Multichannel Carrier Telephone Systems

About 1938 multichannel carrier systems were installed that utilized up to 140,000 cycles. Whereas the type C system gave only 3 talking channels, these so-called multichannel systems gave 12 talking channels for systems on conventional-type lines and cables. Later, 240 talking channels were provided on the original New York-Philadelphia coaxial cable, and 480 and then 600 talking channels were provided on coaxial cables in 1948.

In the design of these systems, much of the terminal equipment is standardized, so that the same type of apparatus may be used on the several systems. The voice-frequency band transmitted is from about 200 to 3500 cycles. The terminal equipment⁴¹ employs copper oxide modulators and demodulators such as considered in the preceding pages, quartz crystal filters, and the magnetic generation of the various carrier frequencies.^{41, 42}

This last development eliminates the necessity of many separate vacuum-tube oscillators in the following way. One vacuum-tube tuning-fork-driven oscillator supplies a single basic frequency to a non-linear network⁴² which by distortion produces an output wave such as Fig. 26. This wave contains a fundamental and a large number of odd harmonics. The desired even harmonics are produced by distorting the odd harmonics in full-wave copper oxide rectifiers. The separate harmonics are selected as desired by crystal filters and are impressed on the modulators and demodulators as carrier frequencies.

Figure 26. Wave form of current in the output of the harmonic producer used to supply the carrier frequencies in multichannel carrier systems. (Reference 42.)