The FM and HD outputs and audio processing paths of a combination on-air processor differ in several ways:

The FM side is pre-emphasized and peak limiting is done via distortion-cancelled clipping. The FM side also has the stereo multiplex generator which converts processed left/right audio into the “composite” domain along with the 19 kHz stereo pilot and any subcarriers which may be in use. 

The HD side shares the ‘front-end’ processing with the FM side (multiband leveling, compression, and limiting) but peak control is done using look-ahead limiting. There is no pre-emphasis or clipping on the HD audio signal; all final processing is done ‘flat’ because not only is such "FM-style" limiting not required, it is also not desired due to the presence of a perceptual codec within the HD audio domain. 

If you have purchased your HD gear but it hasn’t been delivered yet, reach out to the HD equipment vendor and ask for the manuals to be sent ahead of time so you can be familiar with the gear and its hookup requirements prior to it arriving. 

We can and always will assist with the audio processing aspects of an FM/HD system, but consultation on the HD equipment side of things (installation, hookup, time alignment, etc.) should always occur with the experts, the HD equipment vendor.