Why master for iTunes Plus?
When creating a master, mastering engineers take into account the limitations and characteristics of the medium or destination format, as well as the listening environment of their audience. For example, a master created for vinyl is unlikely to be listened to in an airplane or car, and therefore is often mastered for a listening environment where a listener can hear and appreciate a wider dynamic range. Similarly, a master created for a club environment might take into account the noisiness of the intended listening environment.
Because iTunes Plus is a highly portable format, its files have the potential to be listened to in a wide range of different settings. So while one listener may be using white earbuds while riding in a loud subway
What is iTunes Plus?
AAC (Advanced Audio Coding) is a format for compressing and encoding digital audio. AAC achieves the portability and convenience of compressed and encoded digital audio while retaining audio quality that’s indistinguishable from larger digital files, such as audio from CDs.
The iTunes catalog was initially offered in 2003 as 128 kbps AAC files, many of which were encoded from the original CD masters. They sounded great—in fact, these downloads led the industry in sound quality. More than 100 million songs were sold in this format in a little over a year, changing the landscape of legal digital music forever.
But innovation didn’t stop there. Recently, using the most advanced AAC encoder, the iTunes catalog was upgraded to iTunes Plus: a variable bit rate (VBR) 256 kbps AAC encoding format. iTunes AAC encoders are now able to transparently encode high definition audio, creating files that retain the small footprint, portability, and ease of use iTunes is known for. And they sound amazing.
The above are extracts taken from Apples white paper on iTunes Plus to give you a summary of the format without too much tech talk! More detailed information can be found here.