H. SANDLER, T. B. FRYER AND H. L. STONE 
MULTICHANNEL ANIMAL TELEMETRY SYSTEMS 
1199 
chanical terms can presently be obtained by te- 
lemetry. These include the electrocardiogram 
(ECG), pressure, dimensions and flow. In a 
typical system the appropriate analog signal re- 
sulting from the transducer is placed in a form 
or code for transmission, transmitted by radio 
wave and then received and decoded in its origi- 
nal form at a receiving station for subsequent 
storage and analysis. This process is schemati- 
cally illustrated in Figure 1. The choice of oper- 
ating frequency, transmitting and receiving 
antennas, modulation scheme and receiver are 
dictated by the demands of the experiment. Data 
is transmitted using a subcarrier which is an im- 
portant feature since it allows for transmission 
without significant loss of accuracy. The most 
commonly used schemes for these latter pur- 
poses are frequency modulation (FM), pulse in- 
terval modulation and pulse code modulation. 
A schematic view of a single channel ECG 
transmitter is shown in Figure 2. These units 
measure 2.5 cm x 1.2 cm x 0.5 cm, including bat- 
tery, and are constructed from discrete parts. 
They have been used in animals ranging in size 
from a mouse to a dog and have operated con- 
tinuously for over one year under these circum- 
stances, utilizing a single 1.5V mercury cell bat- 
tery. Recently a small atomic source of power 
(promethium) has been used to replace the 
mercury cell battery.'^ The anticipated life ex- 
pectany of these units is 2 to 3 years of continu- 
ous operation. 
Single channel pressure,^ dimensionaP and 
flow^ units have been developed and tested. The 
pressure transmitter is comparable in size to 
the ECG unit and uses a small strain gage-type 
cell as shown in Figure 3. The useful life of 
these units has been 6 weeks of continuous op- 
eration which has been extended to 6 months or 
more by intermittent recordings when an im- 
plantable switch has been incorporated as part 
of the battery pack. 
