MAGNETIC DRUM TRANSLATOR FOR TOLL SWITCHING OFFICES 715 



may also be represented as a function of time. This time-rate-of-change 

 of flux within the coils of the head generates a voltage which is of the 

 order of 50 millivolts peak-to-peak in the case of the translator. This volt- 

 age, after amplification, appears as shown in line C of Fig. 3. The trace 

 shown is that which appears at the "linear output" monitor jack of a 

 translator reading amplifier, and includes a phase inversion, character- 

 istic of a three stage amplifier. Such a curve is readily recognized as being 

 quite similar in shape to the first derivative of the normal error-function 

 and hence we may infer that the magnetic condition of the drum surface, 

 at least as interpreted by the head, may be portrayed by a bell-shaped 

 curve, previously mentioned, similar to the error-function itself. 



The residual magnetic irregularity pictured in cell 4 resulting from 

 writing a "0" over a "i" will induce a voltage in the winding of the head 

 having a different amplitude and wave shape from that occasioned by 

 reading a "i." It is sketched out approximately to scale in Fig. 3 and is 

 seen to be a smaller twinned- version of the "i" signal. Its amplitude or- 

 dinarily lies in the range of }4o to 3^ of that of the " T' signal, and for 

 about the middle third of the cell its instantaneous polarity is opposite 

 to that which a "i" signal would have. These facts suggest at least two 

 means of discriminating between the voltage signals obtained for the 

 two code values: (a) on the basis of amplitude difference, and (b) on the 

 basis of instantaneous polarity difference determined or sampled within 

 a particular epoch in each cell. 



The method adopted for the translator is that of simple amplitude 

 threshold. The threshold value indicated by the dotted line in Fig. 3, is 

 set so that the strongest of the residual signal outputs never exceeds it 

 \\ hile, at the same time, the greatest possible proportion of the positive- 

 going lobe of a " 1" signal is allowed tp produce an output. The threshold 

 output stage of the amplifier is also arranged for limiting and this has 

 the effect of blunting the peaks of the applied signals. The over-all result 

 of these actions is shown by the shape of the signals in line D of Fig. 3. 



Cell packing may be of major economic importance in a large installa- 

 tion. The general effect of making recordhigs closer and closer together 

 is that the presence or absence of one of the recordings in a series has an 

 increasing influence on the size and shape of the signals reproduced from 

 its neighbors on either side. In the translator, the cells are spaced "JO 

 niilli-inches center-to-center along the track and the influence of action 

 in one cell on the amplitude of reproduction from neighboring cells is 

 never more than about 10 per cent. The trace of line C, Fig. 3, is drawn 

 for this cell spacing and shows a slight inflection at the transition between 

 the output voltage occasioned by reading cell 3, and the voltage obtained 



