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Radio & Space Services. HF Communications

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 (SK RIP). The program started after the unfortunate loss of source code for his previous HamPal SSTV program which was destroyed after storm activity.

The EasyPal GUI (graphical user interface) was written with Delphi and compiled on Windows. It works best when run on a two GHz or faster CPU with Windows. Although less stable, it should run on most fast computers with Windows latest releases. It is designed to be easy to setup and use. DRM as used in EasyPal, allows very fast data transmissions with error correction, enabling very accurate decoding, and a means to request missing blocks, this is all done within EasyPal, and very simple to do. Images up to 1280x1024 are sent in a little over 1 minute, Easypal does compress images to shorten transmission times, but you have the ability to adjust the compression if unwanted material is present in the image.


EasyPal Hybrid process

The new EasyPal Hybrid mode allows the transfer of high resolution images, animated gifs and movies.

Procedure. The transmitting  station uploads the file to a nominated server - then an RF data signal containing the file name and the internet address of the file is sent via the amateurs transmitter -- when this information is received by the remote amateur radio station EasyPal initiates a connection to the remote server and that file is downloaded into EasyPal and then displayed as a picture, gif, file or movie file. If it is not displayable it is saved in the Hybrid Folder within EasyPal.   VK2DSG

This mode takes much less time to receive than the normal mode and is a combination of radio  communication and internet data transfer.


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:

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 plus

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 analogue signal and digital signal.

Wikipedia

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).


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. 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

 

80 meters 3.643 MHz LSB
40 meters 7.208 MHz LSB
30 meters 10.142 MHz USB
20 meters 14.233 MHz USB
15 meters 21.340 MHz USB
10 meters 28.680 MHz   USB
2 meters 145.625 MHz FM/SSB
70 CM 433.775 MHz FM

NB: Frequencies are not conclusive or official


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Last updated 01/09/17


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