I too have been struggling with this.

I convert every 6 bits to a byte and convert to the approriate 8 bit
ascii character.

It all works for message type 1.

So, I thought I'd look at message type 5, which is

id 6 bits
repeat 2
user id 30
version 2
imo 30
call sign 42
name 120

It works up until imo, then for the call sign I get gibberish.

It seems characters don't like my approach ...

Does anyone have any code or suggestions?

Thanks