Prelude
Over a period of time 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 DSSTV 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. DRM mode has gained so much
popularity that the RDFT mode has become nearly obsolete. Here
is a little background on DRM:
DRM
Digital Radio Mondiale or DRM means "Digital World Radio". DRM
is a new digital radio standard for use by HF broadcasters. The
DRM standard uses a bandwidth from 4.5 KHz and up to 20 KHz
using OFDM modulation. It provides FM quality stereo audio over
HF as well as the ability to send data. Dream is a software
implementation of a DRM receiver. It is capable of making
perfect DRM transmissions of 10 and 20 KHz bandwidth. Reception
requires an adaptor to be connected to the receiver's IF stage.
It was Created at Darmstadt University of Technology in Germany
and Released under the GNU General Public License. The HamDream
software is a modified form of Dream by Cesco HB9TLK. HamDream
uses only 2.5 KHz bandwidth. HamDream is the basis for all the
2.5 khz DRM programs. The project is outdated and will not be
supported any more.
WinDRM is the current software by Cesco and it uses either
2.3 KHz or 2.5 KHz bandwidth. It also has a digital
voice mode. HamDRM is a Windows DLL program by Cesco based on
his WinDRM program. It serves as an engine to be used with other
graphical user interfaces that wish to support the DRM mode.
DRM Comparison (Amateur Radio)
THE DRM mode
has become very popular. Why has the DRM mode become so
popular? It takes no (or an insignificant) amount of time to
decode/encode. This is not like RDFT. Everyone likes the
decoding on the fly so that you can see instantly how the
picture came through. The file data is sent faster, 3 times
faster than RDFT and 2 times faster than DIGPAL. Now small
standard Jpeg and Gif files can be sent in their original
size. DRM allows larger files to be sent in less time which
means better quality pictures in about the same amount of
time. If you are not convinced that the picture quality of
Digital SSTV is not better than analogue SSTV, then you
should see "Analogue vs. Digital Test" on web page
http://kb4yz.ham.org. When using DRM your ID (callsign) is
sent continually. This would allow others to identify the
transmitting station and turn an antenna for better
reception. It allows viewing images with missing data
(blocks) or progressively viewing as the data is received.
This is somewhat like analogue SSTV. There are no critical
periods. You could miss the start or end of a DRM
transmission and still have enough data to be useful.
Without the problem of "Bad block Zero", it is possible to
expect even large files to make it through in spite of poor
band conditions. A replay is exactly like the original. It
would have the same file name, and the same file size.
NB: "There is also no picture slant adjustment required with
digital TV modes which is often necessary with analogue SSTV."
Repair
If
segments of a file are missing, a station may repair the
picture manually. During a DRM replay, those stations who
have missing segments can also receive them "automatically."
If conditions are very bad. it is possible to get
incremental repair ie the repair data does not have to be
received 100%. A partial repair may be repeated until the
file is complete. The "repair data segments" can be sent
multiple times increasing the chance of getting all the
segments even under poor conditions. Under conditions of QSB/fading
multiple instances may be sent. This makes it more likely
for a successfully received transmission.
During transmission it is advisable to set the
wave level for minimal ALC. If too much ALC is present,
there will be a tendency to overdrive the transceiver signal
resulting in the receiving stations MSC indicator bar
remaining 'RED' (EasyPal)
WINDRM allows as many
as eight different files to be sent in a single
transmission.
DRM is very forgiving. It can tolerate QSB and QRM.
Since it uses real
time decoding, it is possible to monitor the success of the
received file as it comes in. The total number of segments,
the number of segments received, and the last segment number
decoded are displayed as received. The display of the signal
to noise ratio (SNR) allows the user to make adjustments to
the receiver during transmission and see if it improves the
SNR and optimise reception.
A robust mode is
available for use when there is heavy QRM or QRN.
A high
speed mode is available for use on VHF/UHF or when
conditions are very good on the HF bands. A SNR of better
than 18 is required for this 64 QAM mode.
Reed-Solomon error correction
is an error - correcting code that
works by over sampling a polynomial constructed from the
data. The polynomial is evaluated at several points, and
these values are sent or recorded. Sampling the polynomial
more often than is necessary makes the polynomial
over-determined. As long as it receives "many" of the points
correctly, the receiver can recover the original polynomial
even in the presence of a "few" bad points.
Wikipedia
EasyPal
software uses the DRM (Digital
Radio Mondiale) encoding to allow the sending of image files
over voice channels and is the work of Erik VK4AES. It is a
relative new program started after the unfortunate loss of
source code for his previous HamPal SSTV program which was
destroyed after storm activity.
