Okay. Try this:
Start here:
http://www.mathewinkson.com/2013/03...ix-the-picture-quality-of-an-external-monitor
His step one is :
"Download the patch-edid.rb script from the forums thread above, or download Andrew Daugherity’s improved patch-edid.rb script from his github page. Put the script in your home directory."
1) Go ahead and download the script. Make a new folder on your desktop called Temp and put the script inside of the Temp folder.
Disconnect all other monitors besides the one you are trying to use the script on. Only do this one monitor at a time.
2) Open the Terminal app; At the prompt, type
cd ~/Desktop/Temp and press the return key.
2a) You will get a new line. Type
ls -la and press the return key. You should get a line that looks like:
-rwxr-xr-x@ 1 yourusername staff 2237 Feb 25 2014 patch-edid.rb [The x characters are necessary if you do not have them we will have to fix that before going further. See note (*) below for instructions as to how to do that.]
3) Now type
ruby patch-edid.rb and press the return key.
It will spit some information to you on the screen as well as make a new folder in your Temp folder . . .
here is an example:
found display 'ASUS PB238': vendorid 1129, productid 9122, EDID:
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
Setting color support to RGB 4:4:4 only
Number of extension blocks: 1
removing extension block
Recalculated checksum: 0x4
new EDID:
00FFFFFFFFFFFF000469A22336A5010015180104A5331D7822E295A2554F9F26115054BFEF00D1C0B300950F95008180814081C0714F023A801871382D40582C4500FD1E1100001E000000FF0045354C4D54463130373833300A000000FD00324B185311041100F0F838F03C000000FC00415355532050423233380A20200004
In your Temp folder you will find a new folder with a name like: DisplayVendorID-469 and a file inside of it with a name like DisplayProductID-23a2.
4a) Open your startup disk and navigate to the /System/Library/Displays/Contents/Resources/Overrides folder. Inside of here, you will find a lot of folders with names like DisplayVendorID-1e6d and DisplayVendorID-4c2d and . . . etc. Find the one that corresponds to your DisplayVendorID-number and open it. Note: If you do not have this folder, we will have to use the one you already have, see note (**) below.
4b) Drag your DisplayProductID-number file inside of here. Note: You will be asked to enter your password before the finder will move the file.
5a) In Terminal hit the return key a couple of times to make sure you have a prompt. Then type
cd /System/Library/Displays/Contents/Resources/Overrides and press the return key.
5b) Type
cd and your DisplayVendorID-Number and press the return key; for example: cd DisplayVendorID-469 (return)
5c) Type
ls -la and press the return key.
Depending on how many files are in the folder you will have that many lines of information given back to you. They may look like:
-rw-r--r-- 1 yourusername staff 612 Jan 29 19:19 DisplayProductID-23a2
-rw-r--r-- 1 root wheel 681 Sep 16 18:48 DisplayProductID-32a3
5d) You need to change ownership of the new file from you to the system. For this example you would at the prompt, type
sudo chown root:wheel DisplayProductID-23a2 and press the return key.
You will be asked to enter your password. Do so.
5e) Type
ls -la and press the return key.
This is making sure that the ownership of the file have be given to the system. If you do not see the same root wheel listing in the right space for the new file, let me know.
6) Type
cd and press the return key.
This gets you back to your home folder and out of the system folder. You can quite Terminal if you wish.
7) Restart the machine and take a look at your About This Mac menu option. You should see your monitor listed with the notice that it is being forced into RGB mode.
View attachment 233445
Repeat for each monitor you are using.
Notes:
(*) The rwx in the listing corresponds to permission types, r being read, w being write and x being execute — run. There are three groups crammed together along with possible leading information about whether the file is a directory and a possible trailing bit of information called a sticky bit, I never remember what it is for. Having the x part is needed to run. If you do not have x in three places then type
chmod +x patch-edid.rb and press the return key.
(**) The Overrides folder has a lot of folders in it but may not have your vendor in there. Drag your entire Vendor folder into the Overrides folder. You will be asked to enter your password.
(a) Type
cd /System/Library/Displays/Contents/Resources/Overrides and press the return key.
(b) You are going to give the folder and its contents to the system so for example type
sudo chown -R root:wheel DisplayVendorID-469 and press the return key, only use your DisplayVendoerID-number.
You may have to enter your password or not depending on how long ago it was that you used the sudo command.
(c) Type
cd and your DisplayVendorID-Number and press the return key; for example: cd DisplayVendorID-469 (return)
(d) Type
ls -la and press the return key.
Depending on how many files are in the folder you will have the many lines of information given back to you. They should all look like:
-rw-r--r-- 1 root wheel 612 Jan 29 19:19 DisplayProductID-23a2
-rw-r--r-- 1 root wheel 681 Sep 16 18:48 DisplayProductID-32a3
(e) Continue from step 6 above.