Scratchin post, carpet or just wood?

I see these very expensive scratching posts at Petsmart.

I was thinking of making my own.

Does carpet really provide a scratching post for the cats? These things
they sell look like fun toys for the cat but do they really provide a
scrathin post. I thought they do that to sharpen and clean their claws.
Would a soft wooden post be a better choice? I dont see any made of just
wood. Some had rope wrapped around which seemed a better idea the carpet.

Your thoughts appreciated.

TY