BotWave Custom (bw_custom)

A custom FM-RDS transmitter for the Raspberry Pi using hardware PWM/DMA, based on PiFmRds. Built to suit the needs of BotWave.

Caution

This software directly accesses hardware DMA and clock registers. Running it incorrectly can crash your Pi, corrupt filesystems, or damage hardware. You have been warned. We accept no liability.

Warning

Transmitting over FM may be illegal in your country. Always consult local regulations before use.

What it does

bw_custom generates a complete FM multiplex signal and transmits it on GPIO4 (pin 7) using the Pi's internal clock generator. The signal includes:

• FM modulation at any frequency between 76–108 MHz
• RDS (Radio Data System) with PS name, Radio Text, PI code, and real-time clock groups (CT)
• Stereo audio (L+R / L-R multiplexing with 38 kHz subcarrier and 19 kHz pilot tone)
• Mono audio fallback for single-channel sources

Supported Pi Models

The Makefile auto-detects your Pi model and compiles with the correct flags and memory addresses.

Model	Architecture	PLL Freq
Pi 1 / Zero (v1)	ARMv6	500 MHz
Pi 2 / 3 / Zero 2 W	ARMv7 / ARMv8	500 MHz
Pi 4	ARMv8 (aarch64)	750 MHz

Installation

sudo apt install libsndfile1-dev build-essential git -y
git clone https://github.com/dpipstudio/bw_custom.git
cd bw_custom/src/
make

This produces the bw_custom executable.

Usage

Usage: bw_custom [options]

  -freq <MHz>       Carrier frequency, 76.0–108.0 MHz  [REQUIRED]
  -audio <file>     Audio input: WAV file path, or "-" for stdin  [REQUIRED]
  -pi <hex>         RDS Program Identifier code (default: 1234)
  -ps <text>        RDS station name, max 8 chars (default: BWC)
  -rt <text>        RDS Radio Text, max 64 chars
  -loop             Loop the audio file when it ends
  -raw              Read raw 16-bit signed PCM from stdin instead of WAV
  -rate <Hz>        Sample rate for raw PCM input (default: 44100, range: 8000–192000)
  -channels <1|2>   Channel count for raw PCM input (default: 2)
  -h                Print help
  -v                Print version

Examples

Transmit a WAV file on 107.9 MHz:

sudo ./bw_custom -freq 107.9 -audio music.wav -ps "MY FM" -rt "Hello from Pi!"

Loop a WAV file indefinitely:

sudo ./bw_custom -freq 100.1 -audio music.wav -loop

Stream raw PCM audio from stdin (e.g. piped from ffmpeg):

ffmpeg -i input.mp3 -f s16le -ar 44100 -ac 2 - | sudo ./bw_custom -freq 103.5 -audio - -raw -rate 44100 -channels 2

Mono raw PCM stream:

arecord -f S16_LE -r 22050 -c 1 | sudo ./bw_custom -freq 98.0 -audio - -raw -rate 22050 -channels 1

Differences from PiFmRds

Feature	PiFmRds	bw_custom
WAV file input	✅	✅
RDS PS / RT / PI	✅	✅
RDS CT (clock time) groups	✅	✅
Stereo audio	✅	✅
Raw PCM stdin (-raw)	❌	✅
Configurable sample rate (-rate)	❌	✅
Configurable channel count (-channels)	❌	✅
Clock error correction (-ppm)	✅	❌
Runtime PS/RT control via named pipe (-ctl)	✅	❌
Traffic Announcement flag (TA ON/OFF)	✅	❌

Hardware Notes

The signal comes out on GPIO4 (physical pin 7). Connect a short wire as an antenna, or, preferably, run a shielded cable directly to a nearby receiver. Do not connect a proper antenna, as this will radiate far beyond legal limits.

License

GPL v3. See LICENSE.

Based on PiFmRds © Christophe Jacquet, F8FTK.
bw_custom additions © 2025 douxx@douxx.tech.