Rational Acoustics Smaart I-O User Manual - page 6
an impulse response measurements using a reference signal from one
Smaart I-O and a measurement signal from another. It may not be
necessary in all cases to actually use this feature, as the Smaart I-O can
also use time cues from the computer’s USB controller to keep its clock
running at a highly consistent rate, but physically linking Smaart I-Os
to run under a master clock signal provided by a signal designated
device still provides an extra measure of consistency.
To synchronize the sample clocks between multiple I-Os, first make
sure all units to be linked are disconnected from the computer. Then
designate one unit as the clock master and connect an RJ-45 patch
cable to its Sync Out port (the bottom RJ-45 jack). Connect the other
end of the cable to the Sync In port of a second unit. To add additional
Smaart I-Os to the chain, connect the Sync out on the second unit to
the Sync In on the third and so on.
When all the link cables are installed, connect a USB cable from your
computer to the first unit in the chain (the designated clock master),
then the second and so on -- if additional units are to be linked, keep
working your way down the chain until all units are connected to the
computer. The Smaart I-O automatically detects the cable connections
on power-up and configures itself appropriately as the clock master or
a downstream client, depending on which jacks are populated. The
USB LED on the front panel of slaved units changes from blue to purple
on power-up to indicate that they are being externally clocked.
Please note that you may still see some variance in delay times from
one measurement to the next in dual-channel measurements made
using signals from two different devices. This can happen even when
the sample clocks are synched between devices, due to ambiguities
in the timing of multimedia events by the operating system. What
clock linking ensures is that once a measurement is underway, the
delay between signals should remain constant for the duration of the
measurement. You may therefore wish to designate one microphone
as your reference for measuring delays in the system under test and
connect that microphone to the same device being used to capture
the reference signal.