Full CTCSS And Closed Selcall On Simoco Radios

[Rob_Brookes]

As part of the on-going research into what's possible and what's not, I've recently programmed a Simoco SRP9130 portable radio and an SRM9030 base set to operate using full CTCSS and closed selcall on receive and transmit. This boils down to the radios must send the correct CTCSS tone and they must also send and decode the correct selcall sequences in order for the audio mute to open and the transmission be heard at either end.

Once this is set up correctly operation of the radios is completely silent as far as FFSK data and selcall tones are concerned. These are not heard at all. The combination of the correct CTCSS tone and selcall sequence opens the mute gate in the receiving radio and the voice signal is heard as normal. Due to the way in which Simoco radios handle the FFSK data, this is still received by the base radio and decoded correctly. It's carried out in complete silence however. MRMap will continue to receive position reports and correctly poll the portables.

Note that this is not permitted on the UKSAR channels, (unless we all use the same basic selcall sequences which is not beyond the bounds of reason), but is allowed by the UK-General (Business Simple) license and on any non-UKSAR channels you might be using. The level of external interference is significantly reduced using both these methods of muting but as each portable radio has a unique identifying number for the selcall sequence, (I used the MRMap own data address but you wouldn't normally), it can be identified by a 'Status List' look-up table in the base radio and the name of the caller is displayed on-screen. The same can be done at the portable end but this is of less value as the radio is often in a rucksack and the screen is not visible.

By using a selcall sequence unique to each base ( the MRMap 'Base Address' being the obvious one as it's different for each team), then the transmitting base can be identified by the receiving portables if this is required. If you find you're being polled, you would know by whom.

Some teams already have a system similar to this but entailing the use of open selcall where the transmitted tone sequence is not used as an audio muting system. If done this way, the selcall tones are audible and irritating. Used correctly, selcall was not meant to be heard in the radio receiving the call and this means closed selcall has to be used. If the decode sequences for the portable radios include the 'wild-card' status value '*' then all portables will receive calls from each other without first having to dial up the person you wish to speak to. Alternatively the group call letter, usually 'A' can be included in the transmitted sequence and again, will be heard by all radios. Anything else on the channel will be ignored by the radios.

Using the system this way, some fairly lengthy selcall tone sequences can be used so giving an increased level of isolation for your radios as no matter how long they are, you're not going to hear them. I used 5 digits but it would be easy to test it with more. Although it's possibe for the base to transmit a second selcall sequence which re-closes the mute gate, I used the reset timer set to one second. This allows the name of the incoming caller to be displayed for a couple of seconds before the display reverts back to the channel in use. If you don't put a name in the status list then the word 'Mobile' is automatically substituted by the radio itself so something always comes up on screen.

I've not produced any sample configs for this one but if anyone wants some, let me know and I'll send them to you. I just need to change the channel on which these tests were carried out and the configs would be on one of the UK-General channels. Any identifying numbers would be unique to these configs so once you've got the idea, just change them to what you prefer. Same for names. It will be mine you see so just go through the programming software pages until you see it and change it to your own etc etc.