1 Installation of Node

The IRLP node was first installed at Adrian VK5ZSN QTH. It was set up using an spare computer using a Version 3 IRLP board. The node was set up as an experiment to see what all this hype was about with VOIP (Voice Over Internet Protocol).

The photo is of Adrian VK5ZSN and Kids with IRLP node 6214 in the back ground just after it was first set up.

Not long after the Node 6214 was operational that AREG found a larger number of people used the node. It was interesting to hear people dial in from all over the world.

During times of JOTA the Scouts use the IRLP Node over the entire weekend. In more recent times AREG has broadcast the audio from NASA TV using the IRLP node and one of the reflectors in the US that relays the audio.

1.1 Dedicated Repeater Link

As the IRLP node was first commissioned, a dedicated link was started being built to connect the IRLP node directly to the voice repeater.

The link radio equipment was modified and put into a couple of boxes. All that was left to do was wire it up. Then the age old problem happened, the members working on the project got too busy with work and other projects and the link is waiting to be finished. One day it will be I am sure.

1.2 Relocation of IRLP Node

The IRLP Node was relocated to Peter VK5TZX house, as he had spare bandwidth to run the IRLP Node. The computer was moved and installed on top of the computer rack in the back room of Peters house.

It was not long before the minister realised another computer had made it's way into the house. It was hoped that the IRLP node could be moved across to a low power computer based around a VIA C3 board.

A number of experiments were conducted by Adrian VK5ZSN and Peter VK5TZX, a problem existed with the IRLP software / audio codec and the VIA sound processor. The IRLP node would play audio fine, but would not put any audio to the internet. It was just a case the driver were not working on the VIA C3.

To cut a long story short, the two were not compatible sound was not reliable on the new computer. Until a new driver could be found the IRLP node would have to run on the old computer.

All that had to be done was wait, as the Fedor team released a new version, and earlier this year we got the VIA C3 board up and going. Now it is just a matter of finding enough time to finish the updated version of the IRLP Interface and install it in the new computer. Then we should be ready to swap the machines over.

2 Update & Repair of IRLP Node

The idea of putting a dedicated link between the voice repeater and the IRLP node had not disappeared. It had just been delayed. What was needed was a way for the IRLP node to be compatible with the AREG standard 15 pin link interface.

The only thing to do was to build an interface board, that would allow the IRLP audio and control lines to be converted to the AREG standard interface. It would not be to hard as all you would need is a couple of operational amplifiers and the balanced line receivers / transmitters.

IRLP Audio Interface Concept Block Diagram

Then the only problem was the AREG linking / repeater system using a flat audio approach. So the IRLP node would need to have de-emphasis and pre-emphasis built in as well.

The board was designed by Peter VK5TZX and a lot of work was done on the modeling of the de-emphasis and pre-emphasis networks. The frequency response of the shelving amplifiers can be see in the following diagrams. The IRLP Audio Interface also block diagram was developed to include the necessary de-emphasis and pre-emphasis networks, for details see the following diagram.

The de-emphasis and pre-emphasis circuits were developed using a shelving amplifier, and the following diagrams show the circuit and expected response from the filter stage.

De - emphasis Circuit Number

Shelving Amplifier Low Pass Design

As this circuit was tested on the IRLP node, it was found to have not enough cut in the audio being sent to other nodes. With some investigation I found the audio response corner frequency should be around 300 Hz not the corner frequency of 3KHz. Changing the 1nF cap for a 10nF capacitor fixed the issue.

Of course checking the TIA/EIA 603 standard would quickly have told me what it should be. The only problem was I would have to purchase the standard to find out, at a cost of $300 AUD. In all I think it is close enough for the moment and maybe one day when I get a copy of the standard I will adjust the corner frequency a bit to fix the issue.

Pre - emphasis Circuits IRLP Interface

Inverting Shelving Amplifier High Pass Design

As you can see, the plots of the two amplifiers are opposite to each other and provide a good 50uS de-emphasis and pre-emphasis. However after some testing, the frequency response was found to be incorrect, and not enough boost was provided by the filter at frequencies above 300Hz.

Once again I had got the frequencies wrong, the corner frequency should be 300Hz or close to, where I had made it 3KHz. A small change of the 1nF cap to a 10nF cap fixed the problem.

