Section VII. MAGNETIC TAPE RECORDER FOR OCEAN-WAVE GAGES 



1 . Theory of Operation of Magnetic Tape Recorder 



No commercial tape recorders are available that could be modified 

 to serve as an analog ocean-wave recorder for long periods of time. CERC 

 found it necessary to design and build its own tape recorders. The mag- 

 netic tape recorder Model LW-1 is designed to record ocean waves with 

 periods of about 2 seconds through 100 seconds. Wave heights recorded 

 full-signal on the tape recorder will be those from the wave gage that 

 provide full-scale indication on the wave gage strip-chart recorder. 



The recording signal is a d.c. analog to the recording head. Line 

 frequency is used as the recording bias to the tape head. Thus, the re- 

 cording is similar to that used on a standard tape recorder for voice or 

 music, although a higher frequency bias is used in a standard tape re- 

 corder. However, at the wave period (frequency) at which the wave 

 recorder operates, the line frequency (60 cycles) is more than adequate. 



The recording signal (wave-gage signal) from the three types of 

 step-resistance wave gages is a to 30-volt d.c. analog. This signal is 

 equal to zero submergence and full submergence, respectively, of the wave 

 staff, thus the signal is proportional to the water level on the wave 

 staff. The tape recorder has a 10,000 microfarad capacitor to remove the 

 average d.c. signal from the wave-staff signal to prevent this average 

 signal from reaching the recording tape head. This, in effect, removes 

 the change in gage signal caused by tidal changes. Removal of this tide 

 signal from the gage signal allows a wider dynamic range of the wave signal 

 to be applied to the tape head, resulting in a better wave recording on 

 the magnetic tape. 



The wave-gage signal from the pressure-sensitive wave gage has the 

 tide component removed from the gage signal in the amplifier-power supply 

 unit by a high-value capacitor in much the same manner as is done in the 

 tape recorder. The output (wave-gage signal) from the amplifier-power 

 unit is a d.c. analog of 15 +15 volts. This signal is proportional to 

 the trough-to-crest wave height for the respective maximum wave height for 

 which the pressure-sensitive unit is calibrated. This signal will produce 

 full-scale movement of the recorder pen on the strip-chart recorder, and 

 is used as the wave signal to the tape recorder. Therefore, the tide 

 removal capacitor in the tape recorder is not required when the recorder 

 is used with the pressure-sensitive gage. 



To operate the tape recorder with both types of wave gages (staff and 

 pressure), one requiring tide removal, the other not, there is a switch in 

 the tape recorder for bypassing the tide capacitor. It is labeled tide 

 capacitor "in-out". 



The magnetic tape recorder may be operated with other wave-gaging 

 systems provided those gages produce a d.c. analog signal proportional 



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