The ADSP feature is deprecated for v17 and relevant code removed from Kodi v18. The reasons are explained in the following links.
Forum discussion detailing reason for removal - https://forum.kodi.tv/showthread.php?tid=252871&pid=2672651#pid2672651
Pull Request to remove ADSP code from Kodi - https://github.com/xbmc/xbmc/pull/13723
AudioDSP (Audio Digital Signal Processing) add-ons allow for enhanced audio processing in Kodi. It is a technique to transform or modify signals using computers. It relies heavily on mathematics and computer science. It is used in wireless communication and all sorts of scenarios which use audio signals. A common use is to to boost or cut the low frequency range of a subwoofer which may be connected to an AV-Receiver. Currently these techniques are outside of the Kodi core program.