Interfacing VarAC to your Transceiver

Last update: 2026-01-21

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This is the area where most new digital mode users struggle, but once you have configured your particular combination of hardware for digital modes, it is generally easy to migrate between using different digital mode software systems.

It is also important to realise that digital modes operate as a system consisting of different components.  The nature of the components depend on your particular combination of equipment, the first being the transceiver you intend using, and thereafter what additional components you require to have a fully functional digital modes system.

The main components required for a digital mode system consists of:

1. Transceiver - this has the largest impact on how it is done and what is required to complete the system.

2. PTT control.  Controlling the PTT function is accomplished via one of these options:

   • cat control - using specific cat control commands to put the transceiver into TX or RX mode

   • an external digital interface, using either vox (as with the Signalink), or a digital line (RTS or DTR) to control the PTT line (Vox - voice activated system)

   • a USB-to -serial converter to independently control the PTT line of the transceiver

Without this functionality you cannot use digital modes.

3. CAT Control, which controls

   • frequency/mode as the tranceiver has to change between the Calling Frequency and a Working Channel

   • PTT - when controlled via the cat control

4. Audio Signals

This provides the intelligence for digital communication, i.e. the modulation, and it contains the audio signal which is either received from the transceiver, or the signal which is sent to the transceiver.

Caveat:  A strong signal in digital modes does not necessarily result in a good SNR decode result.  A clean signal provides better SNR values than a strong signal !!

The audio levels have to be set correctly to minimize distortion of the signal and to ensure the best possible de/modulation.  Poor audio results in poor SNR values for the encoding or decoding of the signal.  Any distortion of the signal results in less than optimal performance.

Audio levels have to be managed for the received signal and the transmitted signal.

4. • The Received signal volume levels is the easiest to adjust using the Windows Sound Manager

   • The transmitted signal is more difficult to configure.  Ideally, using a transceiver's ALC meter function, set the transmitted signal level to just toggle the first bar of the signal level meter.  

If you don't have an ALC meter, check the power meter and adjust the level to bring the power setting to its optimal value for the current power setting, and then reduce the volume level by 10% to 20% to ensure the signal is not over modulated.  Over modulated signals result in distortion and consequent poor SNR values.

• RFI is the main cause of instability

USB interfaces are prone to the effects of radio frequency interference (RFI) and is mostly the cause of any instability when using VarAC and the transceiver.

Having a high quality USB cable, especially a screened USB cable, assists in minimising instability caused by RFI.

The routing of the USB cable may also assist in minimising the impact of RFI.  Keep the USB cable as far away from the antenna coax cables as possible.  Any antenna system with high SWR will also be problematic with respect to RFI.

It is essential to have torroids on the USB cable to reduce the impact of RFI, ideally one torroid close the the PC and another close to the transceiver.

• The PC which runs the VarAC application

  • Single USB cable to transceiver - for cat control and audio interface

  • USB cable to digital mode interface device, e.g. Signalink, home made interface

  • USB 2 serial converter - for cat control - and external USB card for audio interface

• The Interface

The interface is dependent on the particular transceiver used:

• • Transceiver with built-in USB audio and cat control interface

Having a single USB cable is the cleanest and easiest way of managing the environment.

• • Transceiver with only a USB cat control and a accessory interface

  • Transceiver with serial interface for cat control and an accessory interface

  • Transceiver with serial cat control interface, with not accessory interface, but using the microphone and external speaker interface

  • Transceiver with no cat control

While it is technically possibly to use a transceiver that does not have a computer control interface, this will be tedious to use, since VarAC by its very nature often changes frequencies, especially between the main calling frequency and one of ten working frequencies.  

You can manually change between calling channel and working channel (using either the VFO or changing between pre-set memory channels), but ultimately is slow and therefore delays operation.  In addition, an operator has to always be at the transceiver and be ready to respond to VarAC instructions.

• The Transceiver

Each transceiver is unique in the way it is controlled and managed by the VarAC software and the different options of interface hardware also influences the manner in which VarAC is configured, how it controls the transceiver, and how the PTT and audio signals are managed between the PC and the transceiver.

• Minimum PC Requirements

Your PC has to decode the VARA signal without being negatively impacted by a processor which is over burdened.  When the CPU utilisation exceeds 80% it becomes more and more likely that data being sent or received cannot be decoded without errors resulting and consequently poorly decoded packets may have to be re-transmitted due to the error correcting protocol built into VARA.

Your minimum screen resolution has to be capable of displaying the full VarAC screen interface.  The minimum screen requirements is 1280x768.

• CPU/Performance requirements

Ideally your PC's should not exceed 50-60% utilisation to ensure it has reserve capacity for other Windows tasks.

I would steer clear of older i3 Intel CPUs.

Some of the newer generation CPUs may be of lower spec, since these CPUs are more efficient.

Be particularly specific when trying to match new mini-cube PCs with your requirements.

• Memory Requirements

For Windows to perform adequately, the minimum memory installed should not be less that 8 Gigabyte.  Yes, you can run Windows with 4 Gigabyte, but as with compromised CPUs, your system may start struggling dealing with additional background tasks and consequently not be able to decode VARA data without additional retries or faults.

• USB Ports

Your PC should have USB2 ports as a minimum.  Yes, you can use USB1 ports, but there may be data bottlenecks when specific activities require more data to be sent and received.

This is where the greatest diversity could exist, but analysing it using building blocks it is more manageable.

The main components which has to be controlled consists of:

• CAT control to manage the frequency and mode the transceiver is operating on.  If the transceiver permits the PTT to be managed via cat control, this is the preferred option.

