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Homebrew Phone Patch & SignaLink adapters

posted Apr 2, 2012, 11:13 AM by Charles Boling   [ updated Apr 2, 2012, 12:26 PM ]
Two geeky fun things happened at my house this weekend (besides dragging a couch into my office and watching & listening to the C-band satellite feed of General Conference in all of its hi-def glory with the older kids; I only have one computer in the house with enough "oomph" to handle the new H.264 BYU-TV feed.  Satellites are of interest to radio amateurs too, but it's another topic!):
  1. The kids ran around the house talking to Grandma & Grandpa on their FRS radios -- even though Grandma and Grandpa were in San Diego
  2. I checked into this net from a telephone


For several years I've thought I'd like to connect my radios with my phone system.  I run an Asterisk PBX at home, so it opens up all sorts of fun possibilities.  I have an analog card that connects it to the analong phone line from my telco (and 3 groups of analog phones in the house), but also take advantage of its VOIP capabilities; I maintain an Internet link to my employer's office in Camas, and when my wife and I traveled to China in 2011 to adopt Jonathan, I bought a cheap data plan over there for my Android phone (T-mobile uses GSM, so my phone works world-wide) and used SIPDroid over that or free Wi-Fi connections to log in as an extension on my home system over the Internet.  Free calling both directions, and the casual caller didn't know that I wasn't home.

Connecting the radio to Asterisk means that I can not only provide telephone service over the radio, but that I can access the radio over the Internet (think Echolink) or from any telephone.  The reach and mobility of both services are extended.

Redneck Echolink Interface

My interest in digital modes and telephone system interfacing was revived after purchasing a SignaLink USB to supplement my crude home-made soundcard interface (pictured right; click pictures to enlarge) which I originally created to use for EchoLink.  Your classic phone patch consists of an analog audio matching network directly between the radio and the phone line (similar to the interface between the radio and a sound card) with additional DTMF-decoding circuitry for control.  (They are most often part of a repeater system, allowing not only allowing phone access to the repeater, but also providing out-of-band control of the repeater.)  However, if you already have a computer-radio interface, the computer can connect to the phone system using various VOIP protocols like SIP, H.323http://en.wikipedia.org/wiki/H.323, or IAX.

I had always assumed that I would set up another Asterisk server in a virtual machine, and connect it to the radio.  I suddenly realized, though, that I didn't need another server for basic operation -- a client was sufficient -- all I had to do was run a "soft phone" on my PC (already had several installed), have it log into my main Asterisk server using SIP, and set its "speaker/microphone" devices to the radio interface.

Ekiga softphone


Accordingly, I launched a copy of copy of Ekiga (pictured left), which automatically registered with Asterisk, and changed the audio settings to point to the radio interface.  I picked up another phone, called the appropriate extension, and found myself listening HF QSOs over the phone!  I adjusted the volume levels in Ekiga and on the SignaLink, and was able to talk on transmit as well.  The next step was to take it to other radios.

The annoying thing about Tigertronics' pre-made radio cables is that they only make one per connector type, and you must reconfigure jumper wires inside the interface to handle the different pin-outs.  Not only is this inconvenient, but the wires are delicate and you can't swap things too many times before they break.  Of course, they'd be thrilled if you took the ultimately convenient route and simply bought a SignaLink for each of your radios!  The other option is to make your own custom cables.

Adapter cable


Being both cheap and lazy (and not very good at soldering small connectors, which is one of the reasons I bought a pre-made cable in the first place!) I went another route:  An adapter cable!

I dug into my telecomm junk box and found a slim surface-mount jack enclosure.  By comparing the jumper specifications for the different cables/radios, I figured out what wires I wanted to move where.  A jack, a short cable scrap, a modular plug, and 10 minutes later I had my first assembled adapter cable!  Now when I want to switch radios, I can simply insert/remove adapters as appropriate.

Play Time

The first test was the GMRS radio.  One of the kids was already talking to their grandparents on the phone, and several of the others and I thought it would be a hoot if we connected their call to the radio.  I plugged the adapter into my GMRS base station (WQNY420; yes, in spite of my thrift, I'm one of two people in my ZIP code stupid enough to pay money so we can legally operate w/ GMRS frequencies/power) and conferenced the radio's extension in.  Voila!  The rest of us were listening to the conversation on a little FRS HT!  Next thing you knew, kids were running around the house talking to grandpa on the things.

The quality was pretty bad, not only because there are some audio quality problems between the whole radio - phone system interface (One of these days I'll put a scope on it to see what's up where) but mostly because the sound quality of the little FRS/GMRS radios I have are downright terrible; I can't wait for my kids to get their ham licenses so I can justify buying them real radios so we can understand each other.  But the whole point was that we could do it, not to make them sound better, and we were tickled.

The Net

I had an adapter ready and hooked up to my 2m rig (though not tested yet!) about 2min before the net last night.  I grabbed a nearby cordless phone (I could've used my cell phone just as easily, and it would've probably sounded better than the cheap phone I did use, come to think of it!) and dialed the magic extension, and listened to Rick reading the preamble over the phone.  I kept it muted so the vox didn't accidentally trigger the transmitter until it was my turn to check in.  Knowing that the audio was a bit weak, I spoke fairly loudly into the phone as I checked Michele and I in.  He reported me as weak but quite understandable.

Experiment over, I turned up the radio volume, shut off the SignaLink, and hung up the phone, continuing participation in a more traditional way.

What's next?

First, I need to improve the voice fidelity; it should sound better over the phone than it does.  Then, improving control ability is on the hit list; right now, I have to be at the computer to manually establish a call or accept an incoming call for the radio.  I plan to write some Asterisk dial plan scripts (and maybe use a different client) that will allow the SIP client to either remain connected to the PBX full-time so that it can accept coded commands from the radio, or allow a program (either the SIP client or a separate utility) to monitor the radio for DTMF tones and "pick up the phone" to make a call, and allow initiation of calls (both inside and outside my system) 100% from the radio link, while preventing unauthorized use of either phone or radio.

EmComm Application

Though I did this mostly for fun, and a little for future "casual convenience", I can see potential application under stress too.  While the need for phone patches has waned with the proliferation of cell phones, there can be situations where it would be nice to plug your laptop into a phone line or Internet connection, and provide a phone link to people in the field.  They can make personal phone calls, remote leadership or resource suppliers outside of radio access can talk to people in the field (everyone at once -- remember, open radio was the original "conference call"!) -- I'm sure you can think of more possibilities if you put your mind to it.

Charles Boling,
Sep 11, 2013, 4:35 PM