My previous blog posts about deterministic Doom proved very popular.

The reason I was messing around with Doom's RNG was I was studying how early versions of Doom performed random pitch-shifting of sound effects, a feature that was removed early on in Doom's history. By fixing the random number table and replacing the game's sound effects with a sine wave, one second long and tuned to middle-c, I was able to determine the upper and lower bounds of the pitch shift.

Once I knew that, I was able to write some patches to re-implement pitch shifting in Chocolate Doom, which I'm pleased to say have been accepted. The patches have also made their way into the related projects Crispy Doom and Doom Retro.

I'm pleased with the final result. It's the most significant bit of C code I've ever released publically, as well as my biggest Doom hack and the first time I've ever done any audio manipulation in code. There was a load of other notes and bits of code that I produced in the process. I've put them together on a page here: More than you ever wanted to know about pitch-shifting.