thank you very much.. but does it save space? i don't think it specified on wikipedia
It depends.
Mostly, they claim that a 128 bit AAC file is comparable in quality to a 160-256 bit file in MP3. This is fine if you are converting from raw audio (CD ripping) to AAC.
If you're converting from MP3 to AAC, you are going from a lossy format to another lossy format... dropping parts of the signal twice is not a good idea, so you may convert a 160 bit MP3 to a 160 bit AAC just to lose less data in the second compression.
The again, audio is all in your ear, not anyone elses, so you may hear a difference and not agree that a 128 AAC from raw CD audio equals a 160 MP3 from raw MP3 audio, and so it won't save you space.
Which is why I said to consider your needs, and whether AAC is supported on all of your needs, or if MP3 is a better format for your needs.
But if all your devices are AAC compatible, and you're not an extremely picky audiophyle, I'd try converting your uncompressed audio to AAC, and see if you like it. If you do, then AAC is the newer format, and has some improvements over MP3, so I'd start using that for your new audio files unless you have a reason (an incompatible device) not to.
But I wouldn't bother to convert your MP3s to AAC, since you'll just degrade the signal somewhat by recompressing it in the new format, unless of course, you have a reason (a device that doesn't play both AAC and MP3).
Assuming you have existing CDs, existing MP3s without the original CDs, and are only using Apple products (only products that support both AAC and MP3), then I would suggest:
- Rip all your CDs to AAC.
- Leave all your MP3s which you can't recreate as MP3.
- Rip all new audio to AAC format if it's not already in a compatible compressed format.
- Rip any incompatible compressed audio to AAC, but increase the bitrate to prevent excess losses during the conversion.
If you're really concerned about space over sound quality, try ripping to lower bitrates (for things like audiobooks, these are usually as low as 8-32 bits already).