Samsung SyncMaster T220HD DisplayPort to DVI connection problem

There is a possibility that, if the pixel clock is lowered, 1680 x 1050 can run without any noise in the screen. I say this because you point out lowering the format to 1200 x 900 makes the noise go away.

But in order to lower the pixel clock, you will need to install a program like SwitchRes X (SRX) to create a custom timing. If you want to try it, here are some step-by-step instructions:

Download and install SwitchRes X. For this application, I recommend the Control Panel version instead of the full version, which is called Universal Installer. Control Panel is the second link on the download page here:

http://www.madrau.com/html/SRX/DL.shtml

Once SRX Control Panel is installed, double-click it's icon to open and run it. You may want to do that while in clam shell mode with the MacBook, meaning only the external monitor is running. This will remove any ambiguity about which display is being worked with. It's not a requirement. But if you run SRX in dual-display mode, be mindful of which display it is you are working with within SRX. Regardless, be sure the external monitor is running at 1680 x 1050.

With SRX open and focused on the Samsung, click on the Custom button in the main window. The window will switch to showing a table of available timings. In the bottom left corner click on the "+" to add a new resolution to the existing resolutions. A new window will open showing the current timing. The values should be showing the 1680 x 1050 format timing. Make note of the Pixel clock at the top of that window. I am guessing it will be around 145 MHz. At the bottom of the window, put a checkmark in the box for "Use simplified settings" and in the associated pop-up menu select "CVT-RB". This stands for Coordintated Video Timings - Reduced Blanking and it uses predefined formulas to arrive at a valid timing for displays that can work with reduced blanking (most LCD monitors can but not CRT monitors. CRT monitors require full blanking time).

That's all you need to do. You should see that the pixel clock has now dropped quite a bit. It should be close to 120 MHz or lower. If it's not, check the Vertical Scan rate box and make sure the value in that box is close to 60 Hz. If it is far off from 60, like 70 or something like that, change it to 60. Click on OK and then follow the remaining directions for saving and restarting. Now, after a restart, the newly created timing should be present within Displays preferences of OS X. There may be two 1680 x 1050 timings, the original one and the newly created one. Select the newly created one and hopefully the Samsung will now run from it and be noise free. Good luck.

I have the same monitor and similar MacBook 13" with Mini DisplayPort. I discovered that the DVI cable that came with the Samsung is a single link cable and when I replaced it with a dual link cable, my problem went away.