At what point is the hash value of a file calculated so that it can be compared with others?

For example...If I rip a brand new song from a CD and then place it in my "shared" folder....where is the Sha1 value calculated for other hosts to view when searching for files?

Is it done within Phex by my local host, done within the P2P software on the remote host, or is it done by a third server during the search process?

just wondering
Thanks
Dave