What Technically Do OSX Input/Output Slider Positions Mean?

Question

I'm wondering what the sound output and input sliders in OSX actually mean technically. They're nicely "simple" and "intuitive" -- uh, and give you absolutely no information about what's going on.
This is for a stock Mac with default soundcard, and whatever the audio functionality is called these days (Sound Manager? Core Audio?).
For audio out, is the slider at "maximum" sending out the full bitdepth of audio from an application, and if you pull down the slider, it's cutting bit depth? Or is there an amplification boost at maximum?
For audio in, in the Sound control panel:
• If the slider is all the way to the left, does that mean the incoming sound is not being touched (unity gain), or is being cut in level?
• If the slider is all the way to the right, does that mean the incoming sound is not being touched (unity gain), or is being amplified? If amplified, by how much?
• If unity gain is somewhere inbetween, exactly where on the mysteriously uniformative slider markings do we find it? And what do the other markings mean (how much cut below unity for each mark to the left of unity, how much amplification for each mark to the right?)
• What do the input indicator lights mean? In OS9 the lights were green/yellow/red like a normal meter. In OSX, they're all blue. Is there a difference in functionality or just an overzealous "Aqua" interface designer? Is anything telling me clipping is occurring? What does each light indicate in terms of decibels?
And is there any Apple documentation on any of this anywhere??
Thanks!

Answer 1

If you're talking about the hardware ins and outs on the Mac itself, I don't think it really matters. On the input side, use the meters in Logic - and your ears. Try to get a good level without distortion. Make sure the Pre Fader Metering option is on in LP. (Also make sure the correct input source is selected in the Apple Sound System Preference Pane - probably line as the built-in mic is way noisy.)
On the output side, you can max the Mac's volume control. When I play test tones from i-Tunes, the meter on my interface shows the correct level if I max the i-Tunes volume AND the Mac volume, although some mastered CD's will clip so you may actually want to put the Mac's volume 2 - 3 "notches" below max.
That said, the Mac's built-in audio isn't the best way to record or monitor. It works in a pinch but you're limited to 16-bits on the recording side and the input hardware is pretty noisy. You might want to look into an inexpensive firewire interface...
-John