RTL_TCP_ECHO

RTL_TCP_ECHO is a Go application that acts as a proxy between an rtl_tcp server and its client. It transparently passes control commands (such as frequency, gain, and sample rate), forwards IQ data, and records the IQ stream to a file. Later, you can run the application in playback mode, serving the recorded IQ data as a fake rtl_tcp server—allowing SDR software to connect and replay the IQ stream.

Features

• Proxy Mode:
  Forwards all rtl_tcp commands (including frequency and gain) and IQ data between client and server. Simultaneously records IQ data to a file.

• Playback Mode:
  Serves a previously recorded IQ file as an rtl_tcp-compatible server for SDR software to connect and decode.

• Transparent Command Handling:
  All client commands (frequency, gain, sample rate, etc.) are passed through with optional logging.

• Simple Configuration:
  Easily specify listen/forward addresses and recording/playback file paths via command-line flags.

Usage

Build

go build -o rtl_tcp_echo

Proxy Mode (Record IQ Data)

./rtl_tcp_echo --mode=proxy --listen=:1234 --forward=127.0.0.1:1234 --record=iq_recording.bin

• --listen : Address for rtl_tcp clients to connect to.
• --forward: Address of your actual rtl_tcp server.
• --record : File to save the IQ stream.

Playback Mode (Serve Recorded IQ Data)

./rtl_tcp_echo --mode=playback --listen=:1234 --playback=iq_recording.bin --samplerate=2400000

• --listen : Address for SDR software to connect to.
• --playback : IQ file to serve.
• --samplerate : Sample rate in Hz (must match the rate used during recording).

Important: The --samplerate flag is critical for proper playback timing. For digital modes like DMR, P25 Phase 2, and other TDMA-based protocols, the sample rate must match exactly what was used during recording to maintain proper timing synchronization.

Command-Line Flags

Flag	Description	Default
--mode	proxy (default) or playback	proxy
--listen	Listen address	0.0.0.0:1234
--forward	Forward address (proxy mode only)	127.0.0.1:1234
--record	IQ recording file (proxy mode only)	iq_recording.bin
--playback	Playback IQ file (playback mode only)	iq_recording.bin
--samplerate	Sample rate in Hz (playback mode only)	2400000

Example Workflow

1. Start your rtl_tcp server (e.g., on port 1234).
2. Run RTL_TCP_ECHO in proxy mode to intercept and record IQ data.
3. Connect your SDR software (e.g., SDR#, GQRX, etc.) to RTL_TCP_ECHO.
4. Stop recording and switch to playback mode to replay the IQ data for analysis or demonstrations.

Protocol Support

RTL_TCP_ECHO is designed to transparently pass all rtl_tcp control commands, including:

• Set frequency
• Set sample rate
• Set gain / gain mode
• Tuner commands