If you don’t know what the “A” frame is, then skip this post because it is way too esoteric.
However, if you’re an editor and trying desperately to digitize an HD tape or film transfer that has been down-converted to a digibeta, there are a couple ways to find the A-frame:
1) Digitize the footage with a particular starting timecode. Then check to see if the material looks wonky in the editing system (like an Avid). If it looks wonky, start your TC In Point one frame later and try again. Do this about 6 times and you will find the A frame.
2) Or, use the magic 9 trick. Start with any tc number you want as long as the non-drop frame TC ends in “09.” So like 01:00:12:09 would be a perfect place to start your digitizing. You will ALWAYS hit the A frame everytime. Why is that? I always stunk at math, but I just know it works like a charm.