The Podcast Pedant

(This began another recycled post from the Alley — but then I started elaborating, and now very few of the original words have survived.)

The home recording business is moving more and more these days towards digital interfaces on all their low- and mid-end stuff. This makes sense: almost everybody is recording into a computer now, so if putting a USB interface on your otherwise-average mixer will give it a competitive edge, why not? It simplifies life for the musician or podcaster, and sending a digital signal to your computer means you aren’t bound by the computer’s sound card. ¹

I’m of the opinion that it’s a very good idea to have a USB or Firewire interface for your audio — whether it’s built into your mixer, or a separate interface box, or even a standalone USB microphone. But if you can afford it, get Firewire.

Why?  What’s the difference between Firewire and USB? From the outside, they’re both just cables. You can chain stuff together or put them on hubs, and you can buy a lot of the same sorts of devices for each. USB 2.0 and Firewire 400 have similar speeds “on paper.” (480 megabits per seconds and 400 megabits, respectively.)

But even a casual look will show that USB audio devices tend to be cheap and Firewire devices tend to be expensive (barring the odd exception). And just about all A/V geeks will agree that Firewire is better, and even necessary for complex work. Why is that?

The difference is a fundamental one, but it’s internal to the way these things work, and pretty subtle. The following represents my best understanding based on a few hours of research and being a computer geek for several years. If I get any of it wrong, someone please correct me.

USB

USB is a hosted networking protocol — which means every device talks only to your computer and is utterly reliant on your computer to tell it what to do. You can have up to 128 devices on a USB network, and they might be chained and hubbed in all sorts of interesting ways, but they’re all just passing their bits back to the computer to decide what to do with them.

USB devices are asynchronous, which means that any device has the power to send any amount of data at any time. If two devices decide to talk at once, their data can collide with each other. If the traffic’s not highly time-sensitive, this isn’t a big deal. There are routines in place to manage it, and you’ll never notice if your mouse click happens a couple microseconds later. BUT. There are a few applications where it matters, and one of them is audio. An audio interface is sending a constant stream of sound data back to the computer. It rarely uses up the whole pipe, and so it’s still possible for other devices to talk — but the odds of collisions are higher, and if you get too much other traffic the errors can pile up beyond the computer’s ability to stay “caught up” and you lose some sound data.

The details of the protocol are typically implemented in software. That makes it very cheap and easy, as it pushes the work onto the CPU, and the devices themselves don’t have to be very smart. But it also means that USB traffic has a direct impact on system load, and vice versa. For most common applications this doesn’t matter — the traffic from your keyboard and mouse is so slight that it’s hardly going to bring your system to a crashing halt, and most of us wouldn’t notice if our external hard drives slow down for a second when our screen saver kicks in. For audio, however, it does matter. Same problem as collisions. Audio devices are pushing data out at a constant rate, and if the computer’s too busy running sound effects or switching programs or swapping out RAM to pay attention, some data can get lost. And then you get glitches and/or latency.

In case anyone’s wondering, audio glitches are bad. Latency (the delay between the creation of a signal and the final reception of it) isn’t quite as bad unless you’re trying to monitor yourself through headphones while you’re talking. If you are, a latency of a fraction of a second can be unsettling. Like you’re living in the future.

Firewire

Firewire is a peer-to-peer protocol,² meaning that every device on a Firewire network is equally capable of talking to every other device. Two video cameras on a Firewire network can share data with each other. A Firewire audio interface could save sound data directly to a Firewire hard drive. ³ Your computer is just another peer on this network, and has no inherent special status.

Firewire is always implemented in hardware, with a special controller chip on every device. So the load it puts on your CPU is much lighter than USB communications load, and you’re much less likely to lose any sound data just because you’re running fifteen things at once. Specialized hardware usually makes things faster and more reliable, and this is one of those times. (By the way, it’s also one of the things that makes Firewire more expensive. It’s also a reason Apple dropped it from the iPod Nano — there was just no room for the Firewire chip.)

But the real reason Firewire is more reliable than USB is more fundamental than that. It’s because Firewire allows two operating modes. One is asynchronous, as we described above with USB. The other is isochronous mode, and it lets a device carve out a certain dedicated amount of bandwidth that other devices can’t touch. It gets a certain number of time slices each second all its own. The advantages for audio should be obvious: that stream of data can just keep on flowing, and as long as there isn’t more bandwidth demand than the wire can handle (not very likely) nothing will interfere with it. No collisions, no glitches.

From a practical perspective, this also makes it safer to send a lot more audio via Firewire. That’s why most of the multichannel interfaces (18 channels, 24 channels, etc.) are Firewire devices, and USB devices usually just send a two-channel stereo signal.

So there you have it. For hooking up your mouse, keyboard or thumb drive, USB is plenty fast and plenty cheap. For hard drives, either one will do (although Firewire is somewhat more reliable). For audio devices, USB will do fine if no other devices are competing with it and if you have processor room to spare. But Firewire will always be able to handle more load with lower latency and no glitches, because it has resources it can set aside to make sure your audio gets where it needs to go.

…And that’s why Firewire’s more expensive and taken more seriously.

Just in case you were wondering.

1. Which is good, because most sound cards are crap for recording. And even the ones that aren’t have to worry about electrical noise from the rest of the stuff inside your computer case. ↩
2. Hello, RIAA search bots! No, I’m not talking about that kind of P2P. Go away. ↩
3. Note that it could. I’m not completely sure whether any actually do, but I’d appreciate information on this. ↩