recorder amplifier,. The 3~ke tone pulses are applied to the final stage 

 of the amplifier, the 3-kc input leads being terminated on the two left- 

 hand binding posts of the terminal strip on the rear of the tape magazine 

 The 3-kc signal lead comes from the receptacle on the rear of the time tone 

 amplifier chassis marked The two right-hand ter- 



minals on the strip mounted on the rear of the tape unit are the magnetic 

 tape recorder output leads-, These terminals are connected by cable to the 

 receptacle marked on the rear of the time tone amplifier,, 



80 „ During playback, both the signal and the 3-kc indexing as recorded on 

 the tape recorder are fed to the time. tone amplifier. Here the shot-arrival 

 signal is separated from the 3-kc signal by a low-pass filter and applied to 

 the input of the high-speed sound level recorder when the switch on the front 

 of the time tone amplifier is switched to the position 



The 3-kc tone is filtered out, amplified, then rectified, and finally applied 

 to the time-indexing relay on the high-speed sound level recorder, causing 

 its pulses to reproduce on the paper tape, in its identical relation to the 

 shot signal as recorded originally on the magnetic tape recorder „ 



Dis tinction JBe tween Mirro phone and Caltron 



81, It is desirable at this point to explain clearly the term "Mirrophone",, 

 Actually, "Mirrophone" (Fig, 48) is the Western Electric Company 's trade name 

 for their model of the magnetic tape recorder, When construction of the SOFAR 

 station was begun, these were the only types of magnetic tape recorders known 

 to be available; hence, all name plates and references concerning the tape 

 recorders were marked "Mirrophone" , However, it was discovered that not 

 enough Mirrophone3 were available; consequently another type of magnetic tape 

 recorder was obtained bearing the trade name of "Caltron" (Fig., 48) „ 



82 Basically, the "Mirrophone" and the "Caltron" are identical,, The tape 

 magasines are alike with these two exceptions; 



a., The tape on the "Caltron" is a little longer than that on the 

 "Mirrophone" , 



bo The recording head on the "Caltron" contains a hum-bucking coil 

 in series with the record-playback coil„ 



83 a The major difference between the "Mirrophone" and the "Caltron" is in 

 the amplifier,, The 'Mirrophone" has a twc—stage audio amplifier, and 

 utilizes DC for erasing purposes „ The "Caltron" has a three-stage amplifier, 

 and employs a 25-kc oscillator to furnish AC erase The added stage in the 

 "Caltron" necessitates the insertion of a 40-db fixed attenuator at the input 

 to the amplifier to prevent overloading and distorting of the signal, 



84° In both ''Mirrophone" and -"Caltron" units, erasing is accomplished only 

 during the recording process The tape passes through the erase head just 

 before it reaches the recording head (facing the tape unit from the rear, 



20 



