up to the order of 10 cps, a system has slowly 
been developed at Woods Hole. An early effort 
by Farmer~ on this line was used for the 
measurement of ocean waves, where a record of 
heights and instantaneous slopes at fixed 
points on the sea surface was desired. Ini- 
tial experiments where these variables were 
measured and directly recorded on a multi- 
channel galvanometer recorder soon revealed 
the necessity for a more sophisticated 
approach, 
First, the need for remote measurements 
beyond the reach of a conveniently handled 
cable link was recognized. Second, the task 
of manually reducing the data from a contin- 
uous strip-chart recording was soon found to 
be far too tedious to be practical or economi- 
cal. Further, the requirements of the various 
analyses applied to the data were of such a 
nature that automatic digital computing tech- 
niques were mandatory. Thus at an early stage 
in the wave measurement program the need for a 
telemetry and data processing system, whose 
final output would be sampled data in a format 
suitable for entry into a digital computer, 
was recognized. 
As the system evolved and it became 
evident that it could usefully be applied to 
a wide variety of problems in oceanography 
and meteorology, the following desirable 
characteristics were specified: 
1) Flexibility: The input to the system 
should be compatible with a wide variety 
of transducers. 
2) Analog storage: The data should be 
stored in analog form with minimum 
degradation, and the storage medium 
should be chosen to allow fast, automatic 
sampling and digitization for ultimate 
entry into a computer. 
Functionally, the system may be repre- 
sented by the block diagram shown in 
Figure 1“. It is divided into two major 
parts, which perform the functions first of 
data acquisition and storage, and second of 
playback and processing. The data acquisi- 
tion and storage function is performed as 
follows. 
The electrical outputs of the trans- 
ducers (usually voltages which are analogs 
of the physical quantities being measured) 
are processed by signal conditioners to bring 
them to level and form suitable for operating 
the modulators. The modulators further 
operate on the transducer signals in such a 
way that they may be transmitted and stored 
with minimum degradation. A second function 
of the modulators is to multiplex the sever- 
al signals for transmission over a single 
56 
link. The data multiplex, after passing 
through the transmission link, is stored 
in analog form. Note that at this point the 
raw data is still in existence, degraded only 
by the distortion and noise inherent in the 
system. The analog storage medium is of a 
permanent nature so that the raw data can be 
stored indefinitely, if desired. 
An inspect function is shown, which 
allows the investigator to observe the pro- 
gress of his experiment and determine the 
proper operation of the system. The position 
of this function is shown at the input of 
the modulators, but it may actually be located 
anywhere between the transducer outputs and 
the output of the transmission link. 
Processing begins with playing back the 
raw, multiplexed data. The individual chan- 
nels are separated out by the demultiplexers 
and demodulated. The result of these opera- 
tions is a set of voltages directly propor- 
tional to the transducer outputs. 
At this point the data may be displayed 
for inspection, for example on a graphic 
recorder, to allow the experimenter to exam- 
ine and edit his data for further analysis. 
It is now necessary to prepare the data for 
entry into the computer. The several chan- 
nels are scanned in sequence by the commu- 
tator, and the sampled voltages are converted 
into binary coded numbers, which are entered 
into the computer. 
TECHNICAL DESCRIPTION 
In choosing the techniques and components 
to realize the various functions shown in 
Figure 1 every attempt was made to utilize 
standard hardware and well-developed methods 
wherever possible. At an early stage it 
became apparent that the needs of the aircraft 
and missile industry had fostered the devel- 
opment of standard telemetry systems which 
could be adapted to the needs of our science. 
Accordingly we were able to draw heavily upon 
the accumulated experience of specialists in 
this field and find "off the shelf" components 
produced by several companies specializing in 
the techniques of data transmission. 
The method chosen was a well-proven 
technique of frequency-division multiplexing 
wherein the signal from each transducer fre- 
quency modulates an audio oscillator over a 
rather narrow range. A separate center fre- 
quency is assigned to each oscillator and 
sent over the transmission link. It is gener- 
ally most convenient, where a radio link is 
necessary, to use FM transmission. In this 
case the technique is referred to as FM-FM 
telemetry, signifying that both the subcarriers 
