Sujet Pss-480

Citation :
circuit bending details

simple FM synthesizer

Cut the data lines between CPU and FM soundchip (at this soundchip pin 2, 3, 4, 6, 7, 8, 22, 23) and solder an 8er DIP switch block into the cut data lines in a way that the closed switches re-connect them like before. (I connected DIP switch 1 to 8 with soundchip pins 2 to 23 in ascending order.) Add 8 pull-up resistors (each 22 kOhm) to the soundchip side of these lines. The other ends of these resistors are soldered together and connected with the output of an alternating switch that switches between GND and +5V. This way soundchip bits can be modified by opening any of the DIP switches, which sets them all to the value of the alternating switch (0 or 1) by the low current through the pull-up resistors.
With this simple digital synthesizer now many weird new sounds can be created by selecting a preset sound, switching DIP switches off, selecting a different sound and switching them on again. Depending on the DIP switch, even the rhythm instruments can be alterated this way. Also keyboard presses get interpreted as FM synthesis data while DIP switches are open, though key combinations also change the sounds, but in this case the behaviour is only deterministic so far the instrument is switched off and on again before creating a sound this way. I conclude that otherwise the currently selected sound channel number from the previous key press gets misinterpreted as sound data too, which causes semi- random results. Also the effect of pressing drumpads or the vibrato button after synthesizing a sound (i.e. when the new sound is active and all DIP switches are closed again) often drastically modifies the new sound (e.g. shortens the envelope a lot, but further presses on the same buttons don't change it further). I guess that activating a drumpad or vibrato transfers some new register data from the CPU to the sound chip, those in the case of synthesized sounds don't match the alterated settings in the soundchip and thus change the sound. But what you want?! This mod gives you a synthesizer function for only a few euro of additional hardware, and if you just intend to sample the stuff, its anyway not that important how deterministic or logical the thing behaves. It's fun to get lots of messy sounds by pressing keys with DIP switches open, and since this instrument contains no sequencer RAM (unlike your possibly precious Yamaha PSS...), you don't need to fear to erase its settings. Switch it off and on again and it does always the same.

So far no keys are pressed and no rhythm is running while DIP switches are open, the behaviour of the synthesizer is even so well reproducible that you can write down your own library of synthesized sound by noting the initially selected preset sound, the DIP switch setting, the 2nd selected sound and possible vibrato/ drumpad presses after closing the DIP switches. (You can also select more than 1 new preset sound in sequence to synthesize a new sound.)

This easy modification can be likely added to any old Yamaha home keyboard that has a separate FM soundchip and it provides you a universe of new exciting sounds to explore for only about 3€.