Re: AAC..?
Thank you
!!!, now can you tell me something else?, does AAC is a format own by Apple, did they created it? Because is this is true, then all AAC are protected? Or AAC is just some other kind of compression (assuming that is what it is) like mp3?
As Wes said, AAC is one (currently the only, but that won't necessarily always be true) audio compression system used by MP4. Although Apple was involved in the MP4 standard, they do not own it, nor did they create it. It isn't an open standard in that it's not open source that anybody and everybody can use and mess with freely, but it is accepted by a standards body that any company has access to.
Protected AAC, which is what the iTMS uses when you buy music, is, however, Apple specific (so far as I know, anyway--they're certainly the only company using it). Not all AAC files are protected, though--only the ones you buy from Apple. If you create an AAC file yourself from a CD you own, it's as open as MP3, though there are less players for the format.
If this mp3 mp4 aac are compression?, then when I burn a CD to be listened on my car, a regular CD what is it called, what is that format? Or there is non format on those CD, they are purse sound. Maybe what I am asking is that If to put music into a computer always needs to have some compression, couldnt we just copied exactly, isnt both digitals?
Wes wasn't exactly correct on his answer to this question. Audio CDs, which is what iTunes burns and you play in your car, do NOT use AIFF audio encoding; they use a raw, uncompressed audio stream that I believe is technically called PCM. This is close to what you meant by "pure sound", although ANY digital data contains a format of some sort--"format" just means a way of organizing digital data. For all practical purposes, the format of audio CDs is called exactly that: "Audio CD".
In answer to the second half of that question, and what Wes was getting at, you CAN import uncompressed audio into your computer, and the format Macs usually use to do that is called AIFF. An AIFF file can contain several types of compressed audio, but in this case people ususally use them for an exact, uncompressed copy of a CD--identical to the original.
The reason to compress audio when you import it into your computer is size; an uncompressed CD worth of audio will take up about 600MB of space on your hard drive or iPod, but the same thing compressed into an MP3 or AAC file that sounds almost the same to most people only takes about 60MB.
There are some people who are very picky about their audio who actually use uncompressed audio on their computers, but most people are perfectly happy with MP3s or AAC files.
-)