ADX (file format)
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| Developer(s) | CRI Middleware |
|---|---|
| Development status | Legacy (ADX2) |
| Platform | Cross-platform |
| Type | Codec / File format |
| License | Proprietary |
| Website | CRI Middleware |
ADX is a lossy proprietary audio storage and compression format developed by CRI Middleware specifically for use in video games; it is derived from ADPCM. Its most notable feature is a looping function that has proved useful for background sounds in various games that have adopted the format, including many games for the Sega Dreamcast as well as some PlayStation 2, GameCube and Wii games. One of the first games to use ADX was Burning Rangers, on the Sega Saturn. Notably, the Sonic the Hedgehog series from the Dreamcast generation up to at least Shadow the Hedgehog have used this format for sound and voice recordings.
On top of the main ADPCM encoding, the ADX toolkit also includes a sibling format, AHX, which uses a variant of MPEG-2 audio intended specifically for voice recordings and a packaging archive, AFS, for bundling multiple ADX and AHX tracks into a single container file.
ADX is a compressed audio format but unlike MP3 and similar formats, it doesn't apply a psychoacoustic model to the sound to reduce its complexity. The ADPCM model instead stores samples by recording the error relative to a prediction function which means more of the original signal survives the encoding process; as such ADPCM compression instead trades accuracy of the representation for size by using relatively small sample sizes, usually 4bits. The human auditory system's tolerance for the noise this causes makes the loss of accuracy barely noticeable.
Like other encoding formats, ADX supports multiple sampling frequencies such as 22050 Hz, 44100 Hz, 48000 Hz, etc. however, the output sample depth is locked at 16bits, generally due to the lack of precision already mentioned. It supports multiple channels but there seems to be an implicit limitation of stereo (2 channel) audio although the file format itself can represent up to 255 channels. The only particularly distinctive feature that sets ADX apart from alternatives like IMA ADPCM (other than having a different prediction function) is the integrated looping functionality, this enables an audio player to optionally skip backwards after reaching a single specified point in the track to create a coherent loop; hypothetically, this functionality could be used to skip forwards as well but that would be redundant since the audio could simply be clipped with an editing program instead.
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