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:

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 The project is outdated and will not be supported any more.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.

---

Bandwidth

DRM broadcasting can be done on different bandwidths:

• 4.5 kHz or 5 kHz which are half channels. The idea is to offer a possibility for the broadcaster to do simulcast and use a full 10 kHz channel for AM, plus a 5 kHz half-channel sideband for DRM. However the resulting bit rate and audio quality is less (approximately 8-16 kbit/s).

• 9 kHz or 10 kHz which are the standard bandwidth of an AM broadcasting channel so existing frequency plan can be reused (approximately 17-35 kbit/s).

• 18 kHz or 20 kHz which correspond to a coupling of two adjacent channels. It offers the possibility to offer a better audio quality or to multiplex audio channels in the same transmitter (approximately 31-72 kbit/s).

Wikipedia

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)

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

On the downside.

The use of DRM is limited to faster computers running the latest operating
system. DRM Digital SSTV requires a computer that is 500 MHz or faster and Windows XP or Vista.
Although, at times, files with missing segments may be viewed with some corruption, the file saved may be of little use.

A good antenna and a stable transceiver is required. DRM requires a good signal without distortion. Frequency drift must be minimal and audio levels adjusted carefully. Output power is to be set so as not to cause any distortion. The wider bandwidth may be more than some transceivers can handle.

The sending station should pay careful attention to file size or the transmission as time may be excessively long.
The software of DRM is new and under development.
Sometimes errors in data may pass through undetected.
At times, the software may be slow to establish a synchronization lock.

During difficult reception periods a number of BSR requests may be necessary to the originating station to complete a picture. Once completed, the pictures are normally exceptional quality.

2005 Dayton Hamvention
SSTV Forum Presentation on May 21, 2005
Notes by Dave Jones, KB4YZ

 

DSSTV may be heard on the following frequencies(AU).(List is not complete or authoritative)

Lightning tracker

Mystery of the Missing Sunspots, Solved?

---