SDRTap

A live SDR (Software Defined Radio) tap for the Zako3 Tap Hub. Connects to an rtl_tcp server, demodulates FM/AM radio in real-time, and streams Opus audio to listeners through the hub.

How it works

rtl_tcp server
     │  raw I/Q bytes (u8, 2.4 MHz)
     ▼
SharedSdr — broadcasts 100 ms chunks to all active listeners
     │
     ▼
DDC (per listener) — NCO frequency shift + boxcar decimation → 240 kHz
     │
     ▼
Demodulate — FM discriminator or AM envelope detector
     │
     ▼
De-emphasis (FM) + decimate 240 kHz → 48 kHz PCM i16
     │  streaming WAV via tokio duplex pipe
     ▼
ffmpeg — WAV → OGG/Opus
     │
     ▼
Tap Hub → listeners

A single rtl_tcp connection captures a 2.4 MHz wide slice of spectrum. Each listener tunes to any station within that slice independently, with no additional hardware connections. Audio is never cached — every stream is live.

Audio sources are requested as MODE:FREQ_MHZ, e.g. FM:101.1 or AM:1.080.

Prerequisites

• An RTL-SDR compatible device with rtl_tcp running
• ffmpeg installed and on PATH (used for WAV → Opus encoding)
• A Zako3 Tap Hub account (api.zako.ac or self-hosted)
• Rust toolchain (for building from source)

Start with docker compose

in docker-compose.yml

services:
  sdr-tap:
    build: .
    env_file: .env
    environment:
      SDR_TAP_ID: ${SDR_TAP_ID}
      SDR_API_TOKEN: ${SDR_API_TOKEN}
      SDR_CENTER_MHZ: ${SDR_CENTER_MHZ:-98.0}
      TAPHUB_ENDPOINT: ${TAPHUB_ENDPOINT:-api.zako.ac}
      RTLTCP_HOST: ${RTLTCP_HOST:-localhost}
      RTLTCP_PORT: ${RTLTCP_PORT:-1234}
    network_mode: host
    restart: unless-stopped

if you need pre-built image, use ghcr.io/200mill/tap-sdr

services:
  sdr-tap:
-    build: .
+    image: ghcr.io/200mill/tap-sdr
    env_file: .env
    environment:
      SDR_TAP_ID: ${SDR_TAP_ID}
      SDR_API_TOKEN: ${SDR_API_TOKEN}
      SDR_CENTER_MHZ: ${SDR_CENTER_MHZ:-98.0}
      TAPHUB_ENDPOINT: ${TAPHUB_ENDPOINT:-api.zako.ac}
      RTLTCP_HOST: ${RTLTCP_HOST:-localhost}
      RTLTCP_PORT: ${RTLTCP_PORT:-1234}
    network_mode: host
    restart: unless-stopped

Configuration

All configuration is via environment variables. A .env file is supported.

Variable	Required	Default	Description
SDR_TAP_ID	yes	—	Unique tap identifier
SDR_API_TOKEN	yes	—	Authentication token for the hub
SDR_CENTER_MHZ	yes	—	Center frequency to tune (e.g. 98.0)
TAPHUB_ENDPOINT	no	api.zako.ac	Tap Hub address
TAPHUB_SERVER_NAME	no	—	TLS SNI override
TAP_HEALTHCHECK_PORT	no	—	HTTP health check port (for liveness probes)
RTLTCP_HOST	no	localhost	rtl_tcp server host
RTLTCP_PORT	no	1234	rtl_tcp server port

Copy .env.example to .env and fill in the required values.

Running

Start rtl_tcp with your SDR device:

rtl_tcp -a 0.0.0.0 -p 1234

Then run the tap:

SDR_TAP_ID=my-sdr-tap SDR_API_TOKEN=secret SDR_CENTER_MHZ=98.0 cargo run --release

Once connected, listeners can request any station within ±1.05 MHz of SDR_CENTER_MHZ.

Docker

A Dockerfile and docker-compose.yml are included. Edit .env with your credentials, then:

docker compose up -d

The compose file uses network_mode: host so the container can reach rtl_tcp on the host.

Supported modes

Source format	Mode	Notes
FM:101.1	Wideband FM	75 µs de-emphasis (Americas/Japan)
WFM:101.1	Wideband FM	alias for FM
AM:1.080	AM envelope	slow DC tracking

Project structure

src/
├── main.rs        entry point, env config, tap builder
├── shared_sdr.rs  single rtl_tcp connection, broadcast channel for I/Q chunks
├── sdr.rs         SdrTapHandler — TapHandler implementation, DDC/demod pipeline
├── rtltcp.rs      async rtl_tcp client (TCP, binary protocol)
└── demod.rs       FM/AM demodulator, de-emphasis, decimation, WAV header
references/        reference implementation of the Zako3 SDK tap pattern