How a software modem turns bytes into sound and the sound back into bytes: the modern signal science, examined through a single VARA FM session.
Every digital mode you run uses a modem: something that turns bytes on one end into a sound a radio can transmit, and turns that sound back into bytes at the far end. The same principle powered the early computer networks that sent data over analogue telephone lines. These functions can be performed by a hardware device (TNC), or by software running on a general-purpose computer. Today we are looking at the software modem VARA FM — one of the better ones, and the least documented. It incorporates many of the important techniques used in modern digital communications and because of that it is a great case study for this series.
Before any eyebrows are raised, I’ll acknowledge a couple of well-known issues:
- VARA is a proprietary Windows-only closed-source software, with its higher-speed modes sitting behind a paid license. It doesn’t meet the community’s expectations for the open, interoperable software we would like to see in a hobby centered on experimentation and learning. This criticism, however, would ring hollow coming from somebody who also makes a living selling to ham radio operators. Digirig products are open-source hardware, but physical goods protect a business opportunity far better than software does — releasing the design doesn’t give away the product. My view is that the man put serious knowledge and time into making a software modem that works well and he deserves to be compensated for his work.
- Reflected in the FCC’s rules is the expectation that data on amateur radio bands stays open to inspection. VARA doesn’t encrypt in the traditional sense, but that is not the same as being inspectable. The software doesn’t support a monitoring mode for decoding third-party traffic, and its higher-speed modes sit behind the paywall. This raises the question of whether the software is compliant for use by amateur radio operators in the USA. I’m not going to attempt legal analysis here, but it is worth pointing this out.
- This writeup touches on the details of VARA’s implementation as an academic curiosity and a way to demonstrate the application of signal science in a real-world software modem. I don’t believe it conflicts with VARA’s license agreement. My process for analyzing the modem involved using an AI agent to listen to live VARA sessions, work out their purpose and contents, replay fragments of the audio, and match what it heard against publicly available analysis and existing knowledge in the fields of communications and DSP. Doing this reminded me of the film Project Hail Mary, in which Dr. Ryland Grace teaches the ship’s computer to translate the alien’s language. That said, there is no expectation of the information here being complete or accurate.
This is the first of multiple parts, each covering one layer of a software modem with VARA FM as the case study.
The session
Let’s start with an example of a complete session: here is one entire contact recorded on a single computer running two copies of VARA FM from separate folders, each configured to use its own Digirig, connected with a crossover cable. The whole exchange takes under 22 seconds from the first burst to the last, with 1,024 bytes moved in each direction.
Two kinds of burst
A careful observer will notice that the bursts do not all sound or look alike: at some moments the audio is almost melodic and the waterfall shows a sparse scatter of lines, while at others it is a wall of hiss that fills the band. These are two classes of burst common in communications, and they are the subject of this part; we’ll call them the tone pattern and the phase grid. The two have distinct purposes, and the modem switches between them as the session progresses.
The tone pattern is a kind of pilot signal. Its purpose is to draw the receiving side’s attention and help it prepare for the data that follows. It is conservative with bandwidth and carries little information. Once the receiving side is armed, the phase grid fires up, using as much of the bandwidth as is available to push the payload through as fast as possible.
Both kinds are built from the same raw material. A transmission is a sequence of symbols sent back to back with no gap between them — 24 ms each in a phase grid, a shade shorter (21.3 ms) in a tone pattern — so within a burst there is never an instant without signal. Inside one symbol the modem can put a tone on any of 128 evenly spaced frequencies; the ones it actually uses are its carriers, and what you hear is all of them sounding together.
The two classes side by side
| Tone pattern | Phase grid | |
|---|---|---|
| Role | acquisition and signalling | payload |
| Family | MFSK — the message is which frequency is active | OFDM — the message rides in each carrier’s phase (and, at higher speeds, its amplitude) |
| Information | a few bits, by carrier position | up to 8 bits on every carrier in use |
| Bandwidth | 3 carriers of 49 | the whole channel |
| Receiver needs | an energy detector | symbol timing, a phase reference and error correction |
| Error correction | none; redundancy from repetition and diversity | a rate-1/2 turbo code |
| Failure mode | the detection vote falls short | the checksum fails after decoding |
| Trade-off | robust and cheap to detect, but a very low rate | a high rate, but needs a clean channel |
| First used | the “Piccolo” diplomatic radiotelegraph, 1962; touch-tone dialling a year later | the Collins Kineplex radio modem, 1957; mass use came with DSL |
| Also seen in | FT8, WSPR; Wi-Fi and LTE use similarly sparse preambles for acquisition | 802.11, DVB-T, DAB, LTE and 5G payload; VDSL |
| In VARA | burst detection, addressing, acknowledgements and control frames | all payload, the connect request, the CQFRAME beacon |
Terms used here
| Modem | Modulator–demodulator: turns data into a signal a radio can send, and turns it back at the far end. |
| Symbol | One step of the transmission — in VARA, a slice of sound around 24 ms long. |
| Carrier | One of the evenly spaced frequencies a tone can sit on; a burst uses many at once. |
| Half duplex | One station transmits at a time; the two take turns and never key up together. |
| Pilot signal | A known signal sent to help the receiver lock on before the real data arrives. |
| DTMF | Telephone touch-tones: two tones sounded at once identify a key on the keypad. |
| MFSK | Multi-frequency-shift keying: the message is which of several tones is present. |
| OFDM | Orthogonal frequency-division multiplexing: many slow carriers sent side by side at once. |
| DQPSK | Differential phase-shift keying: data rides in the change of phase from one symbol to the next. |
| Turbo code | An error-correcting code strong enough to rebuild a damaged frame, not merely flag it. |
| Checksum | A short check value that reveals whether a frame arrived intact. |
What’s next
That wraps up the quick overview of the two kinds of burst — in data transmission generally, and in VARA specifically. In the next part we’ll take a deep dive into the tone pattern — the simpler of the two — and see how it works, what information it carries, and how VARA encodes and detects it.
I encourage feedback on the series and welcome questions, corrections and criticism. The comments section is open below.