SOUND RECORDING ON MAGNETIC TAPE 171 



ing of the recording flux within the tape. The recording flux is always 

 at substantially right angles to the axis of the tape and parallel to its 

 surface and is in the opposite direction to the residual magnetization. 

 If W is made quite small or in other words if the pole-pieces Pi and 

 P2 are directly opposite each other with the thin dimension of the 

 tape between them, we have the conditions shown in Fig. 4. The 

 tape is so thin that there is very little spreading of the flux so that the 

 width of the flux path is not appreciably dependent on the strength 

 of the signal. This type of recording is called perpendicular magnetiza- 

 tion in order to distinguish it from cross-magnetization, where the 

 width of the tape instead of the thickness determines the pole-piece 

 separation. The perpendicular method of magnetization permits a 

 relatively low tape speed. The thickness of the pole-piece tips 

 determines the frequency response for a given tape speed. 



Method of Recording with Perpendicular Magnetization 



If the tape is first subjected to a saturation flux which is at right 

 angles to the surface of the tape, it will be left with one side of north 

 and the other of south polarity. If the tape in this condition is 

 passed between recording pole-pieces carrying only AC flux, it is 

 obvious that only half cycles will be recorded. The record is therefore 

 much distorted. The current reproduced from such a record is 

 similar to the alternating current which may be obtained from a 

 single wave rectifier. 



If on the other hand the tape is passed through an alternating high- 

 frequency field which is strong enough to erase the record, it is left 

 in a substantially neutral condition. If it is then passed between the 

 recording pole-pieces, both half cycles will be recorded but there will be 

 amplitude distortion. Figure 5 shows a magnetization curve for iron 

 which has previously been demagnetized with alternating current. 

 The slope of the first part of the curve is small in either direction of 

 magnetization and then increases with increase in the flux and finally 

 becomes smaller again. Small signals will therefore be recorded 

 weakly and strong signals will be recorded relatively higher. Both 

 will have wave form distortion. The same effects would be obtained 

 with longitudinal or cross magnetization. In the past, investigators 

 have often utilized only one side of the magnetization curve, A direct 

 current was used as a bias to bring the recording flux to the most 

 suitable part of the curve such as at n, Fig. 5. 



The method employed here will be made clear from Figs. 6 and 7. 

 As the tape elements enter the field of the polarizing pole-pieces Pi, P2 

 (Fig. 6) they are subjected to an increasing magnetizing force. The 



