Converting AAC to MP3

You wouldn't be able to tell any difference in quality and yes, the files size is greater with mp3 - one of the reasons why iTunes uses .m4a. If you want to upgrade the sound quality you need to convert to Apple Lossless or AIFF format.

You can get by with converting from aac to mp3 and maybe not hear any significant loss in quality depending on the song, but this is a bad idea. Both are lossy formats which mean they have taken a huge song file and run it through a complex process to strip out much of the data stored in the music and keep the parts necessary to make it sound good enough for your ears. Now what you are trying to do is take one highly compressed file and move it to another compressed format. Ideally you would convert to either mp3 or aac from a lossless file format (FLAC, Apple Lossless, etc.) or the original cd - but not convert between them.

Here is a good article on file formats: http://www.stereophile.com/features/308mp3cd

The article has some in depth acoustics data, but you can read it for the main gist. The aac (mp4) file format generally gives a better sound with a slightly smaller file. So unless you have a device that is not compatible with aac, there is no reason to be converting to mp3.

I suspect you mean he would have to create the Apple Lossless or AIFF format from some other source than the aac or mp3 file. Going from a compressed file to a lossless format is like trying to use a B&W photo to create a color photo - the data just is not there.

Contrary to what billtils says you cannot "upgrade the sound quality". If you convert the file to a lossless format like AIFF the sound quality will remain identical but in a much bigger file. Converting to a lossless format cannot restore the information stripped out by a lossy compression algorithm!

Converting from AAC to MP3 will inevitably lead to loss of audio quality because two different lossy compression algorithms are being applied to the original source signal. You may not hear a difference on ordinary in-ear headphones but try listening to the resulting file on a high quality audio system with a pair of good full-range speakers and you *will* hear a difference.

Ideally you should do as jeff1970 suggests, re-encode from the original lossless source (e.g. CD) to AAC.

Sorry for the loose language - I did not mean to imply convert from AAC to AIFF or Apple lossless, but meant what jeff1970 says, namely when converting your source material to a higher quality format to store on iTunes and play through a good quality sound system, you should use AIFF or Apple lossless, not AAC.

BTW, I have two iTune libraries on my MacBook, one for iPod use where everything is .m4v format and one for serious listening where they are all AIFF, and wondered what would happen to the AIFF files when using iTunes Match cloud storage. I have not been able to check thoroughly but the AIFF versions cerainly play very acceptably.

To tell you the truth, I don't really know if the quality will be any different if you convert aac to mp3 with itunes but I, for example have been using this Audials software, don't know if you've heard about it..and converting works great, I mean so far, I did not notice any quality loss when converting aac to mp3 or other formats for that matter. It's definitely worth trying, anyway.

Audials software

That only works under Windows. Frankly I am not sure that one even needs it since iTunes will do the conversion for free.

There is no reason to convert (and several to not convert) AAC to mp3 unless you are using a player that doesn't support AAC.