Basics Question: Tell me about interface i/o's.

What's the price on the MOTU unit? My favorite budget interface a few years back was the Firepod and you should be able to pick one up rather cheap. 8 preamps + converters + a pretty solid overall system that's stackable for additional inputs.

Here's some basic terminology:

Connector types: In the analog world, you have two primary types of connectors, XLR and 1/4".

XLR is three wires, while 1/4" can be two or three wires. XLR cables are called "balanced" cables, because they use that extra wire to cancel out any noise/interference. (See bottom for explanation of how that works.) 1/4" connectors with two wires are called TS ("tip-sleeve") and carry a mono, unbalanced signal. 1/4" connectors with three wires are called TRS ("tip-ring-sleeve") and can either carry: (a) a balanced, mono signal; (b) an unbalanced, stereo signal; or (c) two separate unbalanced mono signals (e.g. send/return).

Signal Levels: Here's where things get a little tricky. Microphones (which nowadays always use XLR connectors) output very low-level signals, hence the need for a preamp. The preamp accepts a mic-level signal, amplifies it with as little added noise as possible (and imparting its unique sonic character), and then outputs a "line-level" signal.

Line-level signals are frustrating, because they come in two flavors: one professional (+4 dBu) and one consumer (-10 dBV). dBV stands for "voltage decibel" and is essentially an electrical measurement comparing the signal to 1V. dBu is basically similar, but different. To be honest there's a thousand different definitions and standards, it can get kinda confusing.

Most consumer gear, like the Firepod, uses -10dBV line levels. Some gear, like my RME Fireface 800, gives options for both +4dBu and -10dBV. A lot of professional rack gear uses +4dBu exclusively, often in balanced configuration (i.e. TRS for a mono signal).

So, what do you do with all those definitions?

To find total number of analog inputs, add up all XLR and 1/4" inputs, counting any "combo" jacks as 1 channel (since you obviously can't use two different cables in one jack).

To find total number of microphones you can use with the interface by itself, find out how many XLR inputs there are. (Note: this isn't always true, but by the time you're spending that kinda cash, you'll understand the differences and reasons much better.)

So, take my Fireface 800 for example. There's a total of 4 XLR inputs and 6 1/4" inputs. That gives me 10 total analog inputs, but I can only use four microphones because that's the total number of preamps I have. What if I need more mic inputs? Either (a) buy a different interface, or (b) buy some stand-alone preamps.

One note: a lot of interfaces, including my Fireface, have digital inputs as well. Most consumer/prosumer interfaces are designed to work with ADAT signals, and there's a lot of boxes available which offer 8 preamps + 8 A/D converters output through ADAT format, so that'd be another place to look.

For example, I use a Mackie Onyx 800R connected via ADAT to my Fireface, which gives me the 4 XLR inputs on the Fireface + 8 XLR inputs on the Onyx + 6 more analog inputs (which I have connected to some stand-alone preamps).

I know this gets confusing at first, but after a while you'll get the hang of it. And then you can start worrying about how all these different preamps, converters, etc. sound, and that's a whole 'nother can of worms.

Dave

P.S. Okay, so here's the balanced cable explanation. You have three wires: one is normal, one is ground, and one is normal but flipped 180 degrees in polarity. If interference is added at any point on the cable, when the two opposite polarity signals are combined (with the flipped side being "unflipped") any interference cancels out. So say you add +4 dB of noise...when the second cable is flipped before combining them, it becomes -4 dB. -4+4 = 0, so voila! no noise.