You may have already noticed that (Mac) OS X tends to recognize external screens as TVs, especially when they are plugged in HDMI. This is not a real problem, but the image is damaged on some screens, cause they are using YCbCr instead of RGB, more effective. Fortunately, there is a solution.
It happens only on some screens, and it’s quite visible: the image is deteriorated, especially at the level of the fonts. You will immediately see if a screen is recognized as a TV: in the preferences, the refresh rate is locked on NTSC, for example.
To switch to RGB, you first have to save a little script, and to connect the screen to adjust only. Next, the script has to be launched with the following command in the Terminal.
A folder will appear with a name starting with
DisplayVendorID- followed by a value corresponding to the brand of the screen. You must copy this folder in
/System/Library/Displays/Overrides (the system should ask you for a password) and restart, it should be enough. Once this is done, the screen should change to RGB and indicate it in its name, in the preferences.