fm-rds-tx

Go-based FM stereo transmitter with RDS. Supports ADALM-Pluto (PlutoSDR) and any SoapySDR-compatible TX device.

Status: v0.6.0-pre — Hardware-ready

What works

• Complete DSP chain: pre-emphasis → stereo encoding → RDS (IEC 62106) → MPX → limiter → FM modulation
• Real hardware TX via SoapySDR CGO binding (PlutoSDR tested)
• Continuous TX engine with Start/Stop/Stats
• IQ resampling (composite rate → device rate)
• HTTP control plane with /tx/start, /tx/stop, /runtime
• 82 passing tests including spectral verification

Signal path

Audio Source → PreEmphasis(50µs) → StereoEncoder(19k+38k) → RDS(57k)
→ MPX Combiner → Limiter → FM Modulator(±75kHz)
→ IQ Resample(228k→528k) → SoapySDR → PlutoSDR RF

Build

# Without hardware (simulation/offline only):
go build ./cmd/fmrtx
go build ./cmd/offline

# With SoapySDR hardware support (requires PothosSDR installed):
go build -tags soapy ./cmd/fmrtx

Usage

List available SDR devices

.\fmrtx.exe --list-devices

Offline IQ file generation

.\fmrtx.exe --dry-run --dry-output build/dryrun/frame.json
go run ./cmd/offline -duration 2s -output build/offline/composite.iqf32

Real TX (PlutoSDR)

# Start with manual TX control via HTTP:
.\fmrtx.exe --tx --config docs/config.plutosdr.json

# Start with auto-TX on launch:
.\fmrtx.exe --tx --tx-auto-start --config docs/config.plutosdr.json

HTTP control

POST http://localhost:8088/tx/start    → start transmission
POST http://localhost:8088/tx/stop     → stop transmission
GET  http://localhost:8088/runtime     → engine + driver telemetry
GET  http://localhost:8088/status      → config status
GET  http://localhost:8088/config      → full config
POST http://localhost:8088/config      → patch config (freq, RDS, etc.)
GET  http://localhost:8088/dry-run     → dry-run summary
GET  http://localhost:8088/healthz     → health check

PlutoSDR notes

• Device rate: 528 kHz (PlutoSDR minimum ~521 kHz)
• IQ format: CF32 (float32 interleaved I/Q)
• Gain range: 0–89 dB (outputDrive 0..1 maps to 0..89 dB)
• SoapySDR driver name: plutosdr
• Requires: PothosSDR or SoapySDR + SoapyPlutoSDR plugin installed

Repository layout

cmd/
  fmrtx/          main CLI (--tx, --dry-run, --simulate-tx, --list-devices)
  offline/        offline IQ file generator
internal/
  app/            TX engine (continuous chunk loop) + simulated transmit
  audio/          sample types, WAV loader, resampler, tone generator
  config/         config schema, validation, PI parsing
  control/        HTTP control plane (/tx/start, /tx/stop, /runtime)
  dryrun/         JSON dry-run summaries
  dsp/            oscillator, pre-emphasis, FM modulator, limiter, Goertzel, IQ resampler
  mpx/            MPX combiner
  offline/        offline composite generation (full DSP chain)
  output/         backend abstractions (file, dummy)
  platform/       SoapyDriver interface, SoapyBackend, SimulatedDriver
  platform/soapysdr/  CGO SoapySDR native binding (build tag: soapy)
  rds/            RDS encoder (IEC 62106, CRC, differential, group scheduler)
  stereo/         stereo encoder (19 kHz pilot, 38 kHz DSB-SC)
docs/
  config.sample.json      default config
  config.plutosdr.json    PlutoSDR-specific config
scripts/
examples/

Legal note

This project is intended only for lawful use within relevant license and regulatory constraints. RF output, deviation, filtering, and transmitted power must be validated with proper measurement equipment.