I found some information at the HFLink website describing what is currently known about a rather old protocol - CCIR 493-4 which was the basis of the later ITU R M.493-xx GMDSS DSC, about which I've learned a great deal over the last few years.
There's scant information about the older CCIR protocol, although it forms the basis for the majority of Selcall systems in use by "land mobile" HF users with radios from manufacturers such as Barrett, Codan etc. These manufacturers have enhanced the basic Selcall with their own in-house functions, some of which are compatible with other radios, some only work with radios of the same make. No technical details seem to have been published of these enhancements.
In its basic form CCIR493-4 is similar to MF/HF DSC - it shares the same modulation scheme (170Hz shift, 100baud FSK) the same FEC interleaving (each symbol is repeated after four other symbols), the same dotting pattern for bit sync, the same basic phasing pattern for word-sync, and a very reduced subset of "format", "category" and "EOS" symbols. I modified my Python script which I recently wrote to generate HF DSC transmissions - removing un-needed functions etc.- so that I could experiment with CCIR493-4 over the air. Unfortunately, without access to a real Selcall-equipped radio, the only way of testing was to find a software decoder that could understand the protocol. YaDD would show the symbols, but of course the overall message format is not understood by YaDD. MultiPSK has a "493-4" function as part of the "GMDSS" decoder, but only very basic messages are understood. The only other software decoder seems to be "Sorcerer" which has a wide range of modes, including various flavours of HF Selcall. I discovered quickly that Selcall has an inverted tone sense, compared to DSC, so the mark & space frequencies have been switched in the generator.
A recording of a Selcall made using the Python generator is here:
The current GUI looks like:
There is also a CW ID function, which sends "FSK CW" of arbitrary text (generally "de GM4SLV") for identification on the amateur bands. It sound like this:
The current version of the Python program is here:
it runs on Windows, and Linux, although I struggle with the arcane details of the Linux sound-system so I tend to use it mainly from a little Windows XP netbook. There is an initial delay before the Selcall message is transmitted, due to the calculation of every sample-value in advance. The FSK modulator uses "Continuous Phase Frequency Shift Keying" to avoid sudden phase jumps at the boundaries between bits, and at the moment the final waveform is calculated in full before being fed via the "Default Soundcard". I use a Signalink USB interface with no need for any software PTT control etc, although I have also written a version that has direct control of an Icom IC7200 using CiV commands, but that's on the back burner at the moment.
I've successfully transmitted Selcall on 60m and 30m, being received by G4VLC, also using Sorcerer to decode:
[2015-03-22 13:23:16] [ BARRETT 22/03/2015 13:23] [2015-03-22 13:23:16] Format: Beacon Request [2015-03-22 13:23:16] Destination Address: 1234 [2015-03-22 13:23:16] Callers Address: 5678 [2015-03-22 13:23:16] Category: Voice [2015-03-22 13:23:16] 123 12 34 100 56 78 117
[2015-03-22 14:13:10] [ BARRETT 22/03/2015 14:13] [2015-03-22 14:13:10] Format: Beacon Request [2015-03-22 14:13:10] Destination Address: 5699 [2015-03-22 14:13:10] Callers Address: 5678 [2015-03-22 14:13:10] Category: Voice [2015-03-22 14:13:10] 123 56 99 100 56 78 117
[2015-03-22 14:13:34] [ BARRETT 22/03/2015 14:13] [2015-03-22 14:13:34] Format: Selcall [2015-03-22 14:13:34] Destination Address: 5699 [2015-03-22 14:13:34] Callers Address: 5678 [2015-03-22 14:13:34] Category: Voice [2015-03-22 14:13:34] 120 56 99 100 56 78 117
Now I'm hoping to find some other amateur radio Selcall users, to get more experience, and find out more about the details of the protocol. Hopefully one day we'll have a software implementation of two-way Selcall, otherwise the only way to use it is to buy a purpose-built radio - but Codan and Barrett radios don't come cheap.