3. Allow measurement signal selection and routing. In many cases, you may be using multiple mic and line signals which you need to select from over the course of your measurement sessions. While one can employ the old stone-knives-and-bearskins approach of just re-patching cables on the fly, multiple, routable preamps (mixers, switchers) make the job easier, cleaner, and less error prone.
4. Provide phantom power for measurement microphones.
There are many ways that these preamp requirements can be met. In touring and permanently installed systems, it may be beneficial to build the measurement preamp requirements into the system’s existing signal preamp and routing scheme (i.e., feeds directly from the mix console or system DSP units).
It’s important however to remember requirements 1 and 2 above, and make sure that the “built-in” measurement signal feeds have their own, separately adjustable levels apart from the main system drives — we can’t very well go asking the mixer to turn up or down during a performance just to make our measurement signals happy.
Computer Audio I/O
Once we have our measurement signals, the final step along the signal transmission path is the analog to digital conversion (A/D) and the journey into the computer processor (sorta sounds like an Orlando theme park ride).
The big question: “How do we get there from here?” The most convenient path is to use the converters built into the computer, their stereo line-level inputs, however, over the past 10 years, most PC laptops have dropped that input from their built-in components in favor of a simple mono-mic input (Mac laptops still have them standard.)
If one is available to you, it is certainly a viable option as those inputs usually meet/exceed our humble requirements (again, see the note on measurement system signal path quality.)
In the all-to-frequent case that your laptop does not have a stereo line-level input, or where your measurement rig requires more than two input channels, the standard solution is an external audio I/O unit.
Over the same past 10 years (not coincidentally), there have been a number of computer audio interfaces that have come on the market that satisfy our requirements — most of which including our required preamps.
When considering an audio I/O unit for a measurement rig, the primary concerns (apart from preamp requirements) in general are:
* Physical Connection Format - USB, USB 2, FireWire (IEEE1394) 400, FireWire 800, PCMCIA card, dixie cups on strings? The question is which is easiest, any will it carry the number of signals you need. USB (1 and 2) are the most commonly available connections built into laptops and are generally the preferred connection type for simple two channel (stereo) input. USB 2 and FireWire connections are required for multi-channel input (3-plus channels).
* Audio Driver Format - Just because the signals get into your computer doesn’t mean your measurement software can use them. It’s very important to determine what driver formats your program can access (i.e. wav/wmd/mme, ASIO, coreaudio). This issue is further compounded by OS version issues and is the source of severe headaches for users and developers alike.
* Powering Mode - bus-powered or externally powered. Simple stereo USB units often utilize the buss power available via the USB connection (500 mW max). This is extremely convenient as it adds portability (no need to plug in to AC) and ease of set-up to your rig. It’s also a great feature when traveling between countries that use different standard AC voltages because the bus-powered unit gets its power from the computer, which normally utilize auto-ranging power supplies. Once you are into multi-channel I/Os, it’s pretty much guaranteed that bus power will not suffice and it will need to be plugged into local AC for power.
* Form Factor - simply put, what type of audio connectors does it have and how big is it. For those of you who need an extremely portable measurement rig, rack-mount gear is most probably too big for your requirements. A corollary to this issue then is ruggedness/roadability — sure it’s portable in size, but is it really built to withstand the transportation demands/conditions placed on it?
The proper choice of audio I/O and preamps is truly defined by the intended use for the measurement rig — what systems are going to be measured, under what condition and whether or not (and how) the rig is going to be transported. No one solution works for every user and use case.
Often, it’s preferable to field a basic set of stereo preamps and I/O, and then supplement that with additional preamps and signal routers (mixers, switchers) when the complexity of the rig and system requires.
Jamie Anderson is a founding member of Rational Acoustics, which provides training courses, hardware products/packages, and professional consulting for sound system measurement, analysis, and alignment. He has been teaching and working in the field of sound system engineering, measurement and alignment for almost 20 years. During his career, Jamie has worked as a technical support manager and SIM instructor for Meyer Sound Laboratories, as a system engineer on tour for A-1 Audio (kd Lang) and UltraSound (Dave Matthews Band), and most recently, as a product manager and instructor for SIA and EAW. Also check out the Rational Acoustics Store for a selection of many of the components discussed in this article.
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