Force stereo audio output to HDMI?

[jt-kgy]

Hi All,

Is there a way to force stereo output?
Most of the DCP assets I received are in 5.1 audio but I only have 2.0 stereo hardware.

Here are what I am using.
Machine running MacOS Tahoe 26.5.1
DCP-o-matic 2.18.42
HDMI output to a Samsung TV.

In the audio preference, if I select Samsung TV, audio will automatically selects 5.1 channels output.
If I select Mac's speakers, then it runs 2.0 stereo output.
However, Samsung only has 2.0 channels for it's built-in speakers not 5.1.
So when I play DCP with Samsung audio, some of the sound will not play correctly.

I know I could fix this by checking boxes to force output of all channels to the first 2 channels.
But some of the background music come out too prominently.
Is there a way to force HDMI output to 2.0 channels?

Thank you!

[Kewl]

Your TV should downmix automatically to stereo. The TV use a standard preset for how this downmix is done (often the Lfe channel is muted in this kind of preset downmix).

If you want to send stereo to your TV, you can change the audio channels levels when the downmix is initiated in DOM.

[jt-kgy]

Hi Kewl,

Thank you for reply.
The Samsung TV didn't downmix automatically.
I couldn't hear any of the dialog, for example, coming from the DCP until I made the audio changes (check boxes) in the Preference.

"If you want to send stereo to your TV, you can change the audio channels levels when the downmix is initiated in DOM."
I'm not sure what you meant by this.
When I changed the audio output to the Samsung TV, it automatically selected 6 channels.

[Kewl]

You can select a 5.1 device and downmix to stereo in DOM so that only outputs 1 and 2 are used.

In this exemple, L and R are lowered to -6, C is lowered to -9 (and send to both 1 and 2), Lfe lowered to -12 (and send to both 1 and 2), Ls and Rs to -6 and send to 1 and 2 respectively.

Capture d’écran, le 2026-06-17 à 19.52.03.png

[jt-kgy]

I figured it out.

On the Mac running DCP-o-matic with HDMI out to Samsung TV, I ran the Mac Utility "Audio MIDI Setup" from Apple.

In the setup, I can see that the HDMI output is set to do 6-channels to the Samsung TV (even though the TV only got stereo speakers).
I set the output to a 2-ch audio instead.

Restart DCP-o-matic... Check the Audio setup.. Now the Samsung TV is doing 2-channel as expected!

[Kewl]

Ok, rather than doing the downmix in DOM, it's done in the Core Audio engine of the Mac.

That being said, your TV should absolutely be able to downmix. TVs have been doing that for, like, 15 years or more. It's probably down to a question of audio settings on your TV.

[Carsten]

DOM and MacOS by default will only output 8 linear PCM audio channels. Unlike e.g. AC-3,Dolby Digital, dts, compressed surround, that's not a well established format for consumer devices, so it may be that some TVs are not able to downmix them properly.

I would use/create a test dcp with channel identificators and play it back over the TV, then try different format settings on the TV (if available at all).

One way or the other will work.