360 BELL SYSTEM TECHNICAL JOURNAL 



slope of the phase characteristic, is the delay. It is sometimes called 

 the group delay or group transmission time as distinguished from the 

 "phase time." ^^ The delay is the only time which can be measured. 

 It does not determine the phase shift of a particular frequency nor is it 

 determined by the phase shift. A phase shifter applied to the net- 

 work merely moves the phase curve intact up or down on the 

 phase axis. 



General Description of the System 



The preceding paragraphs have described features which distinguish 

 the MUSA system from conventional receiving systems. There 

 remain to describe several auxiliary features and to present a unified 

 picture of the whole. 



The experimental system was designed for double side-band recep- 

 tion and all of the results reported in this paper refer to double side 

 band. There has recently been completed equipment which may be 

 substituted for the double side-band equipment for the reception of 

 reduced carrier single side-band signals. The new equipment may also 

 be used to select, with crystal filters, one side band of double side-band 

 signals. 



The delay to be inserted in the low-angle branch as indicated in 

 Fig. 3 is obtained electrically from an audio-frequency delay network. 

 The delay could theoretically be provided at the intermediate fre- 

 quency but no advantage would result. The audio-frequency delay 

 network is a special artificial line composed of forty sections and termi- 

 nated by its characteristic impedance. Each section has a delay of 68 

 microseconds. A special switch is arranged to tap a high impedance 

 output circuit across any desired section, thus providing a delay of 2.7 

 milliseconds variable in 0.068-millisecond steps. A special equalizing 

 network ^^ which makes the transmission loss the same for all steps and 

 which also equalizes the frequency-loss characteristic so that the re- 

 sponse is flat to 5000 cycles for all steps is automatically controlled by 

 this switch. The forty delay sections appear in Fig. 16 just under the 

 shelf on the right-hand bay. The maximum delay which has been 

 required in actual operation is 2.5 milliseconds. 



Both linear rectifiers and square-law detectors are provided for final 

 demodulation and either may be switched into service as desired. The 



1^ This distinction is brought out by J. C. Schelleng in a "Note on the Determina- 

 tion of the Ionization of the Upper Atmosphere," Proc. L R. E., vol. 16, pp. 1471- 

 1476, November, 1928. 



A general discussion of delay distortion (phase distortion) is to be found in three 

 papers appearing in the Bell Sys. Tech. Jour., vol. 9, July, 1930. 



i"This network and the delay sections were designed by P. H. Richardson of 

 Bell Telephone Laboratories, Inc. 



