712 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 



111 order to facilitate an understanding of the action of the translator 

 as a whole, a simplified account of the magnetic recording and repro- 

 ducing process will now be given. 



Magnetic Drum Geography 



The circumferential strip of the drum surface which moves under the 

 pole-tips of any magnetic head is commonly known as a track. On each 

 track will be written magnetic perturbations or spots symbolizing " I's." 

 It is essential that these spots be precisely located so that they may be 

 readily removed or "altered." For this purpose a synchronizing track or 

 some equivalent distribution of equally spaced identifying marks asso- 

 ciated w4th the drum is provided. With the aid of the electronic circuits, 

 the magnetic spots are restricted to a modular spacing defined by the 

 synchronizing marks, and this module is spoken of as a "slot." On the 

 drum surface, each intersection of track and slot is known as a "cell" and 

 a cell may contain only one magnetic mark and therefore only one bit of 

 information. As a matter of economics, the cell density should be as great 

 as possible. The density which may be attained is determined by the de- 

 gree of interference which can be tolerated among neighboring cells. 



Writing Operations 



The first step in preparing the drum to receive a recording is to uni- 

 formly magnetize the tracks to saturation in the polarity arbitrarily 

 chosen to represent the code-value "O." This is a preconditioning opera- 

 tion required only when a drum is newly placed in service. Referring to 

 Fig. 2, this may be done, for the typical head and track shown, by closing 

 the switch marked "0" for the duration of at least one complete revolu- 

 tion of the drum. Enough current must flow through the windings of the 

 head to establish the magnitude of fringing flux, from the pole-tips, re- 

 rjuired to saturate the thin magnetic coating. In the case of the trans- 

 lator drum, the coating is about ^-i milli-inch thick; the clearance 

 between pole-tips and recording surface is about 2 milli-inches; the inter- 

 pole gap is also about 2 milli-inches at the tips, and about 20 ampere- 

 turns of energization are recjuired. 



With the track thus preconditioned, there is virtually no output \ olt- 

 age from the head since the magnetization is essentially uniform and 

 there is no changing flux threading the head to induce a Aoltage in the 

 windings. 



Whenever a " /" is to be written, a pulse of cuireiit from an oIcM'trouic 

 writing amplifier (indicated, for convenience, on Fig. 2 as a switch) is 





