I haven't kept up with all of the new advances in these units since I have not been in the market to upgrade my electronics in a few years. Not interested in spending the $$ to get the very latest, but would like an HD unit.
My question is, what is the difference between a unit that is just "HD" vs. one that is "HD DI"?
Since they both show the structure beneath the boat in HD, what advantage does the "DI" feature bring?
Thanks.

There is lots of information covering this just google it. Now from what I have learned you would be better off with a SI unit first and Humminbird makes some good ones, just small screens for a very reasonable price. I believe HD means the screen is HD it does not mean the transducer is. DI would bring picture like images and HD would just be regular sonar.

I have been using a 798 side imaging unit for several years.
Just impressed with the HD and looking to add one.
Thanks.

"Hd" just means a brighter backlight screen to see better in the sunlight Has very nothing to do about image quality

The backlighting "HD" was for the 800 and 900 series models what were updated from the previous CCFL backlit screens to LED backlighting.

HD with the 798ci HD SI Combo just means it is Ethernet compatable.
The earlier versions had the exact same display. 798c SI Combo and 798ci SI Combo.

Bottom line is ignore HD with any electronics, it can mean anything but High Definition.