According to Wikipedia, The original objectives of converting to digital transmission were to enable higher fidelity, more stations and more resistance to noise, but actually most of stereo stations on DAB sound worse than FM. So, now the DAB system is in the process of being upgraded, and it will adopt the AAC+ audio codec to improve the efficiency of the system.
The Digital Radio Mondiale (DRM), says Wikipedia, "is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcast, particularly shortwave. DRM can fit more channels than AM, at higher quality, into a given amount of bandwidth, using various MPEG-4 codecs".