The Background to the Audible FormatThe Audible format (represented by the file extension .aa, .aax, and .aax+) is a proprietary audio scheme developed by Audible, Inc. It is specifically designed for the secure distribution and use of audiobooks on various software and hardware devices. There are several different Audible formats that are in use which cover a wide range of encoded bitrates. This selection of sound quality levels is designed to give you maximum choice on the sound quality level you want to download your purchased audiobook at. This flexibility is also useful to have when you have an older portable hardware device that doesn't support certain Audible bitrates or you need to limit the size of audiobook files due to storage space constraints. The current Audible formats are:
- Format 2 (.AA): this has a bitrate of 8 Kbps and its sound quality is classed as AM radio.
- Format 3 (.AA): the bitrate for this format is 16 kbps and produces sound that is on a par with FM radio.
- Format 4 (.AA): Sound at this level is encoded at 32 kbps and the quality of sound is categorized at standard MP3 level.
- Enhanced Audio (.AAX / .AAX+): this format has the highest Audible bitrate of 64 kbps and is deemed as having CD quality sound.
How Audible Files are Protected and its RestrictionsTo prevent the unauthorized copying and playing of downloaded audiobooks, the Audible format uses an encryption algorithm (generically referred to DRM copy protection). Interestingly, the actual sound data inside an Audible file is encoded in an unprotected format (either MP3 or ACELP), but is then wrapped up in the encrypted Audible container.
There are several restrictions (like other DRM copy protection systems) when using this audio format. These are:
- Playback Restriction: a username and password mechanism is used to prevent the playback of downloaded files on unauthorized devices.
- Maximum Number of Computers: you can only playback your purchased audiobooks on a maximum of four computers.
- Limited Audio CD Burning: even though you can burn audio CDs via iTunes for example (which incidentally creates an unprotected disc), you only have the option to burn an Audible download once. This is extremely limited when compared to other DRM systems like Apple's FairPlay protection that is used for certain digital products on the iTunes Store.
How Audible Content is Distributed and Played
- Computer (Mac or PC ): this is the common way of purchasing and downloading content from Audible's library. In order to do this, you first have to download the AudibleManager software which then allows you to playback and transfer your purchases to a compatible portable device. As previously mentioned, you can have up to four computers authorized to handle Audible content. While on the subject of software, jukebox applications that can play back files in the .aa format are relatively thin on the ground compared to compatibilities for other more mainstream formats. Apart from the AudibleManager software program, the only other applications that can playback this format are iTunes and Windows Media Player.
- Hardware devices: there is a wide selection of different hardware devices on the market that can be used to play back files in the Audible format. This includes: iPod, iPhone, iPad, portable media players, MP3 players, PDAs, and streaming media devices.
- AudibleAir: Rather than using the traditional method of downloading Audible content to your computer and then transferring files via cable to your mobile device, AudibleAir can be used. This method of distribution utilizes software on compatible hardware devices to make it possible to download DRM copy protected audiobooks via a Wi-Fi connection or straight from Audible's library using your phone's carrier. There are software apps for various smartphone platforms such as: iOS, Android, Palm OS, Windows Mobile, Symbian, etc. There's also a growing collection of PDA hardware devices on the market that have AudibleAir built in -- this enables seamless integration with Audible.com for updating your purchases and even automatically deleting audiobooks when you have finished using them.