Not sure what got me started on this project. Perhaps it was listening to too much Meat Beat Manifesto in the late 80’s. Or maybe it was my wife who got me to sit down and watch the Scratch documentary. Whatever the hell it was, I’ve been wanting to make a decent vinyl scratch app with Actionscript for quite some time now. Now this idea isn’t exactly new. Since the release of Flash Player 10, it’s been possible to manipulate the very byte data of sounds with the SampleDataEvent and many have toyed with this idea including Lee Brimelow, Grant Skinner, Andre Michelle, (unfortunately you can’t actually see that one still), and this guy (sorry – the blog is in French and I couldn’t find an ‘about’ section to provide the actual name). All of these (including a similar one I did myself a few years back) pretty much sucked though – and I’ll tell you why…
The trouble with digital audio is that its, well, too digitized. That is, when you write bytes to an audio stream, no matter how fast or slow or whether you write forwards or backwards, every single one of those bytes will be distinctly played. The result of that then isn’t so much a ‘sccrrrratcchh’ sound, but rather how you might imagine a fast moving tape would sound – sorta like a turkey all hopped up on meth. But back in the analog ‘real’ world, when you slide a vinyl disc underneath a needle, it won’t play every note distinctly – instead it kind of plays the sounds in between and mixes it all together. Fortunately, there’s a way of emulating the mixing of in between values digitally; with the fancy sounding word ‘interpolation’.
Now seeing as how I know as much about audio processing as I do quantum topology (and I don’t even know if that’s a real thing or if I just now made it up), I do what everyone does these days – I turned to Google to see if I could find any ready made examples I could just copy. The most comprehensive thing I could find was this C++ / Obj-C example from Jan Kalis. So I downloaded the source and made a half assed attempt at just a straight port to Actionscript. Not really knowing C++, Objective C, or audio processing really made that a rather difficult task and I soon just gave up and went back to googling. While I couldn’t find any better source code, every blog post, forum thread and stack overflow question I came across all mentioned that magic word: ‘interpolation’ – quite often preceded by the word ‘hermite’. Actually, one other place where source code was available was in the Mixxx project (incidentally, Mixxx is a great open sourced DJ software project – I definitely recommend downloading it and playing around if you’re in to that sort of thing). So after digging around through the Mixxx source, I discovered that they were, indeed, using hermite interpolation to create their scratch sound. That then seemed to be the key. Still I was stuck though. I had no idea how to implement hermite interpolation with sound in Actionscript. So back to Google I went. And that’s when I came across this great blog post from Alex Nino. I downloaded the example files and immediately learned two things – how to use interpolation with sound and the very nifty little trick of extracting float samples from a sound’s byte array and storing them in a vector for easier access.
So I turned back to my own project and tried those two tricks and interpolating between a current position and target position based on mouse movement. I compiled expecting to hear amazing things and… nada. I still had my hopped up turkey. I searched around for other interpolation methods and tried 3 or 4 others. All without any change. At this point I cussed and fumed and kicked the floor and swore to give up on the project.
After that, I went back to the Jan Kalis source code. Now that I knew a bit more about storing and accessing sound float values and interpolating those values with Actionscript, all that C++ suddenly made a lot more sense. So with a bit of tweaking, fiddling, tinkering, and fine tuning that cpp into as, I finally came up with the below..
It ain’t perfect, but it’s the best I’ve heard in Actionscript yet. And if you’re interested in the source, you check it all out over on Wonderfl.