Steve, since it's effectively Monday and I'm only partially working
, I'll try and take this one step further to shine some light on another commonly used technical term, which has been used in this thread, and better describes the actual thing users are trying to optimize when misapplying the term 'unity gain'. By that I mean determining the limits of a comfortable range of gain settings which provide a good
signal-to-noise ratio. A good signal-to-noise ratio simply means getting more of what we want (signal) and less of what we don't want (noise). 'Noise' in this sense is anything we don't want, and it refers in this case specifically to two forms of noise which show up at opposite extremes of that comfortable range of gain settings- Noise in the form of overload distortion or clipping from a signal which is too hot at one extreme and excess hiss from needing to use too much gain on a signal that is too weak at the other.
There is a range of possible gain settings which will provide a good signal-to-noise ratio, not just a single setting. The important thing is to find the comfortable 'safe zone' of the gain settings which avoid the problematic extremes. It's not a single setting but a range, with brick-walling or clipping at one end and excess noise at the other.
The way in which the term 'unity gain' has been misused around here implies a single specific input-gain setting, and so in addition to being the incorrect term for what people are actually trying to optimize, it also is misleading in implying that there is only a single 'optimal' setting.
At the risk of confusing things again, I'll point out that even that mis-implication is incorrect. As established in this thread, 'unity gain' simply means that the input voltage equals the output voltage, which DSatz pointed out somewhere earlier.
[edit- and as Chirs just pointed out as well with regards to gain through mixing consoles] Most small recorders have separate controls for both input gain and for output gain of the line-out or headphone jack. You can set the input gain way low but compensate by setting the output gain high enough so that the device would still be at ‘unity gain’ as long as the input voltage = the output voltage. You could also do the opposite and set the input gain too high and the output gain low enough to achieve 'unity gain'. In either case your signal-to-noise ratio would not be a good as a more reasonable setting for both, and that’s what we’re really concerned about.
Notice that the above description on different settings that all achieve ‘unity gain’ says nothing about actually recording anything on the machine, only about a signal passing through it having the same voltage going out as it did on the way in. That’s the only way ‘unity gain’ applies for end-users. When we get more technical and talk about the different stages of circuitry inside the device and how they interact, we might apply the ‘unity gain’ terminology in describing the gain through those internal circuits, but that doesn’t usually apply to anything the end user can adjust on these small recorders.
[edit- Chris's example of a mixing console is a good one, since in that case the user can adjust signal gain at various points along the signal path on the console.]