Reading about compression rates and everything ususally won't replace some first-hand experience, so I ran a little test today. Might be interesting for those wondering (like I did). 
I encoded two versions of the same song (Trafik - Echoes), one in WAV and one in M4A lossless. Calling the first file ORIGINAL, I then converted the M4A file back to WAV and renamed it to REENCODED. In theory, if lossless would be really that, both files should be identical, right?
Next step, I started Cool Edit and loaded both files into a session, setting their starting point to the exact same position. Now here's the interesting part - I took one of the files and phase inverted it.
Usually, if you invert one file and play it back simultaneously with the original file, both signals will cancel out each other, leaving the listener with nothing but silence since one is the exact opposite of the other.
So I had the ORIGINAL and REENCODED (and now inverted) file in my session, moved the mouse over the PLAY button, clicked once, and...
... nothing. Pure silence. Not a single peak anywhere, just a perfectly flat line and absolutely no sound. Both files had perfectly cancelled out each other.
Which leads me to the conclusion that M4A is truly lossless as you can completely convert it back to its original WAV format without any loss.
[edit] If someone's interested, I could run this little test with some AAC encoded files as well, posting screenshots of what the mixed down tracks look like to visually compare compression quality.
I encoded two versions of the same song (Trafik - Echoes), one in WAV and one in M4A lossless. Calling the first file ORIGINAL, I then converted the M4A file back to WAV and renamed it to REENCODED. In theory, if lossless would be really that, both files should be identical, right?
Next step, I started Cool Edit and loaded both files into a session, setting their starting point to the exact same position. Now here's the interesting part - I took one of the files and phase inverted it.
Usually, if you invert one file and play it back simultaneously with the original file, both signals will cancel out each other, leaving the listener with nothing but silence since one is the exact opposite of the other.
So I had the ORIGINAL and REENCODED (and now inverted) file in my session, moved the mouse over the PLAY button, clicked once, and...
... nothing. Pure silence. Not a single peak anywhere, just a perfectly flat line and absolutely no sound. Both files had perfectly cancelled out each other.
Which leads me to the conclusion that M4A is truly lossless as you can completely convert it back to its original WAV format without any loss.
[edit] If someone's interested, I could run this little test with some AAC encoded files as well, posting screenshots of what the mixed down tracks look like to visually compare compression quality.
