Saturday, May 30, 2009

Sound on the MSP430

Started playing with sound on the MSP430. I'm using an MSP430F2618 because it has an internal DAC, although a '169 or using an external DAC would work too. A couple of notes:
  • Output of DAC feeds a Sallen-Key filter which then drives a TI TPA721 300mW amplifier which drives a little toy speaker
  • White noise is harder than it sounds. After several tries, I ended up using a Linear Feedback Shift Register, specifically a Galois LFSR. See WikiPedia.
  • For sine waves, I made a wave table with 72 points (every 5 degrees from 0 to 355 degrees)
  • To get volume levels I ended up making ten wave tables, one for each 10% volume level. You could of course compute this on the fly but it took too many clock cycles for my implementation.
  • To prevent "zippering" at the start of the code I compute intermediate wave tables (5%, 15%, etc)
  • Learned the hard way that the "rail to rail" op amp ain't. Needed to recompute the wave tables with an offset to keep the bottom of the DAC output above the ~100mV floor of the op-amp.

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