832 EXPLORATION GEOPHYSICS 



track into a variable area track), and a bank of thirteen photo-electric 

 cells which convert light variations through the unmasked portion of each 

 trace into electrical signals v^hich are the counterpart of the originally 

 recorded signals. The two ends of a given record may be spliced together 

 to form a loop, and the data on this loop may be played back repetitively 

 for study and analysis. The data may be rerecorded to conventional oscillo- 

 graph records and during the rerecording operation any one of several 

 special techniques such as filtering, compositing, etc. may be employed. 

 One model of the reproducer includes provisions for introducing discreet 

 amounts of time displacement between the several traces during the play- 

 back. 



Fig. 514. — (Left) Section of Mirragraph 13-channel recording on 35 mm. film. (Right) Sketch 

 showing enlarged variable area track obtained by masking off one-half of a constant width track. 

 (Courtesy of Western Electric Company.) 



The frequency-analyzer unit shown on the right of Figure 515 consists 

 essentially of a crystal controlled band-pass filter of fixed width whose 

 position in the frequency spectrum may be set at any point between 2 cps 

 and 6,000 cps. Crystal filters of various band widths from 2 to 200 cps 

 are available. With this instrument it is possible to make a detailed analysis 

 of a complex signal and determine the frequency components and their 

 relative amplitudes. The device has been well proven in connection with 

 vibration studies, but much remains to be done in applying it to the study 

 of transient phenomena, such as seismic reflections. 



The general functions of the system as described above may also be 

 had in the form of a multi-channel magnetic recording and reproducing 

 unit. When using the magnetic methods for recording and reproducing low 

 frequency signals, it is necessary to compensate for the drop in response 

 at the low frequency end of the spectrum due to the velocity characteristics 

 of the magnetic method. This dropping off in response, which occurs at 

 the rate of 6 DB per octave at any given magnetic film or tape speed, can 

 be partially compensated by recording at a low film or tape speed and repro- 

 ducing at a higher speed. Carrier recording is an obvious, although not 

 entirely satisfactory, solution, and other methods for accomplishing flat 

 low end response are under development. 



