This program and driver allow custom resolutions using built-in video on a PowerMac 7300, 7500, 7600, 8500, and 8600. I have used it to get perfect 1920x1080 on an LCD for over a month now. I have tested it in Mac OS 9.2.2 and Mac OS 7.6.1.
* To use, you first need a "modeline" or the raw video parameters your display uses. When using a modeline, you can paste the full modeline, or strip off the ModeLine "123x456" prefix and also the sync polarities from the end. For example, you can paste
148.35 1920 2008 2052 2200 1080 1084 1089 1125
into the modeline field without problem.
* Put the "73/75/76/85/8600 GraphicsDriver" extension into the Extensions folder, but don't restart yet.
* Run Customize and paste your modeline into the Modeline field and click Decode, or enter the individual parameters into the fields.
* Click Save, then restart.
* Open Customize again and click Apply. This will try the new resolution until you click the mouse. If it doesn't work, you can adjust parameters and click Apply again until you get them right.
* Once you have good parameters, click Save, then open the Monitors control panel and select the new resolution and enjoy (on System 7, hold the Option key when clicking Options... in the Monitors control panel, otherwise the custom resolution won't show up).
The refresh rate, horizontal rate, and pixel clock fields all control the same thing, so only one really matters. If you want to specify a particular one and have the others derive from it, clear the other two.
This driver is based on the original "73/75/76/85/8600 GraphicsDriver". All I've done is decompress the globals in the extension to make them easy to modify, and written the Customize application to modify the parameters. The driver code is unchanged.
Here are a few modelines I've used:
ModeLine "1680x1050" 147.14 1680 1784 1968 2256 1050 1051 1054 1087 +HSync +VSync
ModeLine "1920x1080" 148.35 1920 2008 2052 2200 1080 1084 1089 1125 +HSync +VSync
Download it here: CustomGraphicsDriver.sit