434 MECHANICAL AND ACOUSTICAL SENSES 



accompanied each hourly mean. Circuitry capable of processing a sensor's 

 output and accumulating this much data (approximately 1000 binary 

 bits) is presently planned for the shark transmitters. Upon receipt of the 

 proper interrogate code from the boat, the shark unit would send back, in 

 binary code, its previous 24 h of stored data via a nondestructive readout. 

 The unit would then begin accumulating data as it arrives, discarding only 

 that data older than 24 h. Thus, the shark unit need be interrogated only 

 once each day, alleviating tracking fatigue and freeing the researchers for 

 other work. 



The above telemetry system is for remote, periodic interrogation of the 

 data-storage module while the unit is still on the shark. An alternative 

 (nontelemetric) method would be direct readout of the stored data after 

 recovering the unit at the end of a tracking. For very high capacity storage, a 

 small magnetic tape recorder may be preferable if space is available. The 

 subminiature, micropower, digital-incremental tape recorder developed by 

 Goodman et al. (1973) for animal-borne instrument packages measures 

 about 100 cm 3 , weighs about 100 g, and stores 20 million data bits. Many 

 other data-recording methods exist (mechanical, electronic, photographic, 

 and so forth), some of which are discussed by Mackay (1970). 



Transmitter Construction 



One problem facing the prospective user of telemetering techniques is a 

 source of suitable transmitters. In the past, most research teams built their 

 own units to meet their particular requirements. Nowadays, several com- 

 panies offer basic USTs for sale, some with the capability of sensing tempera- 

 ture or pressure (not both in one unit). Information on most commercially 

 available USTs has appeared at one time or another in the "Underwater 

 Telemetry Newsletter." For many studies, such off-the-shelf units are per- 

 fectly adequate and probably most economical. Other research purposes, 

 however, require specialized transmitters that are not available commer- 

 cially and are therefore usually designed and built by the researchers them- 

 selves or on contract. 



Methods of Circuit Construction— During the last few years, the elec- 

 tronics industry has undergone a "quiet revolution" in the development and 

 proliferation of integrated circuits (ICs), a large variety of which are now 

 available at low cost in various small multipin packages or as tiny chips. Of 

 particular usefulness in telemetry are the CMOS (complementary metal- 

 oxide semiconductor) digital types which have low-microwatt quiescent 

 power requirements and operate over a wide range of unregulated voltages 

 (< 3-1 5 V). Using ICs, the designer can now incorporate a wider variety of 

 functions (e.g., gating, counting, crystal-controlled timekeeping, multi- 

 plexing, data processing, and storage) into small, low-power circuit packages 

 than would have been practical using earlier technologies. However, the 

 primary benefit to biotelemetry of the new IC technology is to permit the 

 addition of new, more complex functions to transmitters, not to achieve 

 power savings in the transmitter's power-output stage. 



