"An HD Tremulous..." is actually correct, though arguing over an or a really shouldn't happen. See http://www.dailywritingtips.com/when-to-use-an/ for some instructions, specifically the section on "Which indefinite article to use before an abbreviation, a numeral, or a symbol, depends upon pronunciation." You'll notice, because "HD" is pronounced one letter at a time, and the letter 'h' is pronounced with a hard a at the beginning (In almost all dialects of English, the name for the letter is pronounced /ˈeɪtʃ/ and spelled ‹aitch›[1] or occasionally ‹eitch›. The pronunciation /ˈheɪtʃ/ and hence a spelling of ‹haitch› is often considered to be h-adding and hence nonstandard), or in IPA, /ˈeɪtʃ/, the word 'an' should be used, not 'a'. Thanks for listening guys. Shouldn't be a big deal. 
Exactly. It depends on how you pronounce HD, and it looks like there are three ways:
-Pronouncing "HD" as one word... maybe something like "h'd"?
-Actually saying "high definition" each time, which makes you sound like a tool.
-Spelling it out: "H-D", which, as per Moose's link, would mean "an HD".
Now that this is over, can we get back to more important things, like
flaming each other the Great Engine Debate?