Recording - January 2011 - 30

Audio geeks can argue for days over what’s best. Some will say that FireWire, a well-established standard, is best for audio, while USB is a poor second and best for disk storage. Others argue that as computers get faster, USB (a “dumb” interface where the computer does all the heavy lifting) becomes less of a load on system resources, and FireWire (a “smart” interface where each device does some of its own thinking about moving data around) loses its advantage and can even be a hindrance, and maybe it’s time for disks to be relegated to FireWire so USB can come into its own as an interface standard. Our best advice is to experiment where possible and see what works best for your system. USB is generally found in two flavors: 1.1 and 2.0. There was a 1.0 standard that wasn’t used for long before 1.1, and we’re not going to say much about the brand-new 3.0 standard here—see below for a few general notes. USB 1.1 is fast enough to handle stereo I/O, but multitrack devices and high sample rates need USB 2.0. Most add-on control surfaces are USB; like mice and computer keyboards, they tend to have a very low bandwidth compared to audio—even if they’re sending MIDI messages—and are a good risk to coexist with hard disks or even an audio interface without hurting anything. If a USB (or FireWire) audio interface has a control surface built in, you can be sure it was designed to behave nicely on the bus as a single device and not worry. The main thing to worry about here is that the presence of a USB 1.1 device on the bus can force every other device down to that very slow speed, which can cause problems for USB 2.0 audio interfaces. Some computers offer two different USB buses; if so, isolate your 1.1 devices from your 2.0 if at all possible. Mac OS X has a nice “device tree” showing the USB hierarchy on your computer; you can find it under “USB” in the System Profiler (About This Mac > More Info), as shown in the large screenshot on the previous page. This is possible because USB is a “dumb” protocol, and the computer is in control and must know the particulars of every USB device connected to it. Unfortunately there’s no equivalent display for Windows. FireWire also comes in two speeds, 400 and 800.The two can be mixed, and a simple adapter cable lets you attach FireWire 400 devices (the standard for audio) to a FireWire 800 port (now all there is on Macs). If you mix the speeds, put the FW800 devices closer to your computer, and hang the FW400 devices off the FW800 devices. This works okay for multiple hard disks, but beware mixing audio interfaces and disks on the same bus! Be careful when using FireWire ports on Windows laptops. Many of them are designed for transferring data from video cameras and not much else; they’re often 4-pin unpowered ports and don’t support isochronous audio transfer reliably. Some would say that using FireWire for audio (or anything else) on any Windows machine is a crapshoot, but there are enough Windows FireWire devices out there to at least partly give the lie to that prejudice. Admittedly, though, Windows machines have a
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much wider set of component and driver possibilities than Macs, and there’s always a chance your particular machine may need coaxing to use FireWire happily, especially if you’re adding a FireWire card rather than using what’s built into the motherboard. Devon Brent, our resident PC-torturer, says that any FireWire card or daughterboard on a Windows machine should use a Texas Instruments chipset in order to work reliably with audio gear, and that FireWire audio is extremely susceptible to disruption by any and all powersaving measures, so you should shut them all off before doing audio work. Hubs Hubs exist for both USB and FireWire, but are much more common for USB. A powered hub can help power a number of bus-powered devices at one time, and any hub adds more places to plug things in. Some hubs are smart enough to isolate USB 1.1 from USB 2.0, but many don’t, and all merely give you more ways to clog a bus with more stuff. There are good reasons to use an external powered hub (see “USB Power Considerations” below), but in general, use hubs with care. Speaking of hubs, you should realize that in many cases, the multiple ports on your computer are basically a built-in hub! You may have two FireWire ports and four USB ports on the box, but the odds are good (especially for laptops!) that under the hood they go to only one of each bus, so plugging in your devices to these ports won’t get you more bandwidth—although you will see benefits of more stable bus power and perhaps a more reliable connection than with an external hub. As mentioned above, some computers actually give you multiple USB buses, which is handy for separating 1.1 from 2.0 devices, but don’t count on that just because you have multiple ports on your computer. USB power considerations USB devices come in two flavors: low-power and highpower. A low-power device, like a mouse or keyboard, draws practically no current at all; a high-power device, like an audio interface, can draw quite a bit, if it successfully tells its host computer “I need this much power” and the host agrees. You can’t do this through an unpowered hub, which is why it’s a good rule of thumb not only to avoid hubs if at all possible, but also to consider unpowered hubs genuinely evil for anything musical, other than maybe copy protection keys... and Devon tells me he wouldn’t even trust one for those. USB provides current in what are called “load units” of 100 mA. A low-power device uses one unit or less; high-power devices can negotiate for up to five units or 500 mA. Since the rail voltage in USB is 5 V, that translates to one load unit being 0.5 Watts of power. That number’s significant, because if you do the math, a properly designed channel of 48V phantom power for a mic preamp draws 0.68 Watts, so it’s impossible to design a phantom-enabled audio interface without negotiating for more power from your computer. That’s why all USB audio devices should be handled as if they’re highpower, because they almost always are. With a maximum power draw of 2.5 W, an interface’s preamps can easily phantom-power two mics and still have juice left over for other functions... like, you know, actually providing gain. Note that some small laptops don’t provide a full 5V on their USB connections, and also note that if an interface

Recording - January 2011

Table of Contents for the Digital Edition of Recording - January 2011