When I get a chance, I will load up P-Spice again and re-model the plots. Just last time I installed the copy I have it crashed my machine in such a way it was a F-Disk and start again from scratch.

Other aspects of the design also included electrical isolation between the computer and the radio of the IRLP interface. This was achieved by using opto couples and 600ohm to 600ohm transformers. The radio side of the IRLP interface is powered by the radio power supply, where as the computer side of the IRLP interface is powered by the computer power supply.

The 9 pin D type connector on the IRLP interface has be configured to match the 9 pin D type on the IRLP Version 3.0 Board, thus allowing a straight through cable to be used.

Refer to the updated IRLP Audio Interface V0_3 Schematic.pdf for detail on the IRLP Interface schematic diagram. The overlay for the IRLP Audio Interface V0.3 PCB Overlay. For the PCB artwork is not been included, as the design has problems that have been fixed in revision 0.4. A new version was developed which solved some audio interfacing issues between opamps. Refer to Version 0.4 for details.

As most IBM compatible personal computers have at least some space inside the case. The IRLP interface can be mounted inside the case and wired to the back panel of the computer. One day when I get time I may get around to designing a surface mount PCB for this board. As it is a lot easier to make and not drill as many holes.

The following photo shows the IRLP interface PCB installed in side IRLP Node 6214 computer.

The power cable and the AREG Standard Interface cable run from the right hand side of the IRLP interface PCB to the rear of the computer.

The power cable has a 2 pin Ultilux male connector installed in the rear of the computer, a spare 12Volt DC feed from the radio power supply is plugged in.NOTE !, the computer side of the IRLP Audio Interface must be power from the computer power supply, the radio side can be powered from either computer power supply, or the radio power supply.

The 15 pin D type female is mounted close to the IRLP radio side DC power connector, for detail refer to the following picture.

The picture on the left shows the outside of the computer where as the picture on the right shows the inside of the computer with IRLP I/O Interface Version 1.0.

The DTMF audio, SQL, PTT signals are normally feed into a Male 9 pin D type on the rear of the computer which connects to the IRLP 10 pin header. As the IRLP I/O Interface Version 1.0 was being installed inside the computer. A new longer cable was manufactured using the same pin outs as the shorter cable and a direct connection between the 10 pin header. The cable is not suitable for the IRLP I/O Interface Version 3.0 board, refer to the IRLP I/O Interface Version 3.0 DTMF Led Mod for details on the cable required.

Refer to the updated IRLP Interface V0_4 Schematic.pdf for detail on the IRLP Interface schematic diagram. The overlay for the IRLP Interface V0.4 PCB Overlay. For the PCB artwork please see the IRLP Interface V0.4 PCB Artwork.PDF, if you are not able to manufacture your own PCB and would like a commercial made board, please contact AREG to obtain a blank PCB. A new version of the PCB is under way, as a DC blocking cap and bias resistors were missed between the low pass and high pass filters. It is not recommended that you manufacture the PCB unless you are intending to modify the board after wards.

A NEW Version is on the way, that will make the connection of the IRLP Audio Interface much easier.

1. Remove the 9 Pin D connector, place a 10 pin IDC with the same pinout as the IRLP IO Interface board, so a straight through cable can then be used.

2. Remove the 3.5mm sockets, and replace with 3 pin connectors, much easier to install into a PC case, and more suitable for direct connection to sound cards.

IRLP Radio Modifications

The radio to be used for the link is a Motorola Syntrx. The radio required some modification to get it to work correctly on the required part of the 70cm spectrum. The installation of a DX Interface PCB inside the Syntrx radio provides the AREG standard interface levels and control signals. The following picture shows detail on where the DX Interface was installed.

As the syntrx was a link type of radio, it came with a 15 pin D type female connector all ready installed on the front. A simple modification to the radio involving two 240 ohm resistors in parallel, turns on the radio with requiring a microphone plugged in.

AREG has professional made PCB's available for purchase, please refer to the sales section of the web site.

Picture of professional made PCB for IRLP Audio Interface & DX Interface PCB's

The boards and radios shown above are for a couple of IRLP nodes in the northen part of South Australia.