TRANSMISSION RUNS 



75 



Figure 7. Oscillograph record of received 50-msec single-frequency signals; R is the radio signal, T is the 60-c timing 

 trace, I and III are rectified traces, and II is the heterodyned trace. The range is approximately 80 yd. 



cord the received signal on film or sensitized paper 

 moving past the oscillograph at a constant speed. A 

 timing trace, usually provided, permits accurate 

 measurements of time intervals on the film or paper 

 strip. 



The signal is transmitted not only as an under- 

 water sound signal, but also as an airborne radio 

 signal over a radio link, usually FM, between the two 

 ships. At the ranges involved, the radio signal arrives 

 practically without time delay and without distortion. 

 In addition to providing a convenient monitoring de- 

 vice, the radio signal serves as a means of accurately 

 determining the range between the two ships. Since 

 it is reproduced as a separate trace on the oscillograph 

 record, it is an easy matter, with the help of the tim- 

 ing trace, to measure the time interval between the 

 arrivals of the radio signal and the sound signal, and 

 thus determine the distance traveled by the sound for 

 each separate transmitted pulse. The resulting record 

 will be similar to the strip in Figure 7. The top trace 

 is the radio trace, the bottom trace the timing trace, 

 and the three traces labeled I, II, and III, are the 

 outputs of three different hydrophones, recorded 

 simultaneously. The outputs of I and III were recti- 

 fied before being recorded, and the output of II was 

 heterodyned down to 800 c before recording, but not 

 rectified. The range can be read off the record with 

 an error of less than 15 yd. In the example shown in 

 Figure 7 the range is approximately 80 yd. 



In the past, transmission runs have also been re- 

 corded by means of power level recorders. These re- 

 corders are electromechanical recording instrimients 

 with a logarithmic response. A stylus records on a 

 moving paper strip the received signal level in 

 decibels above the reference level. Although these 

 records are much easier to read than oscillograph rec- 

 ords, they suffer from a certain unreliability of the 

 instrument. Frequently, the stylus "sticks"; that is, 

 it follows a change in signal level only when this 



change exceeds an appreciable threshold value. Fur- 

 thermore, the stylus travels only a certain number of 

 decibels per sec (50 to 500 db per sec, depending on 

 the model) ; the instrument, therefore, cannot record 

 correctly the level of very short signals. For this 

 reason, power level recorders have not been used in 

 recent transmission work. 



WHOI has developed an electronic device designed 

 to combine the advantages of both oscillograph and 

 power level recorder. It consists essentially of a 

 rectifier, an amplifier with logarithmic response over 

 a range of approximately 80 db, and a galvanometer 

 oscillograph. This device has a time constant of 

 roughlyO.5 msec.^ Up to the present, it has been used 

 only for reverberation studies; whether it will prove 

 useful in transmission work remains to be seen. The 

 amplifier used in this device has been improved since 

 reference 2 was published. 



The output of the hydrophone is passed through 

 filters at some stage before it reaches the recording 

 instrument. The purpose of the filter is to improve 

 the signal-to-background ratio. All the unwanted 

 background (see Divasion 6, Volume 7) contains 

 energy in a very broad frequency band. A band-pass 

 filter centered at the signal frequency will discrimi- 

 nate against the broad-band background in favor of 

 sound at the signal frequency. Most of the filters 

 used are approximately }/2 kc wide. Such a width 

 leaves an adequate margin for possible drift of the 

 driving oscillator in the sending stack and for dop- 

 pler. 



Both the amplifiers and the recording instruments 

 will be linear and otherwise satisfactory only in a 

 limited range of signal amplitude. On the other hand, 

 actual signal levels are likely to change by as much 

 as 80 db between short and long ranges of transmis- 

 sion. For this reason, step attenuators are provided. 

 These attenuators are usually operated by hand; 

 however, in the most recent installation at UCDWR 



