INTRODUCTION

The Internet Radio Linking Project was started back in November of 1997 as an attempt to use the internet to link radio systems across Canada.  The first full time link that was established ran from Vancouver, BC to Saint John, NB. The link had many problems and was shut down in March of 1998 due to the numerous computer crashes and repeater lockups it was causing, and the lack of user control over the system.

I set out to design a better way to use the same technology to perform the same task, while improving usability, user control, and sound quality. My first breakthrough was to replace the existing operating system, Windows, with a more stable and versatile language. I chose Linux, a IBM form of the operating system UNIX, because of its superior networking characteristics, its reliability, and its ease of programming.

I designed my own interface board to interface the radio to the computer. This allowed a large amount of delay to be removed from the system because two VOX circuits were no longer being used . I also wrote my own custom control software, and modified an existing voice-over-IP software package to accommodate the project.

The final product was a combination of hardware and software that created a nearly seamless radio link between two remote sites on the internet. The product works so well that many people can not believe that they are talking through a link at all!

 

How Does IRLP Actually Work ??

The IRLP uses a Voice-Over-IP (VoIP) streaming software called Speak Freely. Speak Freely is very similar to other VoIP software packages (such as Microsoft NetMeeting and VocalTec Iphone) with one difference... It runs under Linux.

Linux is the operating system of choice for the IRLP system as it allows the best in reliability, programability, efficiency, and functionality. Most IRLP nodes use RedHat Linux 6.2 as it is a very stable release and runs very smooth on any 486 computer.

The concept of IRLP's use of VoIP is as follows:

  1. Sample the audio using an analog to digital (A/D) converter. The A/D converter used by IRLP is the input source of a standard PC sound card. This creates a continuous mono 8-bit digital stream of raw audio at 8000Hz (64000 bps).
  2. Compress the audio by downsampling the stream and using an 8-bit ULAW algorithm to reduce the size of the stream by a factor of two (32000 bps)
  3. Split the sample into small chucks (or packets).
  4. Transmit the packets to the remote host using a User Datagram Protocol (UDP) stream. UDP does NOT confirm the reception of packets, so it uses a "fire and forget" method.
  5. Receive the packets on the remote host.
  6. Join the split packets back into a 8-bit ULAW stream.
  7. Uncompress the ULAW stream back into an 8-bit raw stream of audio.
  8. Play the raw audio stream through a digital to analog (D/A) converter (the output device of your sound card).

The control software controls the stream using carrier operated squelch (COS) or continuous tone coded subaudible squelch signals (CTCSS) to start and stop the stream. When COS is present, the computer detects it through the IRLP interface board.

The PTT is controlled by the buffer which joins the split packets back into the audio stream. The IRLP interface board receives a "transmit" signal from the computer while there are packets in the buffer, and a "unkey" command when the buffer is empty.

The user interfaces to the IRLP computer using DTMF signals sent over the radio. DTMF sequences are owner programmable, and can accomplish almost any function imaginable. The DTMF signals are detected on the IRLP interface board and sent directly to the computer in binary, where they are converted into numbers. A DTMF software program which I wrote then runs commands on the computer depending on the code entered.

These commands are what start and stop Speak Freely, basically establishing and breaking the link.

And that is, in a nutshell, how IRLP works.

This information was provided by:

David Cameron
IRLP Designer
VE7LTD