SOUND RECORDING ON MAGNETIC TAPE 



173 



elements are magnetized to the saturation point P as shown by curve a, 

 Fig. 7. As the elements leave the polarizing field they are subjected to 

 a field of decreasing strength so that the magnetic induction drops 

 along the curve b to R, this point being reached when the applied 

 field is zero. In Fig. 7, the magnetizing force // refers to the externally 

 applied field. The tape elements then pass between the recording 

 pole-pieces which carry a flux in opposite direction to that of the 

 polarizing pole-pieces. If there is no signal current present, the 

 magnetic induction will be brought down to the point N by the biasing 

 field. As the elements pass out from between the pole-pieces, the 

 field will decrease to zero and the magnetic induction will change from 



Fig. 7 — Diagram showing the cycles of magnetization through which the elements of 

 a steel tape may pass during the process of recording. 



N to 0, which is a substantially neutral condition. However, if there 

 is a signal current present at the time the tape elements are passing 

 between the recording pole-pieces, the magnetization will be reduced 

 to a point A higher than N if the cycle is in opposition to the bias flux 

 or to the point B lower than N if the signal flux is in the same direction 

 as the bias flux. In either case the elements will retain a magnetiza- 

 tion value corresponding to ^' or B' respectively. This system makes 

 it possible to record over a longer portion of the magnetization curve 

 without appreciable distortion. 



Unless the proper value of biasing field is used to bring the magneti- 

 zation approximately to the point N when no voice current is present, 

 the maximum recording range cannot be obtained without excessive 

 amplitude distortion. For example if no bias is used, it has been found 



