Basically it boosts the top end whilst writing to tape and the cuts the top end when playing back ... i'm sure it's a bit more complex than that. If youreally want something that does Dolby you could probably find an old 16 track that's beyond repair with working dolby bits ... a lotof the big tape machines have seperate units for the noise reduction.

But it seems like a lot of heartache for something relatively unexciting.