I have been fielding more and more questions in the past few weeks concerning the “Apple Digital Masters” (formerly Mastered for iTunes) logo that is positioned on so many artist pages in iTunes. “Apple Digital Masters” is a declaration by Apple that the “songs and albums for sale on iTunes are exactly as the artists and engineers intended them to be heard.” To support this claim Apple has developed a series of software tools for mastering engineers, medicine musicians and producers alike that allow them to preview mastered audio as it will sound after being compressed by Apple’s ACC encoding process. The tools also help the engineer compare and detect clipping within the new ACC file. This can be very helpful to identify audible distortion and other issues caused by the ACC encoding which has been a problem in the past.
Apples latest coding method is a two step process which is emulated by their software tools. First, the stereo file’s sample rate is converted from it’s original format to 44.1khz. Some songs are tracked and mastered at 44.1khz so a conversion is not necessary. However, many HD systems can record at sample rates as high as 192,000khz and need to be down-sampled. The bit resolution is then considered by utilizing all the dynamic range of a 24 bit file which eliminates the need for dithering (adding dither noise) and removes the need to encode this noise.
Does it work? Is it helpful? Yes and yes, but as long as Apple is distributing the 256kbps ACC file, which in my opinion is less than adequate, it is a conservative step in the right direction. Apple keeps a copy of the high quality studio masters and retains the option to re-release music as better codecs become available or even release the original studio master if we could be so lucky – so I am holding off on the excitement just yet in favor of something bigger and better!
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