Chapter 6 — PRINCIPLES OF SONAR 



appearance, but their operating characteristics 

 are the same as the basic model. 



Originally the AN/UNQ-7 had recording speeds 

 of 3-3/4 and 7-1/2 inches per second (ips). Field 

 changes permitted an additional speed of 15 ips. 

 All subsequent models, beginning with the AN/ 

 UNQ-7A, have the 15 ips feature built in. In 

 general, the faster the recording speed, the more 

 faithful is the sound reproduction. 



The standard reel is 7 inches in diameter, 

 holding 1200 feet of l/4-inch tape. At a re- 

 cording speed of 15 ips, 1200 feet of tape 

 will last about 15 minutes. 



OPERATING PRINCIPLES 



Magnetic tape consists of finely ground iron- 

 oxide particles deposited upon a plastic backing 

 (tape). The particles, too small to be seen 

 with the naked eye, are deposited in such a 

 way that they are allowed to move or line up 

 in an orderly fashion as a result of some 

 force applied to them. The force having such 

 an effect on the iron-oxide particles is a mag- 

 netic field. This magnetic field can be caused 

 by a common magnet or by a temporary electro- 

 magnetic field. It results from passing an elec- 

 trical current through a coil of wire. 



MAGNETIC HEADS 



A microphone contains a magnetic device 

 that is capable of producing electrical energy 

 representative of the sounds spoken into it. 

 In a tape recorder, the microphone-produced 

 electrical pattern causes a magnetic field to 

 represent the pattern about a wire coil to which 

 it is fed. The coil is wound on a core that has 

 a gap in one side. Because the magnetic field 

 is most intense at the gap, that is where the 

 signal is placed on the tape. The narrower the 

 gap, and the sharper the gap edges, the better 

 is the high-frequency response. Figure 6-17 

 shows a typical magnetic head. The magnetic 

 tape passes over the head, and the electrical 

 pattern is reproduced upon the tape in the 

 form of regular patterns of iron-oxide particles 

 that line up in accordance with the signal passed 

 through the coil. 



The recorder has three heads. The device 

 in the recorder about which the magnetic field 

 is produced by the microphone is called the 

 recording head. Another head, called the erase 

 head, eliminates the magnetic pattern from the 

 particles on the tape. A third head is for play- 

 back of the recorded signal and is called the 



GAP 



CORE- 



Figure 6-17.- 



COIL 



71.122 

 ■Typical magnetic heaid. 



reproduction head. The three heads are con- 

 tained in a single housing. 



Recording Head 



The recording head is composed of a highly 

 permeable material, which means that the core 

 is easily magnetized, and just as easily demag- 

 netized, by a current passing through the coil. 

 High permeability is a necessity in order for 

 the magnetic field to follow the fluctuations of 

 the signal to be impressed on the tape. 



In reality, the signal cannot be put "as is" 

 directly onto the tape. It must be mixed with 

 another signal for strength and linearity, that 

 is, without distortion throughout the signal band. 

 The bias signal with which the original signal 

 is mixed is of very high frequency — too high to 

 be heard. Electrically, it has unchanging, steady- 

 strength characteristics. The a-c bias assures 

 better acceptance by the tape of the signal to 

 be recorded, and helps reduce distortion of 

 strong signals. 



Erase Head 



Tape passes from the erase head to the 

 recording head, and then to the reproduction 



105 



