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


So I am new to recording. I have been reading a bit of the Tweakheadz.com guide. I hope I'm at least asking the right questions. Feel free to point me to the answer if this is a redundant post.

I am still unclear about the specifics of interface inputs and outputs. I am looking at a used MOTU 828 mkII (found on CL) to use with my Macbook Pro / Garageband (for now).

So, that specific interface has 2 XLR / quarter inch combo inputs and mic pres so I can basically plug two XLR mics in the front and press record in my computer (correct?). Now what does the 8 TRS (1/4" stereo correct?) i/o's on the back do for me?

Also, if I want to record drums with an interface (3 or 4 mics at once) should I look for another interface? Can you only record (at one time) with as many XLR inputs on your interface or can you add on mic pre's as you can afford them and plug them into the TRS inputs?

Thanks in advance.


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.


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.


Dave, thanks for your explanation. It was a very thorough and easy to understand; just what I needed. I have to say learning about recording is pretty fun.

I'll look (more) into the Firepods. I've read about what seems like some reliability issues with those units, which is why I'm thinking MOTU or maybe Apogee Duet. The guy with the 828 mkII is asking $450, which seems high from my little bit of research. I was thinking about seeing if I could get it for closer to $350.

Is there any reason why consumer's line level typicall = -10 dBV and pro = +4dBu?

I had to lookup ADAT signals. This is a digital means of transferring audio, similar to how Firewire cables can transfer audio (I know Firewire isn't strictly audio). So your Mackie Onyx 800R has 8 mic inputs > preamps (the preamp is used to boost the low level signal of the microphone) > converted from Analog to Digital > ADAT output (means of transferring digital audio) > into Fireface interface > out of Fireface into computer > (and with any skill / luck into million's of listeners ears). This is how I understand it.

As far as how stuff sounds. I am trying to pay attention to that upfront so I'm not buying then selling (and repeat) like I do with guitar gear. I went Peavey Classic 20 > Fender Deluxe Reverb > AC30 > HIWATT and now I realize it's too much ($$$) and I'll be fine with a Fender Deville. Guitar amp metaphor alert: With recording I'm trying to start out with the Deville and stick with that.

Again, thanks.

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