MULTICHANNEL MODULATION SYSTEM 3 



repetition rates up to 672 kilocycles, and pulse durations as short as a quarter 

 microsecond. These pulses, obtainable from more or less standard types of 

 vacuum tube circuits, can be distributed from point to point in the equipment 

 without too much difficulty. Amplitude modulators and demodulators at two 

 neighboring carriers — 65 and 66.5 megacycles — then serxt to bring the PCM 

 signals into the intermediate frequency range for transmission to and from the 

 microwave equipment. 



The speech quality of the overall system in respect to such factors as band 

 width, volume range, noise, distortion, and crosstalk more than meets the 

 requirements generally imposed upon such systems. 



Figure 1 is a front view photograph of the experimental apparatus setup with 

 covers remo\-ed from one bay to show typical construction. The two end bays 

 contain intermediate frequency modulators and demodulators required for the 

 two groups. In addition voice frequency terminating sets and jacks are 

 mounted here, together with testing equipment. The center bay and the one 

 to the right of it are identical; each includes apparatus for handling a group of 

 twelve message channels. Transmitting equipment is mounted in the upper 

 half, and receiving equipment in the lower half of each bay. The remaining 

 bay, second from the left, holds all the timing equipment needed to furnish 

 control pulses for operating eight of the message bays, a total of 96 channels. 

 Included are circuits for synchronizing the receiver. Individual regulated 

 power supplies are mounted near their loads on the several bays. 



Figure 2 is a rear view of the same equipment. Cables in the four horizontal 

 ducts shown carry control pulses from the timing bay to the 12-channel group 

 bays. These ducts are large enough in cross-section to handle all the cables 

 required for a complete 96-channel terminal. 



II. Functional Problems Involved 



The broad problems brought together in building this system may be con- 

 sidered under the four classes follow mg: 



1. The pulse code modulation problem; to convert signal waves to pulse pat- 

 terns. 



2. The multiplex problem; to aggregate channels into groups and groups into 

 a supergroup. 



3. The transmission problem; to fit the system into the minimum required 

 band width, and to remove the effects of transmission impairments. 



4. The pulse code demodulation problem; to convert pulse patterns back 

 to the original signal waves. 



These are to be discussed from a functional standpoint to provide background 

 for discussion of the specific equipment. 



