Abstract :
Several types of nonsynchronous multiplex radiotelegraph systems have been developed for use on heavy traffic routes of the Mackay Radio and Telegraph Company. These utilize the advantages obtained through the use of frequency shift techniques. The Twinplex system is based on the combination of two 2-element mark-space channels, wherein each of four frequencies represents one of the possible combinations. The carrier frequency is shifted 400 cycles for one channel and 800 cycles for the other channel, a total of 1200 cycles. The desired shift is obtained in an electronic combining unit. Suitable band pass filters and an electronic converter separate the two channels at the receiver. The Twinmode system employs two Twinplex systems, one shifting the carrier and the other shifting a single tone frequency which amplitude modulates the transmitter. The frequency of this tone was chosen so that the same band pass filters could be used at the receiver as used for the Twinplex system. These are a broad band filter from 1600 to 3400 and narrow band filters centered at 1950, 2350, and 2750 cycles per second. The Polyplex system is intended for not more than eight nonsynchronous telegraph channels. It employs Twinplex equipment as basic building blocks, adding units for shifting a subcarrier at approximately 200 kc by each Twinplex Combiner, mixing these together with a 1.8 mc frequency, selecting the upper side bands centered at 2 mc, mixing these with a harmonic of a suitably chosen crystal controlled oscillator, selecting the lower side bands centered at the operating frequency and amplifying in a linear amplifier. The power amplifier must be designed and adjusted for single side band operation. The equipment at the receiving station is the same as that employed for Twinplex operation, offset frequencies being used to bring the desired group of frequencies within the 16OO to 3400 cycle pass band of the input filter. The complete independence of each telegraph channel from all other channels has proven to be of considerable advantage from an operating standpoint. The flexibility permitted by these systems has been found to be very desirable. Although these multiplex systems prove to be of greatest value on circuits having large amounts of traffic, they may also- be used on a forked basis where the two (or more) destinations are in the same general direction and at such distances that the same frequencies are effective for both. One destination must adjust its receiving apparatus to operate from one channel and the other destination operates from the other channel.
