326 BELL SYSTEM TECHNICAL JOURNAL 



encountered and there is only one adjustment of the delay which 

 gives a straight line on the oscilloscope. 



With the cathode ray oscilloscope there is no indication as to whether 

 the delay in the circuit is too small or too great, but only that it is 

 either correct or incorrect. A direct indication of whether the delay 

 should be increased or decreased can be obtained by connecting one 

 diversity branch to the push-pull input of a balanced modulator and 

 another diversity branch to the parallel input of the same modulator, 

 after having shifted the phase between the two branches by 90°. 

 When the two receiver branch outputs are in the same phase the two 

 modulator input voltages will add in quadrature and the currents in 

 the plate circuits of the two modulators will be the same, but when 

 the phases are not the same the current in one plate circuit will be 

 greater and the other less than in the in-phase case, the sense of the 

 unbalance depending upon whether too little or too much delay is in 

 the circuit. A center-zero meter in a bridge connection in the plate 

 circuits therefore can be made to indicate the sense of the necessary 

 correction. By substituting a voltmeter relay for the indicating meter 

 a motor drive of the variable delay circuit can be operated in such a 

 manner as to adjust the delay to the correct value. 



This automatic equipment must operate satisfactorily with circuits 

 having types of privacy in which the energy bearing components of 

 speech are shifted from their normal position in the frequency 

 spectrum. The equipment is made, therefore, to operate on a band 

 of frequencies from 250 to 750 cycles. Volume limiters are provided 

 which keep the input to the automatic equipment from this band sub- 

 stantially constant. If the delay is never incorrect by more than 

 1,000 microseconds, which has been found to be true under all con- 

 ditions of normal operation, the automatic equipment will bring it 

 to the correct value. For greater errors the equipment tries to set 

 the delay at values about 2,000 microseconds higher or lower than the 

 correct value. 



Since the delay adjustment operates on speech, the relay operation 

 will be intermittent at a syllabic rate and the motor drive relay system 

 must incorporate a suitable hangover circuit to keep the motor opera- 

 tion as constant as possible when the delay is far from the proper 

 value, and still not cause an over-run when the proper adjustment is 

 reached. Freedom from over-run caused by motor inertia is obtained 

 by using a motor having a multiple-pole permanent magnet armature 

 with a speed of 75 r.p.m., which under no load will brake itself in 

 about twenty degrees of armature travel. 



