Apr 11, 2022 07:00:00

What is the identity of the 'blank 500KB' that exists in music downloaded from Apple Music?

Music purchased and downloaded from Apple Music and the iTunes Store contains 500KB files that are not used for anything. Software engineer Daniel Alexandersen discusses what this is and why it exists.

The 0,5 MB of nothing in all Apple Music files

https://www.ctrl.blog/entry/apple-music-nullbytes.html

Music files downloaded from Apple Music etc. are encoded in AAC format and stored in ISO / IEC format containers. This container stores metadata such as subtitles, album art, audio (multimedia stream), etc., but the behavior differs slightly depending on the order of the files in the container.

For example, if you place a metadata block at the end of a file, it will either download the entire song or load it from your local storage before playing the song. To avoid such useless behavior, encoders tend to place metadata blocks in front of multimedia stream blocks.

However, if such blocks are laid out and placed, the user will not be able to change the metadata such as changing the artist name. Therefore, the encoder places a free space 'free block' between the metadata block and the multimedia stream block, which allows the user to modify the metadata.

If you encode a CD with AAC using iTunes etc., about 5KB of free space will be reserved for free blocks. On the other hand, when Alexander analyzed the music downloaded from Apple Music etc., it was said that the free space secured for these was about 500 KB. Also, the music downloaded from the iTunes Store in 2010 also contained 500KB of free space.

Alexandersen prefaces that and considers why Apple has so much capacity. In 2006, Apple made a change to iTunes to not include album art in music files. It is said that this was to implement

Cover Flow , which displays album art graphically. From this, Alexander speculates that '500KB of free space was originally reserved for importing album art from the server.' Apple's server needed this much space to write directly to the metadata on the fly when the album art was delivered, and Apple may have forgotten to remove 500KB of free space when deleting the album art. is.