purchsed a 998c at christmas time for a really good price. But I noticed that the models i've seen on other store sites have 998c HD on them. My new unit only has 998c on it. Is there a difference in the 2 units. I also have a 798 on the bow and it seems to give crisper images then the 998, even with the si range turned down. the 798 reads 798ci HD. Is there a difference in older to newer models. All have the most current software installed.