Sorry for that long title, but it’s hard to say this more succinctly. When you connect your iPod to your computer, the Summary pane gives you a number of options for syncing. One of them is “Convert higher bit rate songs to 128 kbps AAC.” If you check this, iTunes will convert any songs that are higher than 128 kbps, either AAC or MP3, to AAC files at this bit rate.
Normally, you don’t want to convert from one lossy format to another, but if you’re listening to music on your iPod outdoors, you won’t notice the difference in quality. The advantage is that you can save a lot of space. If, for example, all your music is ripped or purchased at 256 kbps, you can put twice as much music on your device.
But you may want to know how much space you can save, especially if you have music files at different bit rates. With iTunes 10, first look at the Capacity bar at the bottom of the window. For example, with my 32 GB iPod touch, I see this:
If you check the option mentioned above, the Capacity bar will immediately change, and you’ll see how much more free space you’ll have:
Previously, the capacity bar would increase in real time as content was added to your device. Now, in iTunes 10, you can see the results of your change even before you start syncing.
Note that making this conversion can lead to a very long sync, at least the first time; I’d suggest you do it overnight. Subsequent syncs are much faster, but are still slower than if you simply copied music files. But if you want to put more music on your iPod, this is a good way to do so. Check the option, click Apply, then wait. You’ll notice that subsequent syncs will take longer if you have new music to put on your device, but after that first sync, unless you change a lot of music, it won’t take too long.
Learn more about iTunes 10 in my ebook Take Control of iTunes 10: The FAQ.