I think the purpose to have two typies of sources(both for offset and noise situation) is counting for circuit thoery, but I just cannot see the obvious reason for this... ask for a simple but in depth explaination why only just one of them is not self consistent in circuit thoery...
ps: I think the problem roots from counting offset for input source with arbitary source impedence...if we use ideal voltage source, only voltage offset source is needed then,,,