SD and HD are 2 different standards. They require two different types of sync.
SD bi-sync and HD tri-sync are different signals.
SD Bi-sync supports computer video, composite video, s-video, and component video, by using 2 voltage levels ( high and low )... systems using bi-syn are triggered by the differential voltage at the leading ( negative) edge of the signal. Bi-sync basically runs at 0 votage ( black) then drops to a negative voltage and then back to zero ( 2 states or BI-sync)
Tri-sync provides a more exacting sync for the 3 component signals. HD has sync info on all three channels ( Y, Pb, and
Pr). Tri sync starts at 0 volts ( video black ) then goes negitive, then a positive voltage, then back to 0 volts ( 3 states, Tri-Sync). This also fixed the introduction of a voltage differential by bi-sync signals into the actual video signal. the tri-sync signal is triggered by the on the negative transition, then on the positive transition. thus 0,-300mv,+300mv,0 3 sync points... This all happens in the Horizontal blanking interval. There is something called the White reference level that is set at 700mv. the 700mv drops to 0 at the start of the blanking period. the signal drops to -300mv, the to +300mv, then to 0 the back to reference white 700mv. This is the HD Analog horizontal timing data where HD tri-level sync is used. ( I don't remember what the white level is for bi-sync.. sorry pulling most of this out of my head... ) There are also a couple of issues revolving around the Progressive vs interlaced HD frame timing that comes into play here.
bi-sync is on the Y-signal on SD video. tri-sync is on all 3 channels for HD. ( check out SMPTE 274M ad SMPTE 296M )
so the two are incompatable for sync .. there are a lot of options out there for devices that support both bi-sync and tri-sync.
the sync IO accepts SD BiSync or "traditional" video black, the HDSync IO accepts Tri-sync signals. As far as gear it all depends on what you are syncing... if its protools to an HD video deck and you are laying back audio, you can use a tri-sync box that offers bi-sync. The bisync can be routed to the protools rig and the tri-sync can be used for the HD deck. If you are dealing with laying back an HD video signal from a video editing system with an AJA Kona3 card to an HD video deck directly connected, you don't need
a tri-sync gen, because the AJA kone3 offers this to the deck via the video signal (read the Kona3 card for details, i don't remember exactly how they do it, but they do) if you are using a euphonix system 5 HDMI IO and a HD video deck you need a tri-sync generator. So it all depends on what you are trying to sync.
I hope this helps a bit with some of the tech background.
cheers
geo