Citing codec (coding/decoding) licensing issues, three free Windows programs for sound card-based HF digital voice were yanked from their download site for a short time recently, surprising hams who are interested in HF digital voice operation; several online groups that supported the software were also closed for a short time.

WinDRM, DRMDV and FDMDV, all written by Cesco Lanza, HB9TLK, used a codec that was developed for the US Department of Defense and NATO. Rights to various forms of the codec are held by several companies; they have never been released for free distribution and use by hams. According to Gary Pearce, KN4AQ, the companies have "winked" at ham radio use for several years, but a recent complaint caused the programs to be pulled from the download site. "Lanza did a quick rewrite to use an open-source codec, and now WinDRM and FDMDV are back," Pearce said. "DRMDV, an intermediate program between the other two, has been abandoned. WinDRM could always use the open-source Speex codec, but FDMDV users will need to download the new version."

Pearce said these three programs all allow hams to transmit and receive digital voice by connecting their PC sound card to an ordinary SSB transceiver: "The result has been surprisingly high quality audio, with virtually no noise -- sort of like listening to FM, but in the narrow bandwidth of a sideband signal. WinDRM, the best sounding program, uses 2.5 kHz of spectrum. FDMDV sounds a little rougher, but uses only 1.1 kHz of spectrum. They both use OFDM modulation, a set of close-spaced carriers that are each modulated with a little bit of data to add up to the final digital signal. The main problem with HF digital voice is that it needs fairly strong signals. FDMDV works with weaker signals than WinDRM."

Pearce said that none of the available open-source codecs work as well as the old one: "MELP, or Mixed Excitation Linear Prediction, was designed specifically for high-quality, low data-rate voice communication. So the on-air audio might suffer some with the new version. Digital voice users have been waiting and hoping for someone to concentrate on developing a codec optimized for ham radio use, but none has been forthcoming."

FDMDV and WinDRM can both be downloaded from Jason Buchanan's, N1SU, Web site.

The AOR digital voice modems and D-STAR radios both use the AMBE 2020 vocoder, and are not affected with the coding changes; the AMBE 2020 vocoder is a proprietary chip that is embedded in each unit.

For more information on WinDRM, check out QST Editor Steve Ford's, WB8IMY, article ["Life Could Be a DReaM," pages 38-40] in the April 2007 issue of QST.