Robust Packet

History and how to access VAPN

Robust Packet

The protocol for all VAPN HF ports is the 500 Hz wide Robust Packet mode.  Some good resources include…

Robust Packet is spectrum friendly

The SCS Robust Packet AX.25 signal fits well within a 500 Hz window making for a good neighbor on the amateur radio bands.  Here is what an example packet sounds like…

 

Here is the spectrum of the above Robust Packet (RP) signal compared with a traditional 300 baud HF Packet (HFP) AFSK waveform…

Spectral comparison of SCS Robust Packet (red) to HF Packet (blue)

Notice Robust Packet’s eight individual tones uniformly spaced around a 1500 Hz center frequency compared to a single tone switched between two frequencies in the HF Packet trace.

Here is a portion of the Robust Packet waveform in time from the same signal that produced the red spectrum above.

Robust Packet audio waveform in the time domain.
Robust Packet audio waveform in the time domain.

This is a good example of a signal with well behaved “baud” transitions to help manage the bandwidth.  In contrast here is the waveform of 300 baud HF Packet…

300 baud AFSK (HFPacket) audio waveform in the time domain showing two tones.
300 baud AFSK (HFPacket) audio waveform in the time domain showing two tones.

The spectral efficiency improvement of Robust Packet over HF Packet is evident when we zoom out in the frequency domain…

Spectral comparison of SCS Robust Packet (red) to HF Packet (blue) across voice channel.
Spectral comparison of SCS Robust Packet (red) to HF Packet (blue) across voice channel.

Simplistic two tone AFSK modulation schemes combined with AX.25 had their day in amateur radio shortwave digital communication.  Nostalgia aside, it’s time to retire these antiquated HF modulations and let DSP techniques produce true narrow band signals purpose built to survive the vagaries of ionospheric radio paths.  The Joy Taylor WSPR tools have redefined narrow band low payload communication near the noise floor.  Pactor III and IV rule the roost with regards to moving relatively high data volume in the voice channel of an HF radio circuit.  Robust Packet occupies the sweet spot in between these two techniques offering functional data speed in a small 500 Hz wide footprint.

Robust Packet once had a cost

Before October 2020 the least expensive way to use the superb Robust Packet (RP) mode from SCS was to purchase their SCS Tracker DSP TNC.  Early 2020 SCS announced discontinuation of the Tracker leaving only the more expensive SCS modems available to work RP.  Rather than letting the thrifty, narrow (500 Hz), efficient (FEC), AX.25 compliant replacement for HF Packet go extinct, one of the SCS engineers worked on a sound card version of the Tracker TNC.

Robust Packet now has no cost

The summer of 2020 produced two ports of the Tracker DSP TNC: WinRPR and TeensyTNC.  The Teensy effort does function, but is far from release.  On the other hand, WinRPR, while currently an alpha release product, provides the basics of a hardware TNC as a Windows executable that will work with any sound card connected transceiver.

WinRPR available to all

Read this release note we published in October 2020 after someone in the EU leaked the news.

Download WinRPR

Exempt WinRPR in your virus checker

As is often the case with esoteric ham radio software, you may have to convince your Windows computer’s virus protection system to allow WinRPR to run by adding it to the exemption list.  I have to do this with Avast.

How to use Robust Packet and WinRPR to access VAPN

One of our patrons graciously put together a handy guide detailing the steps required to access VAPN using your soundcard interface transceiver and Windows.