).