I thought I would send a note to the list about what I've built recently so that if anyone else is interested in building something similar we can trade notes.
I have a ham UHF repeater system in the San Francisco/Monterey Bay area ( more info at www.wb6ece.org ) with 6 sites all sharing the same frequency. For years it has been CTCSS-select of which site you'll go through, but I've long wanted to link the sites together, use a voter to pick the best receive audio, and simulcast the results back out, but it took several clever (at least in my opinion :) ) ideas to get it to happen. (Not to mention a number of false starts with three different attempts to use 420 MHz analog linking, some self-designed and built PIC microcontroller-based audio delay and signalling boards, and other interesting and educational experiments that were ultimately dead ends) At this point we have three of the sites online with the new linking system and it works even better than expected. Audio artifacts even in heavy-overlap areas are almost nonexistent. A "proper" simulcast system would use low-level transmit sites to reduce the overlap, but we've got mountaintop sites that have nearly complete overlap of, for instance, the San Jose area, and it really works well. So what were the clever parts? 1. Transmit frequency control. We use Vertex VXR-5000 repeaters. The transmitter is modified by removing the internal 12.8 MHz TCXO and hooking the PLL chip reference up to a surplus HP Z3801A GPS-disciplined 10 MHz reference oscillator. Then a PIC microcontroller is installed to jam the new divisor settings into the PLL chip over its control lines. No need to synthesize up a 12.8 MHz intermediate, as 441.300 MHz (our transmit frequency) is easily reached with a 10 MHz reference. Cost is the PIC chip, a capacitor, and some cable and connectors. 2. GPS-locked VoIP linking with software voting. The linking is all done over IP, by digitizing the audio at each site, computing the noise level, sending the audio and the noise level to a central site as VoIP packets, and sending the best choice (lowest noise level) packets back out to all the transmitters. But for that to work, the transmitter audio needs to be in phase, and for inaudible voter switching the receiver audio needs to be in phase as well. So what we do is use a 4-channel 96 kHz sample rate audio card (M-Audio Delta 44) at each site, plugged into a Soekris net5501 (500 MHz Geode running Linux, 512 MB of RAM, 2 GB of Compact Flash instead of disk, no fans). One channel listens to the 1 PPS signal from the Z3801A GPS, one channel listens to a looped-back signal from the output of the card, and that leaves two full-duplex channels for linking. A software FLL/PLL algorithm to align the input audio sample time with the 1 PPS and the output audio sample time (as seen via the loopback connection) with the 1 PPS and we get about 10 microsecond phase accuracy in both directions, which is adequate for simulcast. All in software, no specialized DSPs or hardware to clock the audio card or anything. And once the audio is digital, everything else (tone detection and generation, muting, filtering, input and output gain) can be done in software as well. So off-the-shelf hardware, plus a cable built to plug everything together, turns it into a pure software problem... and we've only just started on what's possible in software. Feel free to comment or ask questions. Matthew Kaufman, KA6SQG [email protected] http://www.wb6ece.org ps. Once I catalog everything I have, I plan to sell off all the unused 420 MHz radios and related gear I've accumulated in the learning process. pps. We're also in the market for used VXR-5000 repeaters, and particularly, Pacific Research RI-210 controllers, for system expansion.
