I have a question about how you transfer music in your Ipods

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I have been using Ipods for like 2 to 3 years already, I transfer music from my original cds to my itunes. My question is when you transfer songs to your ipod do you convert all the music into aac format? Because I understand that when it is in aac format you get to edit the text title of the song and properly arranged from artists and he spellings and correct titles. Usually when transferring a cd with mp3 the format is mp3. I am also having a problem with the songs in my itunes it seems to be so difficult to arrange which songs are actually in there and can be played or not. Sometimes there are too many songs in my itunes but only some songs really play.