Lobster Audio Beacon Node

March 7, 2026

Overview (Key Operational Facts)

This system is a headless Raspberry Pi 4 audio node used to broadcast mixed audio through a low-power FM transmitter.

Core behavior:

Spotify audio source using librespot
DTMF beacon transmission every 420 seconds
Text-to-speech announcement after the DTMF
ALSA dmix software mixer allows multiple audio sources simultaneously
USB audio interface (C-Media 0d8c:000c) drives the transmitter
48 kHz audio pipeline
Spotify bitrate set to 96 kbps

Operational tuning:

Spotify client volume ~88%
ALSA PCM mixer ~29%
WAV files pre-rendered at correct amplitude

Audio flow:

The audio path from source to transmitter, as the post describes it.

Beacon timing:

DTMF duration: ~12.6 s
TTS duration: ~6.8 s
Total playback window: ~19–20 s
Cycle interval: 420 seconds

Primary System Components

Hardware

Host system

Raspberry Pi 4 Model B Rev 1.1
4 CPU cores
3.8 GiB RAM
Debian 13 (trixie)
Kernel 6.12.62+rpt-rpi-v8
Headless operation

Audio interface

Vendor: C-Media
USB ID: 0d8c:000c
Product: C-Media USB Headphone Set
ALSA card: Set
Device: hw:Set,0

Supported formats:

16-bit PCM
Stereo
44100 Hz
48000 Hz

System pipeline standard:

48000 Hz
Stereo
16 bit

Connected USB devices

C-Media USB audio adapter
Zebra GK420d label printer
CP210x USB serial adapter
ASMedia USB SATA bridge

Audio Pipeline

Librespot (Spotify Connect)

Service: /etc/systemd/system/librespot.service

Startup command:

/root/librespot/target/release/librespot \
  --name Lobster_FM \
  --backend rodio \
  --bitrate 96 \
  --device default

Important parameters:

Device name: Lobster_FM

Bitrate: 96 kbps

Backend: rodio (ALSA through CPAL)

Output device: default

Using default is required so ALSA mixing works.

Beacon Playback

DTMF beacon file:

/root/.openclaw/workspace/will_you_marry_me_becky_dtmf_48k_conv.wav

Properties:

48000 Hz
stereo
16-bit PCM
12.6 seconds

TTS announcement file: /root/.openclaw/workspace/tts_48k.wav

Properties:

48000 Hz
stereo
16-bit PCM
6.82 seconds

Playback Script

Location: /usr/local/bin/dtmf_loop.sh

Simplified behavior:

loop forever
    play DTMF wav
    play TTS wav
    sleep so total cycle = 420 seconds

Example implementation:

#!/bin/bash

DTMF="/root/.openclaw/workspace/will_you_marry_me_becky_dtmf_48k_conv.wav"
TTS="/root/.openclaw/workspace/tts_48k.wav"
INTERVAL=420

while true
do
    START=$(date +%s)

    aplay "$DTMF"
    aplay "$TTS"

    END=$(date +%s)
    ELAPSED=$((END - START))
    SLEEP_FOR=$((INTERVAL - ELAPSED))

    if [ "$SLEEP_FOR" -gt 0 ]; then
        sleep "$SLEEP_FOR"
    fi
done

System Services

librespot

systemctl status librespot

Service behavior:

Spotify Connect endpoint
restarts automatically
runs continuously /etc/systemd/system/dtmf-loop.service

dtmf-loop

Definition:

[Unit]
Description=DTMF Loop Playback Service
After=network.target

[Service]
ExecStart=/usr/local/bin/dtmf_loop.sh
Restart=always
User=root

[Install]
WantedBy=multi-user.target

ALSA Mixing Configuration

File: /etc/asound.conf

Configuration:

pcm.!default {
    type plug
    slave.pcm "mixed"
}

ctl.!default {
    type hw
    card Set
}

pcm.mixed {
    type dmix
    ipc_key 1024
    slave {
        pcm "hw:Set,0"
        rate 48000
        channels 2
        period_size 1024
        buffer_size 4096
    }
}

This forces all playback to: default → plug → dmix → hw:Set,0

So multiple programs can play audio simultaneously.

Correct playback command: aplay file.wav

Incorrect command that bypasses mixing: aplay -D hw:Set,0 file.wav

Volume Configuration

aplay does not change volume.

Volume comes from:

WAV file amplitude
ALSA PCM mixer
Spotify client volume

Current ALSA mixer level: PCM = 44 (29%)

Operational tuning:

Spotify client volume against the ALSA PCM mixer level.
Control	Value
Spotify client	~88%
ALSA PCM mixer	~29%

Observations:

50% Spotify volume too quiet
100% can drown out TTS
DTMF still decodes correctly even when Spotify is louder

System Status

Temperature observed: 28.7°C

CPU cores: 4

Memory: 3.8 GiB

Filesystem:

917G root
17G used

Network:

WiFi only
192.168.10.103

Power Status

Reported by: vcgencmd get_throttled

Result: 0x50005

Meaning:

undervoltage detected
throttling occurred

Possible effects:

USB instability
audio glitches
unpredictable behavior under load

Power supply quality should be verified for long-term reliability.

Monitoring Commands

Temperature: vcgencmd measure_temp

Power status: vcgencmd get_throttled

Audio devices:

aplay -l
aplay -L
cat /proc/asound/cards

Mixer levels: amixer

Service logs:

journalctl -u librespot -f
journalctl -u dtmf-loop -f

Reference Information

These details are useful for diagnostics but not essential to normal operation.

System info:

uname -a
cat /etc/os-release

Network sockets: ss -tlpn

CPU load monitoring:

top
pidstat -u -p $(pidof librespot) 1

Thermal sensor raw value:

cat /sys/class/thermal/thermal_zone0/temp

USB devices: lsusb

Operational Summary

The Lobster node is a stable headless Raspberry Pi system that:

streams Spotify audio through librespot
injects a DTMF beacon every 420 seconds
plays a TTS announcement
mixes all sources through ALSA dmix
outputs through a USB audio interface
feeds a low power FM transmitter

Correct audio balance is achieved with:

Spotify volume ≈ 88%
ALSA PCM ≈ 29%

The only hardware concern identified during audit is historical undervoltage events, which should be addressed for maximum reliability.