Well could be a few lines.. But it needs processing. And you can genlock it. But you will not know if you have it genlock running. In your camera you need to set pre or post genlock to sync up to the atem.
Problem is the Atem will not tell you if you have it perfect sync or running 20 lines 2 lines or 2 pixel before or after the internal frame buffer.
This means that you can not tell if you are using the minimum delay or are just 2 pixels away from using a complete 1 frame framebuffer..... As the framebuffer is always on. You will always have a picture..
In the old days if you were not sync you would not have a picture at all or have garbage on the screen.
So determening acurate sync is difficult. Also depends on a lot of factors.. If you have fiber converters and a router between it. The sync settings are different then direct cable connections.. All because of reclockers in every SDI device that take just a tiny little bit of time to straighten up the signal.. But if you have enough of them in a line it will trow off your sync.
BUT now a fun fact. So take the Example of 50Fps. That is 1 frame is 20ms. Audio travels with 360 meters per second. There are 50 frames in a second. So we can say 360/50 = 7,2 meters per frame.
Audio travels 7,2 meters per frame!
That means that if you have 3 frames delay in your video. And you are sitting 21,6 meters away from the screen you hear and see in perfect sync. If you are at 7 meters from the screen you hear the audio 60ms earlier then video.. if you are sitting at 43 meters your audio will be 60ms behind the video.
If your venue is long enough (Stadiums, big churches, etc etc.. ) And you sit at the far end of the audio setup. 86m = 120ms delay 143m = 200ms delay.
Audio delay stacks will be placed at such big events.. All delayed with processors to be sure you don't create a delay and so introduce an echo...
Now the most fun fact were every AV company will go wrong.. They place extra screens so the spectators can see it better. But they forget to delay the video....
The result is a completely OFF lipsink..