Interesting...my STM32F4DISCOVERY just arrive Friday...intending to make a wavetable oscillator on it, fist thing...
Are you writing the wavetable to a buffer, then pulling from that at the sample rate? if so, what size? On Apr 9, 2012, at 6:28 AM, Julian Schmidt wrote: > Hello all, > > I'm trying to get an oscillator running on the STM32F4 discovery board. > I'm sending my audio with 44100Hz via I2S to the onboard codec. > However I'm experiencing strange problems. > > my code ist quite straight forward: > > phase += phaseInc(440); // calculate the phase increment for 440 hz and add > it to phase > if(phase >= TABLESIZE) phase -= TABLESIZE; > return phase/TABLESIZE; > > this should give me a non bw limited saw from 0 to 1.f > (I also tried it with an actual wavetable in use. a sine is played fine. but > as soon as overtones are added i get the fluttering) > > My problem is that i get periodic amplitude modulations on the overtones > about 2-3 times a second. > the root frequency amplitude is fixed, but the higher the overtone frequency, > the more it's amplitude drops every now and then. > this screenshot illustrates the waveform. the upper waveform has some ringing > on the discontinuity and the lower has not. > http://i.imgur.com/JyuEr.png (this image is taken from an inverted saw > wavetable. but the flutter is the same, no matter if i use a bandlimited WT > or output the phase counter directly) > the waveform oscillates between the upper and the lower image. > > the amplitude flutter from the overtones is also frequency dependent. > at 439 and 441 Hz its not really audible. > > so my tought is that is has to do with the wrap of the phase counter. but i > can't get my head around whats wrong. > any ideas where this change in overtone amplitude could come from? > > cheers, > julian > > > > > > > -- > dupswapdrop -- the music-dsp mailing list and website: > subscription info, FAQ, source code archive, list archive, book reviews, dsp > links > http://music.columbia.edu/cmc/music-dsp > http://music.columbia.edu/mailman/listinfo/music-dsp -- dupswapdrop -- the music-dsp mailing list and website: subscription info, FAQ, source code archive, list archive, book reviews, dsp links http://music.columbia.edu/cmc/music-dsp http://music.columbia.edu/mailman/listinfo/music-dsp
