John Wells, M5AML. QTH: Derby, IO92GW.

DIGITAL VOICE AND FILE TRANSFER SOFTWARE FOR RADIO AMATEURS

 

Digital Voice (DV)

There are four pieces of software that have been used for transmitting digital voice on the amateur bands - WinDRM, DRMDV, FDMDV and FreeDV. WinDRM and DRMDV are based on the DRM (Digital Radio Mondiale) system adapted for amateur use by Cesco HB9TLK. FDMDV/FreeDV (Frequency Division Multiplexed Digital Voice) is a system devised by Peter Martinez G3PLX of PSK31 fame.

DRM and DRMDV signals are wider than FDMDV/FreeDV which is only 1.1kHz, but they do incorporate a level of FEC (Forward Error Correction) whereas at the moment FDMDV/FreeDV does not. As you can imagine DRM and DRMDV require a more powerful computer than FDMDV/FreeDV.

On a waterfall display DRM has three distinct lines in the signal, DRMDV has two distinct lines and FDMDV/FreeDV has two very close lines in the centre of the waterfall. After some time one can tell the differences by ear.

FDMDV/FreeDV, DRM and DRMDV are incompatable with AOR Digital Voice, D-STAR and each other!

Digital voice on HF has never really taken off.  The audio quality of the decoded transmission is poor when compared with analogue SSB, and the decodeability of the signal suffers badly even with the slightest QRM.  Legality problems to do with the use of audio codecs has also hindered the development of digital voice software, but a free codec called Codec2 is being developed.  See HERE for details on the new FreeDV digital voice program which supports Codec2.  Digital file transfer is growing in popularity all the time (see below).

File Transfer and Digital SSTV

RDFT (Redundant Digital File Tranfer) was the first popular modern file transfer system which was made more user-friendly with the DIGTRX software. An interesting system, but requires encoding of data files into audio files before transmission and decoding of received data after reception which can be a lengthy process. Along came the amateur radio adaptation of DRM (Digital Radio Mondiale) by HB9TLK. Much faster file transfer, more robust system and no lengthy encoding and decoding.  Because of this RDFT is rarely heard now.

Most files transferred are pictures in JPEG2000 format (.jp2) which is supported by IrfanView.  HTML and text have also been transferred using DRM but it is primarily a digital SSTV system.  Unlike analogue SSTV, DRM pictures can be of almost any dimension because the actual image file is transmitted rather than each individual pixel.

The really clever thing about DRM is that if any part or parts of the transmission are received incorrectly due to interference, fading or any distortion the station that has errors in the received file can send a Bad Segment Request (BSR) and then the corrupted and missing parts can be retransmitted to repair the damaged file.

On a waterfall display DRM has three distinct lines in the signal whereas RDFT has a number of wide data carriers only.

KG-STV is a new, completely different digital SSTV system solely for transmitting and receiving pictures similar to analogue SSTV.  Click HERE for more information.

Frequencies and Operating

Main frequencies here in UK and Europe for digital SSTV (DRM and KG-STV) are 3.733MHz, 5.335MHz, 7.058MHz, 14.233MHz, 21.343MHz and 144.525MHz.  14.236MHz is also used in North America. 

The 80m and 40m frequencies are very active in the evenings, 20m is more active than it used to be in Europe and North America.  Activity can be heard on 144.525MHz FM around the English Midlands mainly in the evenings. and weekend afternoons.  In the East Midlands (England) DRM has been heard around 145.400MHz FM.

Digital SSTV use the same sideband as normal analogue SSB speech i.e. LSB below 10MHz and USB above.   

It has come to my attention that on some 80m and 40m digital SSTV nets when an amateur sends a large picture (long transmission time), makes a mistake, uses unusual settings, or uses any other operating method which the net regulars disapprove of then the "net regulars" start making rude and disrespectful comments and even talk over the digital SSTV transmission.  This isn't in the spirit of amateur radio is it?

Something else that is bothersome with digital SSTV is that operators seem to think they have a right to use images regardless of copyright.  The number of pictures downloaded from the internet then transmitted via digital SSTV is unbelievable.  You must not breach copyright law!  Use your own photographs and drawings!  Come on, let's have some originality!

  

Below are some pictures received in DRM (all actual size)

Software

Have a look at N1SU's web site to download DRMDV and FDMDV software.  Also see VK3EVL's web site for information on EasyPal which is the most popular and versatile program for transmitting files in the DRM mode.  A high-spec computer is required to run EasyPal but it has a more user-friendly interface than the old WinDRM program.  EasyPal uses Reed-Solomon encoding to make the DRM transmissions even more robust.  WinDRM can work with Reed-Solomon encoding if the RS Decoder is installed but this seems to be no longer available to download.  DIGTRX is rarely used now.  

NOTE:   EasyPal does not support digital voice! See HERE for details on the new FreeDV digital voice program which supports Codec2.

Have a look at my KG-STV page 
HERE to download the KG-STV digital SSTV software.

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