538 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



5ZJ -40 



O-i 



-\-45 





0-65 



40 



10,000 



60 80 100 200 400 600 1000 2000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 4 — Typical noise spectrum of i-inch iron oxide impregnated molded neo- 

 prene bands measured in 200 cycle bands after neutralization erase with 60-cycle 

 ac field. 



showing the apphcation of this erase method to a cyhnder-type machine. 

 This method of erase results in a background noise level measured un- 

 weighted over a 4000-C5^cle band which is at least 45 db below a 1000- 

 cycle signal recorded with 4 per cent total distortion. A typical back- 

 ground noise spectrum is shown in Fig. 4. 



Fig. 5 — General view of recording machines in 3A announcement system at 

 Cleveland. 



