176 BELL SYSTEM TECHNICAL JOURNAL 



Figure 10 shows several frequency response curves. Curve 1 

 shows a response without the use of equaUzation for a tape speed of 

 8 inches per second. Curve 2 shows the response under the same condi- 

 tions for a tape speed of 16 inches per second. Curve 3 shows the 

 same signals of curve 2 reproduced through a suitable equalizer. 



The ratio of the maximum reasonably undistorted 1000-cycle signal 

 to the noise with typical good tape is about 38 db. The transfer loss 

 is approximately 60 to 70 db. The maximum power required in 

 recording is about 0.3 milliwatt. 



Characteristics of Magnetic Recording 

 Magnetic recording differs from other methods in several respects. 

 Since no processing is required, the record may be reproduced without 

 a long delay. The recording medium may be used over and over again 

 for new records. It is only necessary to subject the tape to a strong 

 magnetic field in order to obliterate a record. The obliteration is 

 conveniently done at the same time that the new record is being made. 

 Where temporary records are desired, magnetic recording therefore 

 has some advantages over other methods. On the other hand it should 

 be fully appreciated that the records may be kept, filed away, or 

 reproduced thousands of times with no appreciable deterioration in the 

 quality. 



The magnetic system is very convenient for use where short delays 

 are desired. A short loop of tape in conjunction with recording, 

 reproducing, and obliterating pole-pieces is all that is required. In- 

 stead of a loop of tape, a disc or cylinder rotating at high speed may be 

 used to carry the recording medium. The latter method makes it 

 possible to obtain very short delays. Where perpendicular magnetiza- 

 tion is used, very long records may be obtained from a medium which 

 occupies a relatively small amount of space. For example, a thin 

 coil of 2 mil tape 9 inches in diameter will give a playing time of 1/2 

 hour with a tape speed of 16" per second. 



There are no moving parts in the modulating unit. The difficulties 

 of obtaining high frequencies due to the inertia of the cutting stylus in 

 mechanical recording are therefore not present. The system is 

 subject to the same difficulties of eliminating flutter that we find in 

 other methods of recording; however, mechanical vibrations due to 

 the motor and other moving parts of the recording system do not have 

 to be filtered out as is the case with mechanical recording. There are 

 of course no shavings. The recording medium cannot be easily 

 scratched and may be handled in any kind of light and subjected to 

 large variations in temperature. When properly wound on reels it is 

 not liable to breakage or damage during transportation. 



