zwfm-encoder

README

ZuidWest FM Encoder

Audio streaming software for ZuidWest FM and Radio Rucphen. Stream audio from a Raspberry Pi to multiple SRT destinations simultaneously. Built for broadcast environments with real-time monitoring and web-based configuration.

Features

• Multi-output streaming - Send to multiple SRT servers with different codecs simultaneously
• Real-time VU meters - Peak hold (1.5s) with Peak/RMS toggle, updated via WebSocket
• Web interface - Configure outputs, select audio input, monitor levels
• Auto-recovery - Automatic reconnection with exponential backoff
• Multiple codecs - MP3, MP2, OGG Vorbis, or uncompressed WAV per output
• Single binary - Web interface embedded, minimal runtime dependencies

Requirements

• Raspberry Pi 4 or 5
• HiFiBerry Digi+ I/O or HiFiBerry DAC+ ADC
• Raspberry Pi OS Trixie Lite (64-bit)
• FFmpeg (for encoding)
• alsa-utils (for audio capture via arecord)

Installation

1. Install Raspberry Pi OS Trixie Lite (64-bit)
2. Configure HiFiBerry following the official guide
3. Run the installer as root:

sudo su
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/oszuidwest/zwfm-encoder/main/deploy/install.sh)"

The web interface will be available at http://<raspberry-pi-ip>:8080

Default credentials: admin / encoder

Audio Input

Connect the digital output of your audio processor to the HiFiBerry input.

Requirements:

• 48 kHz sample rate
• 16-bit depth
• SPDIF format preferred (AES/EBU may work but is not guaranteed)

Codecs

Codec	Encoder	Bitrate
MP3	libmp3lame	320 kbit/s
MP2	libtwolame	384 kbit/s
OGG	libvorbis	~500 kbit/s (Q10)
WAV	pcm_s16le	Uncompressed

Configuration

Configuration is stored in config.json:

{
  "web_port": 8080,
  "web_user": "admin",
  "web_password": "encoder",
  "audio_input": "default:CARD=sndrpihifiberry",
  "outputs": [
    {
      "id": "output-1",
      "host": "srt.example.com",
      "port": 9000,
      "streamid": "studio",
      "password": "secret",
      "codec": "mp3",
      "enabled": true
    }
  ]
}

Architecture

flowchart LR
    subgraph Input
        A[Audio Source<br>S/PDIF]
    end

    subgraph Capture
        B[arecord<br>Raw PCM]
    end

    subgraph Distribution
        C[Go Distributor<br>+ Level Metering]
    end

    subgraph Encoding
        D1[FFmpeg MP3]
        D2[FFmpeg MP2]
        D3[FFmpeg OGG]
    end

    subgraph Output
        E1[SRT Server 1]
        E2[SRT Server 2]
        E3[SRT Server 3]
    end

    A --> B --> C
    C --> D1 --> E1
    C --> D2 --> E2
    C --> D3 --> E3

On Linux, arecord captures audio from ALSA with minimal CPU overhead. The Go distributor calculates RMS/peak audio levels directly from the PCM stream and fans out the audio to multiple FFmpeg encoder processes. Each encoder streams to its own SRT destination.

On macOS (for development), FFmpeg with AVFoundation is used for capture instead of arecord.

Post-installation

Optional cleanup to reduce attack surface:

# Disable WiFi
echo "dtoverlay=disable-wifi" >> /boot/firmware/config.txt

# Remove unnecessary packages
apt remove bolt bluez ntfs-3g rsyslog telnet

SRT Resources

• SRT Overview (IETF)
• SRT Deployment Guide
• SRT 101 Video

Related

• Liquidsoap Server - Companion server software for receiving SRT streams

License

MIT License - See LICENSE.md

Documentation

Overview

Package main implements an audio streaming encoder that captures audio from digital input and streams to multiple SRT destinations.

Usage:

encoder [-config path/to/config.json]

If -config is not specified, the encoder looks for config.json in the same directory as the binary.