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1. 

   David Rock

   June 25, 2015 at 1:09 am

   Here are some more details about the echolink project.

   Echolink is a technology to internet-connect ham radios. The main URL for it is: http://www.echolink.org

   The basic concept is to allow RF radios to send/receive audio through PCs that are connected to the internet. This allows an operator on a radio to talk to another operator on a radio anywhere else in the world.You can also talk directly through the echolink software, similar to skype audio. The audio flow in my specific example is:

   RF handheld RF Repeater RF receive radio RPi Internet iPhone

   So when I talked to my friend, my audio went from my iPhone to his
   handheld along the above path, and his came back in the opposite
   direction. Normal repeater traffic would be:

   RF handheld RF Repeater RF handheld

   The issue I have is the official echolink software requires a Windows
   PC. Not only is that overkill, it’s an unnecessary power waste and
   takes up a lot of space. I got a deal on a Pi B rev 2 so I figured I’d
   try using that as the PC portion of the echolink connection. It turns
   out that there is an open-source echolink clone called svxlink that runs on linux (it actually does more than echolink; it’s a general-purpose VoIP software). svxlink is the brains for the whole system. It monitors the audio from the radio and the internet, determines when to transmit a signal, and controls the radio through an interface board.
   The URL for svxlink is: http://www.svxlink.org

   There were still a couple challenges with connecting the radio to the
   RPi. The biggest ones being; how to control the radio to tell it to transmit audio coming in from the internet, and how to know that audio is coming from the radio that should be sent out the internet. svxlink is the software, but you need hardware, too. This is handled by an interface board that has push-to-talk (PTT) controls and the ability to detect active audio signals. The interface I am using is called Easy-Digi. http://goo.gl/5ZYMx8

   It’s a very inexpensive kit developed by a Ham that can be used for a
   variety of digital radio modes. It can be bought as a kit or assembled. It’s brain-dead simple and very effective. It uses an RTS signal to trip an Opto-isolator to engage the PTT on the radio, and takes incoming audio and feeds it to the RPi.

   Another issue is the RPi does not have mic input, so I’m using a cheap USB soundcard for audio in/out to the radio.

   One last thing that I’m doing that I did not mention during Show and
   Tell is I’m using a PowerSwitch Tail II as a failsafe. Because I’m licensed by the FCC, I’m responsible for any transmissions going through the system (and potentially retransmitting through a repeater and jamming the airwaves), I have the RPi configured so I can physically kill power to the radio using the PST II if something goes horribly wrong, even if I’m not at home.

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