Making Rational Gear Choices
Anatomy of a Measurement Rig: Probes, Preamps and Processors (continued)
Preamps (Signal Transmission)
OK, this section should really be called, "Preamps, Cables and Audio I/O" - but that would defeat our cute alliterative naming scheme. Also, while "signal transmission" includes all the connecting cables in your measurement rig, we will, for the purposes of this discussion, assume they are of professional quality and functioning properly (but don't just go making that assumption in practice — check'em), and focus on the preamps and computer audio IO (interface.) Often these two functions are combined in one device, but not in all cases. Here, we shall address the two functions separately. (Also, please read the measurement system Signal Path Quality Note.)
Our measurement rigs require preamps to perform four critical tasks:
- Allow adjustment of incoming measurement signals to appropriate levels for our computer audio interface. In determining our choices of preamps, we must consider what type/level of signals we will be accessing (mic, instrument, commercial line level, pro line-level), and what type of connectors will be needed (XLR, 1/4", RCA, BNC).
- Allow adjustment of measurement signals for appropriate levels for our measurement purposes. Throughout the course of standard measurement processes, it is often desirable to be able to finely adjust the levels of multiple measurement signals relative to each another.
- 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.
- 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 is important however to remember requirements 1&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 is, "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 ten 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 our 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 IO unit. Over the same past ten 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 IO unit for your measurement rig, the primary concerns (apart from your preamp requirements) are generally:
- 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&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+ channels).
- Audio Driver Format - Just because the signals get into your computer doesn't mean your measurement software can use them. It is 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 - buss-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 is also a great feature when traveling between countries that use different standard AC voltages because the buss powered unit gets its power from the computer, which normally utilize auto-ranging power supplies. Once you are into multi-channel IO's, it is pretty much guaranteed that buss power will not suffice and you will need to plug 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?
