To expand on something jwillis mentions, or explain it another way...
An external framesync TBC designed for VHS/S-VHS/Video8/Hi8/Betamax sources is ideal but unfortunately not always easily accomplished these days. Not easy to find, and sometimes costly when they are located. I knew this day was coming several years ago, which is why I invested both time and funds into TBC alternatives.
Genlock isn't TBC, but they are related. Having genlock doesn't guarantee TBC, or vice versa.
The DVK-100 chroma keyer is a unit that required multiple sources be timed, and so does contain both TBC and genlock abilities, but it's also not made for VHS. It was for live cameras, BetacamSP, even digital sources. So it's not as tolerant to bad source signals, and is why it must be paired with an ES15 to pre-process signals cleaner. The ES15 isn't a true TBC either. However, in a pairing, it can be. In some ways, it's still not as good as "true" TBC (TBC-1000, AVT-8710, etc), but in other ways it's better. The ES15 gives strong line correction, something not found in the external framesync TBCs. The ES10/15 framesync (probably not even a framesync TBC, just framesync) is swiss cheese, due to Macrovision, but the DVK fills in those gaps.
As alluded to, TBC is a wide term, and can mean almost anything. Again, I sometimes ponder if my toaster has a TBC!
If somebody wants to send me an SE-500, I'll give it a thorough test, head-to-head with units like TBC-1000 and green AVT-8710, plus others. I have a battery of harsh test tapes that can stress even the best TBCs, test their mettle.
After testing thoroughly, I acquired a lot of units last spring (not from eBay
, via my contacts), and started to resell them after my TBCs ran out. As usual, eBay
sellers have seen this, and are now trying to sell theirs for even more than I did -- and theirs are in worse condition, no power supplies, not tested, etc. As long as they don't copy my descriptions from this site, I don't care, but swiping my writings verbatim pisses me off (and I have reported some to eBay in the past, had the listings removed). And a couple of people are reselling the units I sold them (aka my advice of "buy it, use it, resell it"). I'm down to my last DVK-100, and probably won't be getting any more.