INTRODUCTION TO SONAR 



head (in that order). The erase head demag- 

 netizes both channels simultaneously. It is un- 

 necessary to erase a tape individually before 

 it is re-recorded, because when a tape is being 

 used for recording, the erase head is energized 

 so that a new recording cannot be ruined by 

 superimposing it over an older one. Thus, tapes 

 having recordings that no longer are needed for 

 retention can be used directly. The machine 

 simply is set to record, the old tape Is put on, 

 and, aiter it passes the erase head, it is "cleaned" 

 of the earlier recording. It next passes the 

 recording head, which puts a new magnetic 

 pattern on it. 



Reproduction Head 



The reproduction head, which is next in line, 

 is similar to the recording head in construction. 

 That is, it has a broken ring of permeable 

 material wound with wire coils. When the re- 

 corded tape passes over the gap, the permeable 

 material is affected by changes in the tape's 

 magnetic field, and an electric current is induced 

 in the coils surrounding the ring. The induced 

 signal is amplified and fed to phone jacks for 

 audible presentation. 



OPERATING THE EQUIPMENT 



The top half of the tape recorder, as seen in 

 figure 6-16, is the actual recorder and repro- 

 ducer. The lower portion is the amplifier section. 

 It includes controls and indicators that directly 

 affect the recording and playback of the tapes. 



Each recording track has a separate channel. 

 On the equipment they are labeled channels A 

 and B. Channel A is used for voice recording 

 and reproduction; channel B, for sonar informa- 

 tion. Both channels have separate controls for 

 recording and reproduction. The recording con- 

 trols are to the left of the amplifier section. 

 Playback controls are at the right of the amplifier 

 section. 



Threading Tape 



The roll of magnetic tape to be used for re- 

 cording purposes is placed on the left reel-hold 

 assembly on the upper section of the recorder. 

 It is placed in such a way that, when the equip- 

 ment is in operation (recording or reproducing), 

 the reel rotates in a counterclockwise direction, 

 and the magnetic tape leaves the reel from the 

 bottom. 



Other parts are on the upper section to guide 

 the tape and to pull it through the head device. 

 When the tape unwinds from the left reel, it is 

 threaded around the guiding assemblies, fed 

 through the head assembly, around the capstan 

 (which actually pulls the tape through), and then 

 it is taken up by the right reel. A pictorial diagram 

 of the path followed by the tape from reel to reel 

 is printed on the face of the head assembly. By 

 adhering to the diagram, you will have no diffi- 

 culty threading the equipment properly. 



Recording 



After the tape is threaded, the tape recorder 

 is energized by turning the power switch to the 

 ON position. You must wait for a short warmup 

 period. 



Next, select the speed at which recording is 

 desired. As mentioned earlier, there is a choice 

 of 3-3/4, 7-1/2, or 15 inches per second. Select 

 the faster speed for critical recordings, during 

 which the pitch of the echo may have an important 

 part in later evaluation, A recording made at 

 a higher speed can be studied more completely 

 by reproducing it at the lower speed. (An800-cps 

 tone, recorded at 7-1/2 inches per second, is 

 heard at 400-cps when reproduced at 3-3/4 inches 

 per second.) Hence, recordings that may require 

 later analysis should be made at the higher 

 speed. The disadvantage of the higher recording 

 speed is that the tape is used twice as fast as 

 when operating at the next slower speed. Informa- 

 tion on speeds to use for recording and playback 

 are contained in the technical manual supplied 

 with each recorder. 



Although the higher speed provides the highest 

 quality reproduction, differences in recordings 

 made at each speed are not detected easily. 

 Only when a critical analysis of the recording 

 is made does the faster speed prove its worth 

 over the slower one. 



The recording level is selected next, and 

 each channel's level must be adjusted individually. 

 An indicator on the amplifier assembly is labeled 

 record level indicator. Directly below it is a 

 two-position toggle switch labeled channel selec- 

 tor. The positions of the switch are marked A 

 and B, each representing a channel. Set the 

 toggle switch to either channel, and observe the 

 vertical amplitude on the face of the indicator. 

 The record level controls, below the channel 

 selector switch, are marked for channels A and 

 B. Adjust the level control for the channel 

 chosen until the signal peaks (shown in the level 

 indicator) occupy the space between the upper 



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