NEW RECORDING MEDIUM 535 



When ring tj^pe magnetic heads are used for recording, these bands 

 exhibit frequency response characteristics quite similar to coated tapes 

 using the same magnetic oxides, although the bands are of homogeneous 

 magnetic material up to | inch thick and the tapes have magnetic coat- 

 ings less than O.OOl inches thick. This is because the field from the record- 

 ing gap becomes ineffective at a distance of about 0.001 inches, and the 

 signal is recorded only on a thin siu'face layer of the medium, regardless 

 of its total thickness. 



The noise characteristic of this medium is somewhat unusual. It has 

 been shown* that the reproducing process is not restricted to the surface 

 layer of the medium, but that to a first approximation, when the medium 

 has low permeability, the signal from a magnetized element at any depth 

 ill the recording medium will be attenuated with respect to the signal 

 produced by the same element in intimate contact with the reproducing 

 head by the factor: 



55 decibels X S 



where S = distance between magnet and head 

 X = "wavelength" of magnet 



This indicates, for example, that the signal from a magnetized element 

 at a depth of X/2.75 will be attenuated by only about 20 db and may 

 therefore make significant contribution to the total output. 



In the Bell System telephone applications, where a transmission band- 

 width of 100 to 4000 cycles per second is required, the belts are run at a 

 speed of about 6 inches per second. The wavelength at 100 cycles per 

 second is then 0.060 inches, and at this frequency significant output can 

 be obtained from a layer about 0.02 inches thick. The desired recording 

 is limited to a layer about 0.001 inches thick, but a layer of about twenty 

 times this thickness may contribute to noise. As a consequence, at low 

 frequencies this medium tends to exhibit higher background noise than 

 do the co.ited tapes. The magnitude of the noise is appreciably affected 

 by the method of erasure. 



Two methods of erasing a magnetic record are known to the art. 

 These are the saturation erase, in which the magnetic record is exposed 

 to a unidirectional magnetic field of saturation intensity, and the neutral- 

 ization erase in which the magnetic record is exposed to an alternating 

 field which reaches saturation intensity and decreases cyclically to zero 



* R. L. Wallace, Jr., "Reproduction of Magnetically Recorded Signals," Bell 

 System Tech. J., Oct., 1951. 



