What Do I Need to Operate Digital Modes with FLEX Radios?

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What Do I Need to Operate Digital Modes with FLEX Radios?


What Do I Need to Operate Digital Modes with FLEX Radios?

  System Dependencies
  Minimum PowerSDR Version:     1.6.0  
  Applicable Hardware:     Any SDR transceiver 

Content provided by: FlexRadio Systems Engineering

Operating digital modes with sound card based software using a FlexRadio Radio SDR is a little different than using a traditional  analog transceiver

With a software defined radio, it is the software (PowerSDR) that directly interacts with the sound card based digital mode program, so the primary concern is how do I get the audio from PowerSDR to the digital mode program that is running on the same computer.

There are two ways to accomplish this task. 

One is to use a traditional method of interfacing directly with the radio hardware (in this case, any of the FlexRadio Systems SDRs) using a physical wired interface to send audio (AF) to and from a sound card in your computer.  There are several reasons why this approach, while feasible, is not the preferred method of operating digital modes.  Below are a list of those reasons
  • A wired interface provides multiple ingress points for RF to enter into the radio system allowing for the RFI getting into the computer and the radio hardware.
  • With a wired interface, you must convert a digital signal in PowerSDR to an analog signal and then convert it back to a digital signal for the digital mode sound card program.  These multiple A/D and D/A conversions adds noise and distortion to the digital mode signal, decreasing signal quality.
  • Requires a sound card in the computer to interface with digital mode program
  • The quality of the digital mode signal is directionally related to the dynamic range and bit depth of the sound card performing the A/D and D/A conversions.
When using a software defined radio, we can eliminate all of the deficiencies listed above by sending the AF signals, which are already in a digital format, directly to the digital mode sound card program using a digital audio transfer method

In order to accomplish this, we need  to make in AF input (receive) and AF output (transmit) from PowerSDR look like a sound card to the digital mode application.  Currently, this is accomplished using a third-party application called Virtual Audio Cable or VACVAC creates a point-to-point conduit or audio bridge where audio signals that enter into one end of the "cable" come out the other end.  Each end of the "cable" looks like a sound card device in Windows.  So to transfer audio signals from PowerSDR to a digital mode sound card program, we defined two (2) VAC "cables"; one for the receive audio and one for the transmit audio.

Therefore all you need to operate digital mode with PowerSDR and any of FlexRadio Systems software defined radios is to obtain a legal copy of VAC.

CAUTION: Do NOT use the demo version of VAC with PowerSDR.  The demo version injects noise into the audio stream making reception difficult and generating QRM if you attempt to transmit with it.

For more information on VAC and how it works, please see the following Knowledge Center article, What is VAC and What Does It Do?

In addition there are several very good "how to" articles on how to configure PowerSDR, VAC and various digital mode program with your FlexRadio Systems software defined radio. One of these is How to Setup Virtual Audio Cable (VAC) 4.0x with PowerSDR 1.x

As noted above, VAC is a third-party application and is not provided by FlexRadio Systems as a standard component of a software defined radio.  This software package must be purchased directly from NTONYX by the user.  You can find out more about VAC and how to purchase it from their web site.


This KB article may reference additional files that are available on the FlexRadio Systems web site Downloads page. Please use the URL(s) below to download the referenced materials.

An Adobe Acrobat Reader may be required to open the file. You can download Adobe Acrobat from here.

KB Source Document(s):

None Referenced



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Last Modified:Thursday, May 7, 2009
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Type: INFO
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