• The PTT may be controlled via three options

  • via CAT control - as above

  • via a serial port line toggling either the DTR or DSR line, which controls the PTT

  • via a vox facility built into the digital interface, e.g. the Signalink digital mode interface. Any signal on the output from the PC activates a PTT circuit in the digital mode interface.

• Audio input and output from the PC to the transceiver.  Managing of the input and output levels are vital to minimise distortion of the digital signal.

• A Single USB Cable

  • Ensure the transceiver settings correctly controls:

    • the PTT line

    • audio output from the PC goes to the correct input on the transceiver

    • audio input on the PC is received from the correct output option on the transceiver

• A Single USB cat control cable and a Digital Mode Interface

  • The CAT control line manages the frequency and mode the transceiver uses

  • The digital interface manages the PTT line and the audio to and from the transceiver

• A Single USB cat control cable with a home made digital mode interface connecting to the transceiver accessory interface

As the previous option, the building blocks are similar, however how this is achieved differs between the various possible interface options

With this option the audio connects directly to the transceiver's de/modulator, resulting in better de/modulation of the audio signal compared to when connecting directly to the microphone and external speaker interface.

• A Single USB to Serial Cable cat control and a digital mode interface connecting to the mic and speaker on the transceiver

Only use option with transceiver that do not have an accessory or data mode interface capability.

Using this option has the drawback that the microphone and speaker interfaces optimises audio to make it understandable resulting distorting the audio and consequently this option will provide lower performance with the de/coding of digital modes compared to when connecting directly to the de/modulator of the transceiver.

Configuring VarAC to talk to the transceiver
Use these built-in test capabilities within VarAC to test whether the selected options function correctly for your specific system.

From within the VarAC settings option, on the Transceiver tab, you need to be able to complete the following functions once you have selected the appropriate options:

1. Select alternate frequencies from the dropdown list and select the TEST-option.  The transceiver should tune to the selected frequency and mode.  If not, resolve.

2. PTT operation.  Click on the PTT TX and RX option buttons consecutively.  The transceiver should alternate between TX and RX as you select the options.  If not, resolve.

3. Close the VarAC settings screen once the two above options have been successfully completed.

   1. Check the volume level on the VARA screen.  This is the volume level of the audio output from the PC to the transceiver.  The volume level should not exceed 80% on the volume meter.  Use the Windows Sound Manager to set the correct "Recording" level by adjusting the volume slider.

4. Open the Windows Sound Manager screen and open the window tab for the output level.  Select an appropriate calling frequency on the VarAC frequency dropdown list.   

Transceiver with an ALC meter

When ready to commence with the test, ensure you can click on the BEACON button on VarAC, while you can also see and adjust the volume level of the signal output to the transceiver.  

Click on the BEACON button.  The system commences a down-counter and eventually transmits.  The transceiver should go into the transmit mode, and the ALC meter should show some deflection.

Ideally only the minimum change of the ALC meter should result.

If no movement on the ALC meter - increase the volume level of the output signal from the PC to the transceiver.

If too much movement of the ALC meter - reduce the volume level of the output signal from the PC to the transceiver.

You may have to repeat the above process a couple of times until resolved.

Transceiver that does not have an ALC meter

Use the power meter to determine the transmitted signal output from the transceiver.

Check the maximum power setting used, and determine where that reading will be on your power meter as a reference point.

Click on the BEACON button, When the transmitter starts transmitting, check the power level reading.

When too low:  increase the output signal level from the PC to the transmitter to have an adequate power level.

When at the maximum power level:   Reduce the output level by 10% to 20% to ensure your transceiver does not over-modulate with consequent distortion of the transmitted signal.  Find an optimal value.

That's all Folks !!

While this has been addressed indirectly in the above sections, this is highlighted again, and pertains more specifically to the smart transceivers using a single USB cable for digital modes, which provides services for the cat control and the audio interface.

I would recommend that you check existing online configuration information for your specific transceiver as to the configuration settings for digital modes.

Some important aspects to configure in the transceiver's menu system, and these varies widely between different brands and models:

• Select the appropriate output from the radio to the relevant output connector 

• Select the appropriate input (the signal that will be transmitted by the transceiver) 

• Some transceivers require the main mic to be disabled explicitly

• Some transceivers you need to specify which input line triggers the PTT, while on other transceivers the PTT is triggered via CAT commands

• You are able to set the volume levels for both the input and output signal used for digital modes

Windows Sound Control Shortcut

Initially when setting up the audio interface, and later when managing the audio interface you require regular access to the Windows Sound Control screen where the volume levels for the radio audio interface card is configured.

Since it is tedious finding the path to the multi-layer Windows interfaces, it is much better to set up an icon on your desktop - or elders - on which you may double click and get directly to the Sound Interface Control screen.

To set up an icon on the desktop, righclick anywhere on the desktop and select NEW, and then "SHORT CUT" - as in the diagram below., 

As shown in the above image, enter the text below into the Target field:

C:\Windows\System32\control.exe mmsys.cpl sounds

Note the space between the commands, one space after the 'exe' and another after the 'cpl'

And in the "Start-in" field, enter the text as shown in the above diagram.

Finally click on the OK-button.

You are prompted to fill in a name for the icon.  Call it "Windows Sound Manager"

And click on the OK-button again.

The new icon appears on the desktop.

To open the Sound Manager application, double click on the newly created icon.

Note the PLAYBACK and the RECORDING tabs on the top of the screen.

• PLAYBACK tab

This displays all the available sound interface cards used to output audio from the PC.  Select the one applicable to your transceiver.

Double click on the icon representing the output from the PC to the transceiver.

• RECORDING tab

This displays all the available sound interface cards used as input to the PC.  Select the one applicable to your transceiver.

Double click on the icon representing the signal input from the  transceiver.