The UMIK-1 Measurement Microphone
I’ve been following MiniDSP for the last year or so. They specialize in low-priced, programmable
DSP modules – primarily for tuning home stereos and building custom active
loudspeakers. My introduction to them
was by purchasing a MiniDSP 2x4, which is reminds me of a $100 miniature Meyer
Galileo but with unbalanced I/O and DC power.
You choose routing, delay, filtering and parametric EQ on every in and
out. It’s a great little box (I’ll try
and write about it later).
But what really caught my attention is a new MiniDSP product
called the UMIK-1. It’s advertised as a omnidirectional measurement microphone but with a built-in preamp and D/A. It connects via USB and is recognized as a
class-compliant (driverless) audio interface. Even more tantalizing is the
price at $75 list. The thought of being
able to do transfer function (TF) measurements with nothing more than a laptop
and a mic was also really appealing (that would require using the headphone
jack on the computer – but for backpack studio junkies it’s really
tempting). When the mic arrived, let’s
just say I was really excited!
The UMIK-1 comes with a substantial kit: microphone,
windscreen, mic clamp, large-thread mic stand adaptor, miniature tripod, USB
cable and a padded plastic box. This is quite
generous for a mic of any price. The mic itself is all metal, feels solid, and is
about the same size as most P48 measurement mics (like the Earthworks M30). You can look up your mic’s serial number on
the MiniDSP website and download a custom correction file for trimming up the response
in your measurement program. I retrieved
that file with no issues.
Testing time
The flattest mic in our arsenal is the DPA4007. It’s standard for the Meyer SIM-3 system: with
ruler-flat response and minimal phase shift. Measurement dorks come to fisticuffs when discussing the best mic – but the 4007 is my gold
standard – and I chose it as the reference to do a standard mic comparison TF
measurement against the UMIK. SMAART
v.7.5 and a Focusrite Scarlett 2i4 completed the test rig.
Here is the mic
compare setup. Measurement police
kitties hard at work in the background.
For a source, I used a single JBL LSR4328P studio monitor.
For a mic compare test, the only thing that matters is how broadband the speaker
is, but it’s not a bad monitor (technically, it’s -10 down at 35 Hz
and 25 KHz). I fed it pink
noise, launched SMAART and I was off to the race. The UMIK is in one USB port and the Focusrite in the other - and both are set to 48K.
Then came a big letdown.
The TF phase trace showed the UMIK sitting
180-degrees away from the DPA. I know my
workflow with SMAART and the Focusrite/DPA combo is absolute-polarity correct. I then took a look at the UMIK impulse
response (IR) trace and the spike went downward. The
UMIK is polarity reversed! With a regular analog XLR mic, this is only moderately
egregious since one simple adaptor fixes it. But how do you un-flip the
polarity on a USB plug? This is inexcusably bad engineering –
especially for a measurement mic. Fortunately, polarity won’t
affect single frequency response TF measurements – but the phase trace will always sit 180-degrees off across the whole band. So I set my OCD aside and continue.
Three bad thoughts.
I won’t call the next anomaly another big letdown – mostly because
it was pilot-error – but it stumped me for an hour. As soon as I started the mic compare, I let
SMAART add in the appropriate delay compensation on the reference source. The
phase trace snapped to 180 (this would be zero if the damn polarity was right –
but it was a nearly straight) and the coherence cleared up to >95%. But then the phase trace slowly started to
unwind! And the coherence trace started
dripping red blood all over the screen – starting in the HF and slowly cutting
across the band. I didn’t panic because
I’ve seen this before. When a rigged
loudspeaker is slowly swinging you can see the same phenomena happen. But how could my tiny home studio be
collapsing? The delay time was slowly
slipping about 0.1ms of delay for each second of clock time. That would mean my
monitor was sliding back 1.5” every second!
Of course, the monitor stayed put. So my second bad
thought was that there must be some stupid DSP problem in the mic that is adding delay. (To me that's plausible, given that someone designed it with
wrong polarity). So I engaged the delay
tracking option in SMAART. Sure enough,
it was able to compensate, and I watched with fascination as the delay time
started a slow countdown.
Watch this video. The first half is with no
tracking – and you can see the phase trace slowly unwind as the bad coherence
start to slide left. The second half is with
the delay tracking engaged and you will see the offset grow. Sexy time! You can download a high-res version here.
