HzDrop

Spectrum · audio inactive

Microphone

Not opened

Input level

—

Link

—

Packets

—

Screen lock

Inactive

Protocol v8

One-way broadcast, no acknowledgements. The sender streams coded packets; the receiver collects until it can rebuild the data, then shows Done. Missed packets never force a restart — any sufficient set works, in any order.

Radio: coherent OFDM at 48 kHz, 2048-point symbols (23.4 Hz spacing) with a 10.7 ms guard against room echoes, 320 carriers from 0.94 to 8.4 kHz, every 8th a rotating pilot. QPSK or 16-QAM with per-carrier channel and noise estimates feeding soft decisions. Clip-and-filter shaping raises average loudness.

Coding: 5G NR LDPC (base graph 2) with soft decoding. Each packet carries 180 bytes plus a CRC-16 bound to its index. Files use a systematic GF(256) fountain code: the first pass sends the original packets once. If the receiver misses some, pressing Send again with the same content sends fresh repair packets (never repeats) — any K packets rebuild K. Short messages are one codeword; sending again adds new parity that is combined with what was already heard.

Sync: every ~3.5 s a burst (40 ms chirp, two identical training symbols, a strongly coded descriptor) announces session, mode and packet index. Receivers can join at any burst; two independent detectors (chirp matched filter and training autocorrelation) find it. A low-level pre-roll wakes power-saving speakers first.

Payload: 1 flag byte, content (optionally compressed with Deflate or Brotli, used only when smaller), CRC-32. Uncompressed text appears live on the receiver as packets arrive. Images are re-encoded to the chosen size before sending.