Recording - November 2017 - 53

It's a cool tool, but there are some caveats. First, the accuracy of your measurements is limited by the linearity of
your interface, though most contemporary recording interfaces are adequate
for most audio measurements.
Second, connections between the DUT
and interface are mostly straightforward,
but be aware that there's no single best
way to mix balanced and unbalanced
interconnections-grounding or floating
the wrong lead might degrade your test
results. Remember, too, that every connection is a potential noise source, so
avoid stringing adapters together and
take the time to make or modify cables
for the most direct connections.
Third, it's also important to understand headroom and manage gain
and signal levels appropriately (see
Bill Stunt's article, also in this issue).
REW, by default, expects an input level from the DUT that allows about 12
dB of digital headroom and will complain if it detects clipping (0 dBFS). You
probably allow more headroom than
this when recording with your DAW,
so treat your REW setup with the same
care. Driving a mic preamp into the
"warm-and-phat" range is easy, but
your interface's maximum output level
may be incapable of driving a line input transformer into saturation.
Many of today's interfaces offer
"zero latency" input monitoring that
sends the input signal directly to the
output, avoiding a trip through the
computer. For our use here, an interface's direct input monitoring must be
turned off. REW needs to see only
the test signal coming from the DUT,
and engaging direct monitoring can
cause feedback.
Lastly, recognize that you're in Computer Wonderland here, a place where
no two computers are alike, and sometimes differences can cause unplanned
behavior. I've only used REW on Windows XP and Windows 7 and James
uses it on Linux; it's fair to assume that
the other OS versions work similarly,
but one never knows. I have some tips
for setting up and using REW for measuring your audio gear and James goes
into more detail, but this is neither a tutorial nor a review of the program.

REW SETUP AND CALIBRATION

Setting up REW to work with your
interface is essentially the same as setting up a DAW. Open the Preferences
window (Figure 2), click the Soundcard
tab, and choose the input and output to
which you'll connect the DUT. REW is a
single-channel processor, so you'll need
two passes to measure a stereo device.
Select the ASIO driver if your interface
has one; otherwise, select Java, which
uses the standard Microsoft sound card
driver. The Mac version uses the macOS
Core Audio device selection.
44.1 kHz sample rate is adequate for
measuring across the conventional audio bandwidth, but with a higher sample rate, you can measure at frequencies
above 20 kHz. The rest of the settings
can be left at their defaults for now.
The next step is to calibrate the interface. First, connect a cable between the
interface's output and input. If your interface has input gain and output level
controls, set them to their normal working positions to approximate unity gain.
Click the Check Levels button and, if
necessary, fiddle with the interface's
input and/or output controls to make
REW's meters match within a couple of
dB. If the interface has a Clip indicator,
be sure it remains off. If both the Output and Input meters on the Calibrate
screen don't indicate roughly -12 dB,
something's wrong with your interface
setup and you'll need to sort that out before proceeding. Try a different driver.
Click the Calibrate button, and instructions at the bottom of the page will
walk you through the procedure. REW
measures the frequency response of
your interface (Figure 3) and creates a
correction table so that irregularities in
your interface's response won't affect
your measurements.
Since the calibration sweep starts at
0 Hz, low frequency rolloff is normal,
as is the sharp drop-off above the Nyquist frequency (half the sample rate). If
all is well, the frequency response will
be flat within a few tenths of a dB over
the normal audio range. Finally, click
the Make Cal button (that's short for
"use this as the calibrated interface"),
name and save it, and your test system
is ready to go.
RECORDING November 2017

http://www.ADKMIC.com

Table of Contents for the Digital Edition of Recording - November 2017