Then came the third bad thought – which was that I
was just a presumptive idiot There is an inherent problem having two
uncorrelated (no digital sync) digital input devices connected to the same
program. SMAART can handle as many
simultaneous interfaces as you can plug into the computer – but (rightfully so) it has no method of sample-rate or sync correction. So… how in the world can you sync two digital
sources without word clock? But then I remember a stupid Avid product that
never had any practical use to me... until now.
Every time you install ProTools, a rather obscure audio engine choice called
“Pro Tools Aggregate I/O” shows up in your control panel. This driver allows you to consolidate
multiple interfaces into a single virtual device, which is required for ProTools 90’s-vintage input limitations. What I remembered about the aggregate device is
that you can access a hidden layer that provides “drift” correction for
exactly this problem. It creates virtual
digital sync. I’m now beginning to have
second thoughts about my years of Avid-bashing.
You can only access this functionality when you call up the Aggregate
via the “Audio MIDI” utility in OSX (the preferences audio pane won’t do
it). I clocked both devices to the Mac
internal D/A output by engaging “drift correction”. Fortunately, this fixed the issue -- and kudos to SMAART for allowing the Aggregate choice as input.
Here are the settings
that allowed Avid to fix the sync problems going into SMAART.
Finally I am up and running with a legitimate mic compare TF. I can work around the polarity problem and now
concentrate on discovering just how flat the UMIK is. BTW, it’s not. I ran all my TF measurements with 1/24 octave smoothing and it’s
rough. From 2k to 8k there are multiple
resonant peaks – about 16 of them – upwards of +2.5dB. My best guess is that this is the resonance
of the housing or a problem with the grill. Whatever is the
cause, it’s not good for a measurement mic – but it does have a tolerance of
about +/- 3dB and that’s acceptable deviation and common in many popular recording microphones.
Here is the native
frequency response on the UMIK-1
I imported the correction curve provided by MiniDSP into
SMAART and ran another measurement. It
made the response a bit better – but still not great. MiniDSP claims corrected tolerance to be +/-
1dB and my measurements confirm this.
I then decided to make my own correction curve. A correction
curve is a comma delimited text file of frequency in one column and deviation
in the other. SMAART is also able to export this data, which you can then paste
into Excel and export as a file.
Watch this video to
see the export and creation process for my correction curve. You can download a high-res version of this video here.
The result of the correction curve was quite flat.
Here are the three
curves together. Pink = native, Blue =
mfgr corrected, Yellow = custom correction
With a reasonable correction curve, the UMIK-1 can be used
as a reference mic in non-critical applications. Impulse response (delay timing) and quick system
conformity tests should be just fine with this mic. Because I already had the JBL 4328 set up, I
decided to run a final comparison by measuring an actual TF of the loudspeaker
with each mic. The results are shown
below.
Pink = DPA4007, Blue =
UMIK-1
It’s not bad. Some of
the HF shift is likely due to the mics being about 2” apart with diffractive
reflections stemming from being so close to the JBL waveguide.
SOME OTHER THINGS TO NOTE
Calling this mic omnidirectional is a stretch. It’s really a
“generously wide” cardioid. It exhibits
significant HF shading at 90 degrees – so much so that the mfgr. also provides
a 90-degree correction curve. You can
also adjust the gain of this mic via the MacOS “Audio MIDI” control panel. However, I could not get the sample rate to
change even though MiniDSP claims it is both 44.1 and 48 – the dropdown doesn’t
work in OS 10.11.
CONCLUSION
What sells me on this mic is the convenience it provides at
a very low price point. It suffers from
some bad engineering and a substandard correction curve, but it travels very
easily and costs about 75% less than a decent entry-level (no correction
needed) analog measurement mic. If you
can borrow an excellent mic from which to create your own correction curve, you
have the means to correct for sample rate drift, and you don’t mind seeing your
phase trace 180 degrees off, this is an acceptable device. If you need it to work right out of the box, I
would stay clear.
-Mike
UMIK-1 street price appx. $98 on Amazon with shipping
From the ADR Stage, we moved on to Rerecording Stage 4. Two editors were prepping for a meeting with Lionsgate executives to come in and listen to the final mix for an upcoming film.
Our next stop was to Rerecording Stage 1, where the Audio Editors and Music Editor walked us through their workflow and fielded questions. Here, two editors are able to work simultaneously on the console; one mixing dialogue while the other works on SFX and ambiances.
