Improved Multiplex Voice Frequency Carrier Systems Using New Audio Filters

Using resonators built with high " " ferrite-cored inductors and suitably selected plastic film capacitors, improved band-pass and band-elimination filters have been designed which allow the accommodation of duplex high-speed telegraphy channels in a mid-band-speech carrier circuit, with small degradation in voice quality and with negligible cross-talk. It is shown that in such a circuit, the removal of a 400-cps frequency band from the voice spectrum, from about 1100 to 1500 cps, offers the best compromise as far as intelligibility of speech is concerned. The basic elements of a complete system are described. The improved tone channel filters accept, with small distortion and ringing, frequency shifts of ±42.5 cps and allow: 1) the reduction of the usual 170-cps channel spacing to 145 cps, without increase in cross-talk, and 2) the location of 145-cps spaced tones up to 6000 cps in suitable links. For systems with frequency shifts of ±30 cps, similar units with a narrower acceptance band reduce the required channel spacing to only 85 cps.