6432 HYDROGRAPHIC MANUAL PaGE 598 



6432. Keying Circuit 



The purpose of the keying circuit is to cause the radio transmitter to operate when 

 the bomb signal arrives at the sono-radio buoy. The keying circuit is actuated by the 

 output energy of the audio ampHfier. It shoidd be designed to have a threshold of 

 operation, so that spurious sounds of low amplitude will not operate the radio trans- 

 mitter, but so that, when a bomb signal of sufficient strength is received, the transmitter 

 is operated at nearly full-power output. But the operating threshold must not be so 

 high as to preclude operation by weak bomb signals. Normally the wave front of the 

 bomb signal is sufficiently steep to make the lag, due to the threshold, negligibly small. 

 The keying circuit must be designed with a view to keeping the time lag at a low and 

 constant value. For use in R.A.R., it has generally been found desirable to have the 

 keying circuit actuate the transmitter so that the signal will be an unmodulated pulse 

 of nearly constant amplitude. However, some sono-radio buoys have been used by 

 the Coast and Geodetic Survey in which the bomb signal modulates the radio trans- 

 mitter. Undistorted modulation is not attempted. 



Under certain conditions the bomb signal received at the hj^drophone may be 

 prolonged as much as 7 seconds or more, and in such cases it is advantageous to limit 

 the length of the radio signal to a shorter time interval. Such prolonged bomb signals 

 are due to the numerous reflection paths of the sound between the source of the bomb 

 explosion and the hydrophone (see fig. 123). Also, when the bomb explodes close to the 

 hydrophone, reverberation lasting several seconds may occur. The disadvantage of 

 long radio signals is the interference that occurs between two or more signals from two or 

 more sono-radio buoys at nearly equal distances from the survey ship, the signal from 

 the nearest blanketing those transmitted immediately afterward. In practice, the radio 

 signals have been made as short as 0.12 second, for use where such interference occurred 

 frequently; as, for example, where the sounding lines were near bisectrices (see 6814). 

 For ordinary use, a signal from 0.25 to 0.5 second in length has been found desirable. 

 The method of signal shortening usually makes the sono-radio buoy inactive for 3 

 to 5 seconds after the radio signal has been cut off. The advantage of keeping a sono- 

 radio buoy inactive for a short time after transmission is that interference with survey 

 operations from other buoys is in this way prevented. Otherwise, faulty operation 

 of the circuits, spurious noises caused by passing ships, or by other causes, could keep 

 the transmitter circuit in continuous operation for long periods, or until serviced. 

 Sono-radio buoys equipped with silencing circuits when subjected to undesired noise 

 will transmit interrupted signals of short and evenly spaced duration, which, though 

 annoying, does not entirely prevent the use of the other sono-radio buoys in the same 

 vicinity. 



There are certain disadvantages in the use of signal-shortening-and-silencing cir- 

 cuits. When all the radio signals transmitted are of uniform length, signals caused by 

 bombs cannot be distinguished from those caused by water noises. Furthermore, if 

 the sono-radio buoy is actuated just before the bomb signal arrives, the silencing circuit 

 prevents the bomb signal from operating it. Therefore, wdiere prolonged signals are 

 not particularly bothersome shortening-and-silencing circuits are not incorporated in 

 the sono-radio buoy. The operation of these circuits is considered in 6522. 



6433. Radio Transmitter 



The signal of the radio-frequency transmitter used in sono-radio buoys must be 

 relatively constant in frequency. The frequency should be as constant as can be 



