Ref the Deadlift video, i do concur about some noticable back-rounding.
What i would suggest just for starters, and mind you im no deathlift expert is if you are using smaller than olympic size weights, put a plate underneath on each side to elevate the starting position, or atleast remove your shoes, that gives you a good 1-2 cm less rom alone.
Also try thinking about puffing your chest out when you are in the starting position, that will automaticly put your back in a concave line and not a convex one.
if you make sure the bar is at the center of the totalt length of your foot, then grab it (at this time you will have back rounding) Then make your chins touch the bar by bending your knees. Now if you puff your chest up, that will automaticly set you (and your back) in the right starting position.
Other than that, have a great weekend and keep lifting