rtl-sca

A command-line tool to detect, demodulate, and play back FM broadcast subcarriers (SCA) from an RTL-SDR. Written in Zig.

US FM stations carry more than the stereo program: above the audible 0–53 kHz region, the composite MPX baseband often hides narrowband audio subcarriers — commonly at 67 kHz and 92 kHz (reading services for the blind, background music, niche broadcasts). rtl-sca recovers the FM main carrier, reaches into a chosen subcarrier slot, demodulates it (FM or AM), and plays it back or writes a WAV.

What it does

• scan surveys a station's MPX baseband — pilot, subcarrier slots, modulation (FM / AM-DSB / data), bandwidth, and SNR.
• play demodulates a chosen subcarrier and plays it live out the speakers.
• rec writes the demodulated audio to a WAV.

Each command reads from a local RTL-SDR (just give a frequency), an rtl_tcp network server, or a recorded IQ file. Verified over the air on both macOS and a Raspberry Pi 4 — a blind band sweep + scan locates a live 67 kHz SCA, which play and rec then decode to intelligible audio, drop-free in real time.

Requirements

• Zig 0.16+ — macOS: brew install zig.
• librtlsdr — required to build (the program links against it).
  • macOS: brew install librtlsdr
  • Debian / Raspberry Pi OS: apt install librtlsdr-dev (or build from source)
  • Found on the default search path on Linux; via brew --prefix on macOS.
• libc, plus the CoreAudio/AudioToolbox frameworks on macOS for live playback (wired automatically in build.zig). miniaudio is fetched by the package manager (build.zig.zon) — nothing to install.
• An RTL-SDR dongle for live use; files need no hardware. To drive a radio on another host, run rtl_tcp -a 0.0.0.0 -f <freq> -s 1024000 there and pass --remote.

Build

zig build              # builds zig-out/bin/rtl-sca
zig build test         # runs unit tests
zig build run -- scan 89.9M

Usage

rtl-sca <command> <input> [flags]

<input> auto-detects: a frequency (89.9M, 89900000) tunes a radio — local by default, or an rtl_tcp server with --remote; anything else is a recorded IQ file. Live commands run until Ctrl-C (which finalizes the WAV).

rtl-sca scan 89.9M                                      # survey a station's MPX for subcarriers
rtl-sca play 89.9M --sub 67k                            # demodulate the 67 kHz SCA live
rtl-sca rec  89.9M --sub 67k -o out.wav                 # record the SCA to a WAV
rtl-sca play 89.9M --sub 0 --bw 15k --deemph 75us       # main mono program (a quick front-end check)
rtl-sca scan capture.cu8                                # survey a recorded IQ file
rtl-sca play 89.9M --remote raspberrypi.local --sub 67k # drive a remote radio over rtl_tcp

flag	meaning	default
--sub HZ	subcarrier center (67k, 92k, …); 0 = main channel	67k
--bw HZ	bandwidth recovered (slot width for a subcarrier, ±bw/2 → audio)	8k
--mod MODE	fm | am-env | am-coherent	fm
--deemph TAU	de-emphasis time constant (150us, 75us, off, …)	150us
--remote H[:PORT]	tune over an rtl_tcp server (IP or hostname)	—
--gain DB | auto	tuner gain in dB, or auto for AGC (radio only)	0
--scan-seconds N	scan integration window (longer catches fading SCAs)	4
-o FILE	output WAV path (rec); - streams to stdout	—
-v, -vv	diagnostics to stderr	off

Run rtl-sca with no command for the full flag list. See GUIDE.md for the details — bandwidth semantics, de-emphasis, output streams, diagnostics, and operating notes (gain/overload, the tune-in transient, rtl_tcp behavior).

Project layout

• src/main.zig — entry point; wires the CLI to the pipeline.
• src/cli.zig — argument and subcommand parsing.
• GUIDE.md — full usage reference and operating notes.
• SPEC.md — original design notes & signal-chain reference (Carson bandwidth, de-emphasis, the rate chain). The shipped code is now authoritative; kept for DSP background and rationale.
• CLAUDE.md — conventions for working in this repo (incl. Zig 0.16 notes).

License

MIT — see LICENSE.