Although Apple only sells music through the iTunes Store in the 256 kbps AAC format known as “iTunes Plus”—throwing away 97% of a recording’s original data in favor of smaller file sizes—the company is attempting to show off its “higher quality” selections with a new Mastered for iTunes section in the iTunes Store. Songs from within this section are processed using tools and guidelines that attempt to maintain as much of the original sound quality as possible, while still cutting off that 97% of data.
This is all done by downsampling the original recording from 24-bit samples at 96 or 192 kHz (depending on a system’s processing power and available storage space) to 44.1 kHz using a 32-bit floating-point intermediary file, which is then converted to the AAC iTunes Plus file. Performing this process apparently uses every bit of resolution available, keeping the dynamic range of the original recording in tact. This highly impacts the overall sound of a compressed track purchased from the iTunes Store, making it sound as though it’s of much higher quality than it actually is.
You can read more about the technology behind mastering at Apples’ new Mastered for iTunes web page, and you can check out tracks that make use of this technology in the new Mastered for iTunes section of the iTunes Store.