Patent GB-349972-A (GB)
Title: Signal band reduction system
IPC: H04B1/66
Filed: 1930-03-05
Abstract: 349,972. Compressed-band transmission systems. STANDARD TELEPHONES & CABLES, Ltd., DUNHAM, C. R. and ROCHE, A. H., Columbia House, Aldwych, London. March 5, 1930, No. 7207. [Class 40 (iv).] In wave transmission systems in which the width of a frequency band such as the speech band is reduced before transmission, the side bands which result from the modulation of a carrier of varying frequency by the initial speech band pass through a network whose delay varies with frequency and then modulate a second carrier of varying frequency. The signal band is expanded after transmission in a reverse manner. The action is explained by the frequency-time chart of Fig. 1, in which the element DE of the initial speech band modulates the first varying carrier represented by AB to produce the band GH, which emerges from the variable delay network as the band JK, and this band by modulation of the second carrier represented by AC gives rise to the element NO of the reduced band. Similarly the element D<1>E<1> gives rise to the element N<1>O<1> of the reduced band. Thus each element of the initial band is skewed in frequency-time plane. Since it is impracticable to provide carriers whose frequencies increase continuously with time, a number of modulators are employed which are effective in turn. Fig. 3 shows a practical arrangement, in which speech enters a fixed frequency modulator 2, and the lower side band selected by a filter 3 passes to a set of three first varying frequency modulators 4.1, 4.2, 4.3. The lower side band passes through a filter 5 and the delay network 6 to a set of six second varying frequency modulators 7.1 .. 7.6. The lower side band is selected bv a filter 8 and modulates a fixed frequency modulator 9 to form the reduced speech band. The subsequent expansion of the band is effected in a similar manner, the characteristics of the delay network employed being complementary to those of the delay network 6. The varying frequency modulators of each set are supplied from valve oscillators which are controlled by suitably shaped condensers mounted on the same rotating shaft and consecutively displaced. The varying frequency modulators and delay networks may have non-linear characteristics. Fig. 15 shows how a single oscillator OSC controlled alternately by variable condensers C1, C2 can supply six varying frequency modulators M1 .. M6 by the use of delay networks D1 .. D5 with suitable amplifiers or attenuators A1 .. A6. The introduction of a network of delay increasing with frequency enables the carriers for both stages to be obtained from one oscillator. In a transmission system, speech band reduction and expansion may be effected at each terminal station without duplication of the apparatus by the use of two delay networks having equal and opposite slopes and voice operated switching devices. Alternatively, if two sets of modulators are used, only a single delay network is necessary. When the invention is applied to multiplex working the y speech channels are arranged to operate into fixed modulators and filters to cause the input to the first varying frequency modulators to be a continuous band of width y times that of each speech band. Corresponding apparatus is necessary to separate out the speech channels after expansion. The working range of the delay network and other elements must be y times that required for a single speech channel.