How to Control External Monitor Brightness, Sound Volume on a Mac with MonitorControl (Even Over HDMI)

MonitorControl is a free, open-source macOS app (MIT licensed) that allows you to control external monitor display settings — brightness, contrast, volume, etc. — more conveniently, often via keyboard keys, menu bar sliders, or custom shortcuts

Key features include:

• Supports multiple control protocols:
  • DDC/CI (Display Data Channel / Monitor Control Command Set) for hardware control of brightness, contrast, volume.
  • Native Apple display protocol for built-in or Apple displays.
  • Software/gamma/shade dimming when hardware control via DDC isn’t available — e.g. overlay or gamma adjustments.
• Allows setting keyboard shortcuts, customizing behavior, grouping displays, etc.
• Menubar slider, OSD (on-screen display) that matches macOS native look.

How It Works (including over HDMI)

The core of its external monitor control relies on DDC / DDC-CI protocols. These allow the computer to send commands over the video connection to change settings like brightness/contrast/volume etc.

For HDMI

• If the monitor supports DDC/CI over HDMI (i.e. the monitor side and the HDMI cable/channel permit those signal exchanges), MonitorControl can use that to send commands.
• However, not all HDMI connections on Macs allow full DDC communication. Some Macs (especially with Apple Silicon, HDMI ports on those machines, etc.) have limitations.

For example:

• On many M1 Macs, the built-in HDMI port does not allow DDC control. That means MonitorControl can’t send hardware brightness/contrast commands via HDMI; instead it will fall back to software dimming (gamma overlay etc.).
• Same goes for other machines with HDMI ports that are not fully “DDC-compatible” in hardware.

Limitations & Things to Watch Out For

Here are some caveats / things to verify if you plan to use it with your setup, especially over HDMI:

1. Monitor must support DDC/CI
   • DDC/CI must be enabled (if there is a setting for it in monitor’s menu). Some monitors ship with DDC disabled.
   • If the monitor does not support DDC (or it is disabled), MonitorControl will be more limited — only software dimming, not full hardware control. 
2. Cable/Port path
   • The HDMI cable / adapter / converter / dock between the Mac and monitor must transmit the DDC channel. Some adapters or hubs or docks may block or not pass through the DDC protocol correctly.
   • If using DisplayPort / USB-C / Thunderbolt variants, sometimes the DDC channel is more reliable. Many users report better behavior over USB-C than HDMI in Macs.
3. Mac model-specific HDMI limitations
   • As mentioned, Mac minis, MacBooks, and Mac Studios with HDMI ports on Apple Silicon / recent Intel Macs might have HDMI ports that don’t support DDC properly. If your Mac is one such, you may need to connect via USB-C / DisplayPort instead.
4. Software version / macOS version
   • Newer macOS versions might introduce compatibility issues. MonitorControl’s GitHub repo mentions specific macOS versions and warnings. 
   • Make sure you install a relatively recent version of MonitorControl, especially if your macOS is newer (if there are updates / fixes for your macOS version).

How to Set It Up

If you want to try using MonitorControl, here’s a typical setup flow:

1. Download & Install
   • Get the latest release from GitHub (the .dmg) or via Homebrew:

brew install --cask monitorcontrol

1. Move to /Applications, open it

   

2. Grant Accessibility Permissions
   • If you want to use keyboard media keys (brightness/volume keys), the app needs permission under System Settings → Privacy & Security → Accessibility. 
3. Check Monitor’s Settings
   • See if DDC/CI is enabled on the monitor (check its on-screen menu).
   • If using HDMI, ensure cable & port support DDC.
4. Configure MonitorControl Settings
   • In the app’s preferences, enable “advanced settings” if available to reveal more options.
   • Set how brightness control should work (hardware via DDC if possible, fallback to software dimming).
   • Decide whether keyboard, menu bar, custom shortcuts are what you want.
5. Test
   • Try changing brightness / volume via the keyboard, or via the menu bar slider. See if monitor responds physically (hardware).
   • If not, you’ll see the software dimming effect (e.g. screen dimming / gamma / overlay).

What to Expect Specifically Over HDMI with MonitorControl

Given all of the above, if you connect a monitor via HDMI:

• If everything is compatible (monitor supports DDC over HDMI, Mac HDMI port allows DDC, cable passes the control), then brightness/contrast/volume control should work as if via DisplayPort / USB-C.
• If not, then you might only get software dimming – meaning the app simulates dimming by adjusting the gamma / overlay etc., not instructing the monitor hardware itself. This is less ideal (might reduce color fidelity, etc.).
• Volume control may or may not be available depending on how the monitor is receiving audio and whether the OS sees it as an audio output device. Hardware mute/volume via DDC might work if supported.