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If you wish to use Apple loops in any type of professional or commercial capacity
you must purchase those loops from the original sample source to be licensed.
Apple Loops are specially formatted AIF files which contain meta data within the file. This data tells your DAW package essential facts about the file, such as instrument type, tempo, key reference, genre and transient points. Garageband and Logic both utilize this format to the fullest.
Membership is completely fre, simply signup by entering a username and your valid email address and your password will be emailed to you instantly, the only thing Macloop gets out of it is that it add your email address to its "Loops & Samples" mailing list which gives you updates on new free samples that are available for you to download.
To load Apple Loops into your copy of Garageband simply drag the files you've downloaded into your Loop Browser directly from Finder and Garageband will index the loops into your database of apple loops. You should then be able to search for the new samples using the Loop Browser filters.
If you wish to use Apple loops in any type of professional or commercial capacity you must purchase those loops from the original sample source to be licensed.
If you only want to use these loops for personal use which will never be used commercially then you do not need to purchase a license.
All the samples either come from Silicon Beats or Platinumloops. The page where you download the files displays the "Original Sample Source". The meta data within the files also contains copyright information..