zenpeace69 said:
I'm sorry if this is a really dumb question, but what is "line level?"
Benny broke it down really nicely.
Just to avoid any possible confusion for newbies, when Benny talks about the amplifier needed by microphones, this amplifier is what is referred to as the "preamp", or in slang, "mic pre". Not to be confused with any other kind of power amplifier.
On a more pragmatic level, most, if not all, of your gear in your signal chain that has visual meters marked on a "VU" scale will have these meters calibrated for 0VU to represent line level. There are exceptions (e.g. some tape machines, some consumer or communications gear), but for the most part, the meat of the signal chain meters should be calibrated this way.
This means, simply, that your chain usually works the most efficiently if you keep your meters bouncing somewhere around the 0VU mark. Again, there can be some exceptions; sometimes one may like that "saturation" sound botten by pushing a preamp just a couple of dB too hot, other times some gear just does not sound as good if pushed too hard and may actually sound better and cleaner a couple of dB low. But it'll take some practice and expirimentation to learn the individual personalities of the different pieces in your chain in this way. In the meantime, in general, you can get a great signal path keeping the level at line level (near 0VU on the metering) all the way through.
Untill you cross over to digital via your converter(s). Then the system is designed so as line level converts to somewhre in the mid-negative teens on the digital dBFS metering scale. Typically this conversion level is from line level in analog to -18dBFS in digital, but there are some converters that may approach -14dBFS on the digital outs.
This level may seem low, but on a 24-bit digital recording, this is quite normal and servicable, and is how it should be. Hence the zero/unity gain setting on the digital input; no need or desire to boost the digital signal.
G.