Patent GB-466327-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: H04B1/66
Filed: 1936-10-06
Abstract: 466,327. Band - width compression. ELECTRICAL RESEARCH PRODUCTS, Inc. Oct. 6, 1936, No. 27053. Convention date, Oct. 30, 1935. [Class 40 (iv)] [Also in Group XL] In a speech transmission system, syllablefrequency signals indicative of the energy in each of a number of bands into which the speech is arbitrarily divided and preferably also a syllable-frequency signal indicative of the momentary value of the fundamental frequency, if any, are transmitted instead of the speech itself. These signals are sent by multiplex carrier methods, have zero intelligibility, and can be accommodated in a narrow band e.g. 10-360 cycles. At the receiving end, a resistance noise generator and a multi-vibrator, the fundamental frequency of which is kept equal to that of the distant speech, feed a number of channels, one for each of the bands into which the speech is divided at the transmitting end, and the transmission of each of these channels is controlled in accordance with the corresponding signals. These channels lead to a common circuit in which the re-constituted speech appears. If desired the multi-vibrator may oscillate at a fixed frequency and its controlling channel is then unnecessary. The resistance noise source is omitted and the resulting speech sounds like chanting. Alternatively the multi-vibrator may be replaced by the resistance-noise generator and the resulting speech then has the sound of a whisper. As shown, the frequency-determining signal is derived from the 250 .. 500 cycle sub-band of the original speech since this band contains two or more harmonics of the fundamental frequency when it has so low a value that it is not satisfactorily transmitted by the incoming line L. A portion of the speech in this range traverses a rectifier D, a network E1 in which the loss increases with the frequency, a constant output amplifier LA and a further network E2, the output of which decreases with frequency and is rectified at DO. The output at LA is substantially the fundamental frequency of the speech, fluctuating at syllable frequency but at a constant power level regardless of frequency. The effect of E2 is to cause the amplitude of the syllable frequency output from DO to vary with the fundamental frequency. This syllable frequency wave is fed with a 30-cycle carrier to a balanced modulator MO and a single side-band is selected for transmission by Co. The amplitude determining signals are derived from the sub-bands selected from the speech after delay in a network DE by the filters F1 .. F10. Currents in the circuits AP1 .. AP10 are rectified, passed through syllable-frequency filters F31 .. F40 and modulate carriers 90 .. 360. The carriers are derived from a multi-vibrator MV fed with 60-cycle current which is also transmitted over the line for the control of a multi-vibrator MV1 at the receiver for feeding demodulators DMO .. DM10. The frequency-controlling signal produces a paralyzing bias at B1 for the amplifier of the resistance noise currents generated at FPS and a bias at B2 for controlling the effective resistance of a pair of valves VR. The resistance of this pair determines the fundamental frequency generated by a multivibrator MVO comprising a pair of valves the plate circuits of which are associated with the output circuit in series and in parallel to ensure the output of both odd and even harmonics. The outputs of both generators are fed via an equalizer E4 to filters having the same pass bands as those dividing the speech into subbands at the transmitter and the respective amplitude controlling signals control the gain of amplifiers SN1 .. SN10 connected to the output of the filters. Reconstituted speech appears in the common circuit 4. The equalizer E4 may be omitted if the coupling between the windings of the output transformer of MVO is loose. As an alternative to the multivibrator MVO shown, one working at half the fundamental speech frequency and producing only even harmonics may be used.