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Hello dear RPi friends,

an HDMI cable is connected to an audio extractor at HDMI 0 (near USB-C) to tap off the audio signal for further output. No video signal is used by the audio extractor.

/boot/config.txt:

# uncomment if hdmi display is not detected and composite is being output
hdmi_force_hotplug=1

# uncomment to force a specific HDMI mode (this will force VGA)
#hdmi_group=1
#hdmi_mode=1

# uncomment to force a HDMI mode rather than DVI. This can make audio work in
# DMT (computer monitor) modes
hdmi_drive=2
...

aplay -l:

Karte 0: b1 [bcm2835 HDMI 1], Gerät 0: bcm2835 HDMI 1 [bcm2835 HDMI 1]
  Sub-Geräte: 4/4
  Sub-Gerät #0: subdevice #0
  Sub-Gerät #1: subdevice #1
  Sub-Gerät #2: subdevice #2
  Sub-Gerät #3: subdevice #3
Karte 1: Headphones [bcm2835 Headphones], Gerät 0: bcm2835 Headphones [bcm2835 Headphones]
  Sub-Geräte: 4/4
  Sub-Gerät #0: subdevice #0
  Sub-Gerät #1: subdevice #1
  Sub-Gerät #2: subdevice #2
  Sub-Gerät #3: subdevice #3

If you need any more information, just write. I'm grateful for every solution or hint how to force or display hdmi 0 to get my audio signal over it.

Greetings from germany!

1 Answer 1

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The Pi4 (in contrast to earlier models) has NO video output unless a monitor is detected. See https://www.raspberrypi.com/documentation/computers/config_txt.html#hdmi-mode

The OS CAN be forced to set resolution even if no monitor is detected.

You don't say which OS but Bullseye adds additional limitations. See What are the pros and cons of the new Raspberry Pi OS Bullseye?

The setting you list don't work with kms driver. See https://www.raspberrypi.com/documentation/computers/configuration.html#fake-or-full-kms-graphics-driver

I have enabled the in-built VNC server to use headless (which disables Mutter, although I can't detect any difference).

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  • "The Pi4 (in contrast to earlier models) has NO video output unless a monitor is detected" Do you mean NO "audio" output... ? Cause i don't need video. Or do you mean, i need a display behind the AUDIO extractor to have a video+audio signal?
    – cheat.008
    Jul 4 at 9:28
  • If there is no video output obviously there can be no HDMI output. You can force HDMI with no monitor but the RASpberry Pi Ltd. documentation is rather vague and inconclusive.
    – Milliways
    Jul 4 at 9:47
  • Ok is see, so no video cable (hdmi) connected to the audio extractor will bring up no audio signal. So the only thing i could test would be the hdmi safe mode, which includes the hdmi_ignore_edid=0xa5000080 to hope, that the video output will be switched on.
    – cheat.008
    Jul 4 at 10:45
  • Try adding hdmi_force_hotplug=1 to config.txt. See raspberrypi.stackexchange.com/a/138183/8697
    – Milliways
    Jul 4 at 10:54
  • Please see my config above, there i already added it. But when it's true what you say and the RPi needs an active HDMI Monitor, could this be a solution to let the Pi think there is a display connected to extract the audio? Amazon: "HDMI Dummy Plug Headless Ghost Display Emulator Premium Aluminum PC"
    – cheat.008
    Jul 4 at 17:33

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