Patent GB-721346-A (GB)
Title: Multichannel carrier telephone systems
IPC: H04J1/04
Filed: 1952-12-19
Abstract: 721,346. Multiplex systems. WESTERN ELECTRIC CO., Inc. Dec. 19, 1982 [Dec.29, 1951], No. 32232/52. Class 40 (4). [Also in Group XL (b)] In a frequency multiplex transmission system in which carrier frequencies are each modulated by two speech signals, and the upper sideband resulting from the modulation by one is transmitted with the carrier and the lower side-band of the modulation by the second, magnetic core inductors are used and interference due to inter-modulation arising from their non-linearity is eliminated by a compandor. In such a "twin-channel" system, channels LGTR1, 2, Fig. 1A, modulate a 184 kc/s. carrier and are coupled with a second pair modulated on a 192 kc/s. carrier to form a group which is modulated on a 236 kc/s. carrier to produce a transmitted signal of 40-56 kc/s. A similar signal is transmitted from the distant end and at an intermediate repeater the 40-56 kc/s. bands are transposed to produce bands of 60-76 kc/s. The channels are in consequence inverted and less equalization and amplifier gain is necessary. Speech from the switchboard is applied via hybrid network 21, Fig. 5A, and compressor 22 to a low-pass filter 23 having a cut-off of 3100 c/s. which serves to prevent false operation of the V.F. signalling system which uses a 3700 c/s. signal. The V.F. signal is keyed in a circuit 42 and applied with the intelligence signal to a modulator 28, Fig. 5B, where it modulates the 184 kc/s. carrier serving channels 1 and 2. The lower side-band is selected by a band-pass filter 29 and applied with the upper side-band selected from the modulation of the same carrier by channel 2 and a similar " twin pair" on a 192 kc/s. carrier to a group transmitting circuit 12 wherein it is modulated in a balanced modulator 31. A potentiometer 1 is provided so that the output power of all channels can be made equal. The 40-56 kc/s. in the output of the equip. ment 12 is selected by a filter 32 and amplified in a two-stage feed-back amplifier 34<1> before being fed to line 16 through directional filter. At an intermediate repeater station, Fig. 4 (not shown), wherein the amplifier gain is controlled by the difference between a voltage derived from the signal output and a reference voltage, the signal is modulated on a 116 kc/s. oscillation and thus transposed to a band of 60-76 kc/s. At the receiver the signal is selected by a directional filter 33, amplified in an amplifier 36 wherein the gain varies inversely as the input, and brought back to the original 180-196 kc/s. band by a modulator 35 and band-pass filter 39. After further amplification in an amplifier 38 the signal is applied to " twin-channel " circuits wherein the carrier is taken out by a crystal filter 70 both to supply the channel demodulator 41 and to serve as a pilot for the gain control of channel amplifier 68. The wanted side-band is selected by a filter 40, and after demodulation a low-pass filter 75 is provided to block the V.F. signalling current and any spurious 8 kc/s. signals from the speech path.. An expander 77 restores the signal to its original volume range before it is amplified and passed to line. The V.F. signalling current which is taken from the output of channel amplifier 35 is applied to a limitermulti-vibrator 82 via a band-pass filter 80 and an amplifier 81, and a square-wave signal of constant amplitude for a wide range of inputs is produced. The square wave is applied by a cathode follower 83 to a voltage-doubler rectifier 84 and through a D.C. amplifier 87 to operate a mercury contact polarized relay 86. Spurious operation by short bursts is prevented by a delay network 85. The filters 29 and 33 both use high-Q ferrite-cored coils and the channel filter 29 includes also a high-Q crystal to secure the necessary cut-off. The filters are housed two in one can with plug-and-socket change-over arrangements for changing the channel side-bands. The method of core construction is described in detail and Specifications 707,081 and 712,808, [Group XXXV], are referred to. The system is said to be suitable for short open-wire lines.