Over the last three years, RDFT or Redundant Digital File
Transfer
has been the primary means to send digital SSTV pictures
over ham radio.
Barry Sanderson KB9VAK developed the RDFT mode.
Since then, the mode has been adopted by DIGISSTV and
SSTV-PAL Multi Mode from Erik VK4AES in Australia by the
DIGTRX program from Roland PY4ZBZ in Brazil
and by the DigiACE software from Martin Emmerson G3OQD in
the U.K.
In just the last six months, the DRM mode has gained so much
popularity
that the RDFT mode has become nearly obsolete. Here is a
little background on DRM:DRM can deliver FM-comparable sound quality, but on frequencies below 30 MHz (long wave, medium wave and short wave), which allow for very-long-distance signal propagation. VHF is also under consideration, under the name "DRM+". DRM has been designed especially to use portions of older AM transmitter facilities such as antennas, avoiding major new investment. DRM is robust against the fading and interference which often plagues conventional broadcasting on these frequency ranges.
The encoding and decoding can be performed with digital signal processing, so that a cheap embedded computer with a conventional transmitter and receiver can perform the rather complex encoding and decoding.
As a digital medium, DRM can transmit other data besides the audio channels (datacasting) — as well as RDS-type metadata or program-associated data as Digital Audio Broadcasting (DAB) does. Unlike most other DAB systems, DRM uses in-band on-channel technology and can operate in a hybrid mode called Single Channel Simulcast, simulcasting both analog signal and digital signal.
