TELEMETERING UNDERWATER 



207 



ANALOG TO 

 FM CONVERTER 



POWER 

 AMPLIFIER 



TRANSDUCER 



Fig. 82. Block diagram, basic transmitter 



Transmitter Details (Mechanical) 



The transmitter is housed in a 2 -in. -diameter pipe with an overall length of 14 in. A sin- 

 gle five-pin connector serves as the input for the three electrocardiographic leads and as the 

 power switch. The transmitter turns on automatically whenever the electrocardiographic leads 

 are plugged into the connector. About four hours of operation can be obtained from each bat- 

 tery set. Batteries can be reached by removing the instrument casing. 



Receiver 



A block diagram of the telemetry receiver is shown in Fig. 83. Acoustical energy from 

 the water is converted into electrical energy by the receiving transducer. Signals from the 

 transducer are amplified by a three-stage tuned amplifier with a center frequency of 25 kc/sec. 

 Output from the tuned amplifier drives a variable-gain stage. Normally, the gain is set to 

 maximum. However, when the diver is very close to the receiver, clearer signals can be ob- 

 tained by reducing the gain setting. The sinusoidal signal from the variable-gain amplifier is 

 converted to square-wave form by a Schmitt trigger, whose triggering level is set to zero volts. 

 Output from the Schmitt trigger synchronizes a multivibrator to exactly half the frequency of 

 the received signal. In the absence of a signal, the multivibrator oscillates at a natural fre- 

 quency of 12.5 kc/sec. Therefore, there is always a signal coming from the multivibrator, 

 which prevents excessive excursions of the output of the FM detector in the absence of received 

 signal. FM detection is achieved with a monostable multivibrator followed by a low-pass filter. 

 The monostable multivibrator generates a 40-microsecond pulse each time the multivibrator 

 output swings positive. The average output level from the monostable multivibrator increases 

 with received signal frequency. This average value is extracted by the low-pass filter (80 cps 

 cutoff frequency) and is taken as the electrocardiographic output after passing through an emit- 

 ter follower and gain control. 



SEALAB INTERFACE 



The system layout for the equipment as installed at Sealab II is shown in Fig. 84. 



The hydrophone and receiver were placed within the Sealab habitat. As the diver swims 

 in the water, the received electrocardiographic signals are relayed topside for immediate ob- 

 servation. Cables between Sealab and the Berkone were shared with a wedge spirometer. The 

 signals were recorded in the medical van on a Sanborn recorder. Preparations for relay to 

 Philadelphia via Bell Dataphone were also made. 



