Acoustic data link · measured over the air
Cyrinx is a research modem that moves real data through the air as sound — from an ordinary laptop speaker to an ordinary phone microphone. No radio, no pairing, no network. Measured, byte-verified goodput: 36.6 kbps MacBook → Pixel 7a, and 39.3 kbps decoded by the shipped C library itself.
One transmitted frame 48 kHz · OFDM, NFFT 2048 · 1.1–23 kHz · 16-QAM r¾ · K=7 Viterbi · CRC-32/block
Every number below is ordered byte-verified goodput: a block counts only if its CRC-32 passes and its bytes match the transmitted payload at the same position — and the clock runs across everything (preambles, pilots, FEC, CRCs, gaps). Bench: MacBook Pro M4 transmitting and receiving in ordinary rooms, phone resting near the palm rest.
For scale: the fastest dial-up modems reached 56 kbps over a copper phone line. This is more than half of that, through open air, on hardware that was never designed for it.
Sound is pressure waves; a speaker can shape them and a microphone can read them. Cyrinx splits the audible spectrum into hundreds of narrow slices (like lanes on a highway) and wiggles the phase and loudness of each lane a little bit, many times per second. Each wiggle carries a few bits. A sharp rising chirp at the start tells the receiver exactly when to start listening, and known reference tones let it learn how the room distorts each lane so it can undo the damage.
Because the transducers lie. The raw channel measures ~52 dB SNR, but the speaker-air-microphone chain has an effective error floor around EVM 0.15 (~15 dB SINR) that no amount of volume fixes. 16-QAM rides comfortably above it; 64-QAM drowns — measured 0 of 339 blocks at EVM 0.173. Drag the slider to see the difference on the constellation.
EVM 0.105
Acoustic channels are moody: move the phone six inches and a clean 48 kbps cell becomes a reverberant mess. A naive receiver simply collapses. Cyrinx is built to lose gracefully — measured across placements on the same bench:
The rescue that matters most: maximal-ratio combining across the receiving phone's two microphones. The mics sit at opposite ends of the device, so the room's dead spots differ between them; combining per subcarrier fills each mic's nulls with the other's signal. At the worst coherent placement measured, neither microphone could decode a single block alone — combined, the closed adaptive loop carried 11.6 kbps with all 75 of 75 blocks recovered by MRC, through the same portable C library that ships, pinned by cross-implementation test vectors.
And when even that fails, a non-coherent multitone floor mode — energy detection with symbols longer than the echo tail, Reed–Solomon-coded with two-microphone energy combining — keeps a measured 138 bps flowing (twice the earlier repetition-coded floor) where phase-coherent modems get nothing at all. Slow is a mode. Silence is a failure.
Data-over-sound is a well-trodden field, and Cyrinx borrows its techniques gladly — OFDM, pilot tracking, convolutional codes are textbook material.
What Cyrinx adds is the measurement discipline: end-to-end byte-verified throughput on named consumer hardware, a catalog of the four physical-layer defects that actually limited the link, a published log of negative findings so dead ends stay dead, and a receiver architecture whose graceful-degradation claims were demonstrated placement by placement — then ported into a portable C core that reproduces the headline number itself.
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