Apple Digital Master (ADM, previously “Mastered for iTunes”) is a mastering procedure to ensure that there are no clipped samples in the encoded audio files. Clipping can occur for two different reasons:
1) Intersample overloads. A sharp peak in the audio signal can have its True Peak between two samples at maximum amplitude. As the signal is converted back to the analog domain, the analog signal would have a peak that is higher than 0 dBFS and is therefore clipped.
2) In an encoded signal using perceptive coding such as mp3, aac or ogg, signal components that we supposedly do not hear are removed – not encoded – in order to save bits. As a result, the time signature is altered and may result in overloads although the original audio in PCM format is not overloaded.
When mastering for iTunes, the audio files are encoded using the AAC codec in the OS X operating system and are then checked to make sure that no samples are clipped and that there are no intersample overloads.