Auto-enable system audio mode when switching HDMI-CEC sources

Automatically wake up your AVR and enable System Audio Mode when switching to gaming consoles or media players via HDMI-CEC.

Why this exists

When you turn on a console connected via HDMI-CEC, your TV switches inputs automatically, but the AVR often doesn't wake up or enable System Audio Mode - leaving you with TV speakers. This script monitors CEC traffic and sends the necessary command to activate your audio system.

Features

• Watches CEC bus passively without interfering
• Only acts when playback devices (consoles) become active
• Cancels injection if the system resolves naturally
• Rate-limited to prevent command spam
• Dry-run mode for testing

Requirements

• A Raspberry Pi (or some other CEC-capable device) connected to your AVR or TV with HDMI (USB-HDMI adaptors won't work) on the CEC-enabled port (usually 0)
• Python 3.6+
• cec-client from libCEC: sudo apt-get install cec-utils

Installation

git clone https://github.com/jlian/cec_auto_audio.git
cd cec_auto_audio
chmod +x cec_auto_audio.py

Configuration

Edit these variables in cec_auto_audio.py:

CONSOLE_LAS = {0x4, 0x8, 0xB}  # Playback device addresses
DENON_LA = 0x5                 # AVR address
PENDING_TIMEOUT_SEC = 0.5      # Wait time before injecting command
DRY_RUN = False                # Set True to test without sending commands

Find your device addresses: echo 'scan' | cec-client -s -d 1

Usage

Run the script:

./cec_auto_audio.py

The script monitors CEC traffic and automatically sends System Audio Mode Request (tx 15:70:00:00) when needed.

You can temporarily set DRY_RUN = True while sniffing behavior; you’ll see log lines like:

[AUTO 00:18:19] Playback/console at logical B became Active Source (phys 36:00).
[AUTO 00:18:20] [DRY RUN] Would send: tx 15:70:00:00

Once you’re happy, flip DRY_RUN = False.

Running as a systemd service

On the device, create a service file:

sudo nano /etc/systemd/system/cec-auto-audio.service

Example unit:

[Unit]
Description=CEC auto audio helper (Denon + consoles)
After=network.target

[Service]
Type=simple
ExecStart=/usr/bin/python3 /opt/cec-auto-audio/cec_auto_audio.py
Restart=on-failure
User=pi
Group=pi
WorkingDirectory=/opt/cec-auto-audio
StandardOutput=journal
StandardError=journal

[Install]
WantedBy=multi-user.target

Then:

sudo systemctl daemon-reload
sudo systemctl enable cec-auto-audio.service
sudo systemctl start cec-auto-audio.service

# Tail logs
journalctl -u cec-auto-audio.service -f

Because we print both our own [INFO] / [AUTO] lines and the raw cec-client output, the journal doubles as a trace buffer. journald handles rotation automatically; you don’t need to babysit log files.

How it works

1. Monitors CEC traffic using cec-client -d 8
2. When a playback device sends Active Source (0x82), starts a timer
3. Waits briefly (0.5s) to see if AVR naturally sends Set System Audio Mode (5f:72:01)
4. If not, sends System Audio Mode Request (tx 15:70:00:00)
5. Rate-limits to prevent spam

Troubleshooting

Script doesn't start: Check cec-client is installed and CEC adapter connected (ls /dev/cec*)

AVR doesn't wake: Verify addresses in config match your devices. Find addresses with: echo 'scan' | cec-client -s -d 1

Too many commands: Increase MIN_INJECTION_INTERVAL_SEC or check for duplicate addresses

License

MIT License - Copyright (c) 2025 John Lian - see LICENSE file