Talking book cartridges come in multiple sizes, check out the 8GB or 16GB versions. This 4GB cartridge can hold about 400 hours of talking book audio because each gigabyte holds approximately 100 hours of talking book audio, formatted for the National Library Service of the Library of Congress (NLS) for use on the NLS Digital Talking Book Player. It can also be used to store & play books that are downloaded from the NLS BARD website. It also works with the APH's (American Printing House for the Blind) Book Port DT, and APH's Joy Player (Note: APH's Joy Player is not enabled to play NLS Talking Books). It is also compatible with Humanware's Victor Reader Stream (adapter required). It is compatible with MP3 and Daisy file formats. It is important to know that in order to load data onto this cartridge; it does require a data transfer cable to connect it to your computer. This cable will work with any computer that has a standard USB port. Standard USB extension cables will also work to transfer data to this cartridge. To download books from BARD (the NLS book download website) you must be an authorized NLS patron.