Topic: new engine: StringKS
Wow, appearantly it's been almost a year since I published a new beeper engine. So it was about time for some good ol' t-state squeezing.
StringKS is an experimental engine that implements Karplus-Strong inspired string synthesis. It's more a proof-of-concept than an actually useful engine (hence no converter is provided), however it does prove that physical modelling is possible in 1-bit, and I think it's worth exploring this concept further.
Synthesis is done by creating an initial ring buffer from various sources (at the moment, ROM noise, rectangle wave with variable duty, and saw wave are supported), and then continually running a simple low-pass filter over the buffer. The size of the buffer determines the pitch. It is also possible to source from user-created data (so theoretically one can start from a pre-filtered buffer to create softer attack transients). Additionally, I threw in PWM sample playback on one of the channels, and regular rectangle wave playback (also on one channel only). There's also a (rather brutal) overdrive mode. All synth methods except the saw wave one support a somewhat crude 3-bit volume control.
source code
An extremely uninspired demo tune is attached.
Limitations:
- 8-bit frequency counters only, so the available note range is rather limited.
- At higher notes, tones will fade out very quickly.
I believe it's possible to rectify these issues, but more research is needed. One possible approach I experimented with was to generate data on the fly with the usual add-and-compare method while keeping track of the low-pass cutoff. It works but so far sound quality is worse than with the buffered approach. Another way might be to pre-scale the speed of buffer iteration to reach lower frequencies (e.g. update buffer pointer only every other sound loop iteration), and to slow down decay by only running the filter on every other buffer iteration. Still need to find some free t-states for that, though. Perhaps splitting updates so only one channel gets updated per sound loop iteration might be doable. Well, I'm open to ideas, of course 
