In a recent issue of QST, Steve Ford, WB8IMY, took a look at the forgotten Digital Radio Mondiale (DRM) system. Digital broadcasting was supposed to be the life preserver for international shortwave broadcasters facing the reality of rising costs and shrinking audiences. In 1998, broadcasters, equipment manufacturers, regulators, and others formed the DRM consortium to create a specification for digital shortwave broadcasting that might stem the growing shift to internet broadcasting and revive listener interest.
DRM promised an FM-quality signal that also could convey text information such as program titles and news headlines.
DRM signals heard on a conventional AM receiver sound like wide-band noise. The DRM signal carries three separate channels -- a primary audio channel, and two subsidiary channels, one for essential decoding data and a third service description channel.
Unfortunately, DRM failed to halt the decline of shortwave broadcasting; it was too little, too late. In addition, consumer electronics manufacturers lacked enthusiasm for the new format, so an audience for DRM never coalesced.
Some international HF broadcasters still use DRM on a regular basis. The list includes the BBC, Radio France International, and All India Radio. Decoding a DRM signal is far easier today than it was a decade or so ago, and the rise of software-defined radio (SDR) has provided new avenues for DRM listening. Many SDRs specifically include DRM as a reception mode. Radio amateurs early on experimented on HF using DRM-derived software called WinDRM. Much has changed over the intervening years, and today the HF digital voice application of choice is FreeDV https://freedv.org/. If you hear buzzing signals at 14.236 MHz, chances are it's a FreeDV QSO.
For more information on this topic, see "Eclectic Technology" in the April 2019 issue of QST (p. 65).
https://www.eham.net/articles/42861