and the RF carrier are frequency modulated. 
At the receiving terminal the subcarrier 
multiplex is recovered from the RF carrier 
by means of a conventional FM receiver and 
directly recorded on magnetic tape. 
The advantages of the FM-FM technique 
are several: 
1) The use of frequency-division multi- 
plexing makes possible the simultaneous 
recording of several continuous time-depen- 
dent variables without introducing the sam- 
pling errors encountered in time-division 
schemes. 
2) Frequency modulation of the sub- 
carriers not only carries with it the well- 
known advantages of signal to noise ratio 
improvement in the transmission link, but 
also overcomes the problem of intolerably 
large amplitude fluctuations caused by lack 
of uniformity in the magnetic tape. 
3) FM subcarriers allow the recording 
of DC levels on magnetic tape. 
A limitation of the FM-FM system is 
imposed by its large bandwidth, which re- 
quires the use of a very high frequency 
carrier. Atthough there is a valuable re- 
duction in atmospheric noise in this part 
of the radio frequency spectrum, the trans- 
mission distance is limited by the line of 
sight. It is, however, possible to employ 
the FM-AM technique, where the frequency- 
modulated subcarriers amplitude modulate a 
carrier at a lower frequency suitable for 
trans-horizon communication. The bandwidth 
allowable at these frequencies severely 
limits the number of channels, and trans- 
mitter power must be increased to offset 
the increase in atmospheric noise. 
Figure 2 shows the actual components 
of a typical data acquisition system, repre- 
senting merely one of several variations 
which are compatible with the data reduction 
equipment. In this case the transducers are 
AC excited bridges, and it is required that 
the sense of the bridge unbalance be known. 
The bridge output voltages are therefore fed 
to differential amplifiers in order to bring 
them to a level suitable for operating 
phase-sensitive demodulators. The outputs 
of the demodulators are DC voltages propor- 
tional to the AC transducer outputs. Their 
polarity is determined by the phase of the 
amplified transducer signals with respect to 
the reference voltage, which is derived from 
the transducer excitation source. The DC 
output voltages from the demodulators vary 
the frequency of voltage-controlled sub- 
carrier oscillators, and the frequency 
58 
multiplex modulates an FM transmitter operating 
in the VHF telemetry band. An FM receiver at 
the receiving terminal recovers the frequency 
multiplex, which is recorded on a single track 
of magnetic tape. 
There are many cases where the telemetry 
link is not necessary, and it is merely desired 
to record the data in a compact form at the 
site of the experiment. Here, of course, the 
mixed output from the subcarrier oscillators 
is directly recorded on the magnetic tape. 
The individual components shown in the data 
acquisition system are all either available 
commercially as standard transistorized plug- 
in units or as designs developed at WHOL. 
Thus data acquisition units for field use may 
easily be designed using the modular concept 
with a minimum of circuit development. Both 
voltage-controlled and resistance-controlled 
oscillators are available for a wide range 
of sensitivities. Where DC excited trans- 
ducers are used, the amplifiers and demodu- 
lators are, in most cases, unnecessary. For 
AC systems a specially developed differential 
amplifier-demodulator combination is now 
available commercially. The inspect function 
would probably be omitted in remote, unattended 
installations, but a monitoring arrangement 
then becomes desirable at the output of the 
FM receiver or cable link. Calibration sig- 
nals are inserted in the usual manner at the 
input to the system by substituting standard- 
ized signals for the transducer output. 
In the recording process a certain 
amount of variation in the transport speed 
of the tape is always present. Tape speed 
variation frequency modulates the recorded 
signal, and in an FM system results in noise. 
A well-known method is used to overcome this 
difficulty. In addition to the data multi- 
plex a signal from a frequency-stabilized 
oscillator operating at a different frequency 
from any of the subcarriers is recorded on 
the tape. During play-back the deviations 
in the frequency of this signal are detected 
and used to compensate for variations in the 
data signals caused by tape speed variation. 
The playback and processing system 
known as ADDReSOR, (Analog to Digital Data 
Reduction System for Oceanographic Research) 
has recently been put into operation at the 
Woods Hole Oceanographic Institution, sup- 
planting earlier apparatus of more limited 
scope. Figure 3 shows a much simplified 
block diagram of the device which was manu- 
factured by Tele-Dynamics Division of American 
Bosch Arma to WHOI specifications. A total 
of twelve channels may be accommodated. De- 
multiplexing and demodulation are accomplished 
by standard subcarrier discriminators, each 
of which contains in one unit an appropriate 
band-pass filter for selecting the desired 
subcarrier, an FM discriminator, and a low- 
