Important: When you apply an MPEG-2 setting to your source file, Compressor automatically assigns many of the setting’s properties (including the frame rate, which determines the “GOP structure” and “GOP size” properties) based on the properties of the source file. These automatically assigned properties are strongly recommended. However, if you’ve manually set the “Frame rate” property of the MPEG-2 setting, you can also set the values for the “GOP frame order” and “GOP size” controls.