DIGITAL VOICE AND FILE TRANSFER SOFTWARE FOR RADIO AMATEURS
Digital Voice (DV)
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.
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.
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, comes from a country that one of the operators doesn't like or uses any 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 creativity and originality!
Below are some pictures received in DRM (all actual size)
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 and it has an over-complicated user inerface. 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.