Yeah they're the 3 other music sources other than the now traditional CD. From reading reviews and the like, iPod connectivity at the moment is generally quite poor especially on units with loads of songs (I'm thinking full 20GB ipods). I heard navigating through them using the head-unit is a pain, especially if navigation is through the use of buttons rather than a knob. Of the reviews, Alpine seems to get the best recognition for iPod connectivity; others just seem to pale in comparison.
Line-in for me was very useful to me before getting an aftermarket head-unit. I had a front AUX-in input on my factory Mk3 cassette head-unit and it worked great with my Creative MP3 player, as I had a remote with it. Navigating was a breeze as it meant I wasn't any more distracted than changing songs on the actual head-unit in the case of CDs. Fast-forward to what I have now (RCA -> Line-in input), I don't have to use my MP3 remote anymore as I could just connect it from my centre console compartment. I hardly use that now cos it's much harder trying to change songs while driving as it meant I had to navigate through the actual MP3 player itself. Needless to say, USB became the new MP3 player for me and I like it so much that my next head-unit/s must have a front USB input.
I like the Sony ones in that they have both line-in and USB inputs on the front. Argh! It's so hard to find functionality, looks, and reliability in these things! Actually, in any gadget!
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