C64Music!

Commodore 64 Music in the real world & other related SID stories

Thursday, March 15, 2007

Simple DCF filter in BASIC

This project just started as a joke while discussing the possibilities of using the Commodore 64 SID filter as a realtime FX unit at the prophet64-forum.

The main purpose was to have a really simple DCF application for the breadbox which was easy to realize in plain BASIC.

As you may know Commodore didn't forget about the SID's external in, even if it's hiding behind PIN #5 of the Audio/Video jack. It's difficult to find an adapter cable but easy to DIY. Unlike some people think it would be a sample-input it's just a direct connection to the SID's DCF/DCA in (aka EXT-IN).

To use the filter one has to switch on the EXT-IN in register #17, set the appropriate filter mode and DCA volume in register #18. The values for cutoff and resonance are set through registers #15/16 and the last 4 bits of #17. As long as there're no hardcore calculations required the BASIC language is suitable to perform this tasks ;).

Since I came up with that application at the prophet64-forum, where most people already got POTs attached to their breadboxes, I also went for a set of paddles to control cutoff and resonance. Pimping a C64 with a set of pots doesn't demand more than a pair of 470K potentiometers, a female 9 PIN SUB-D (aka "joystick plug") and just weak soldering skills. If you're up for it check the Getting Started w/ the Prophet64 manual which has the schematics...

Also, prophet64 enthusiasts tend to have the SID2SID, so support for a 2nd SID chip installation is granted as well. Last but not least, there are 4 filter modes to choose from: low-, high- and band-pass and notch.

Allright, once your Commodore got his input cable and pots you're ready to download and transfer the program. You may use Star Commander, anything else or just type in the short basic listing.

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rest of the article...
via robotsinmotion

thanks for posting this Ron !

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