You got your math wrong. What you are calculating is: sin(2*pi*(1000+15*sin(2*pi*6*t))*t) = sin(2*pi*1000*t + 2*pi*15*sin(2*pi*6*t)*t) The 'instantaneous frequency' can be calculated by differentiating the argument of the sine and dividing by 2pi: x = sin(phi(t)) -> f_inst = d (phi(t)) / dt / (2*pi) So in your case: f_inst = 1000 + 15*sin(2*pi*6*t) + 2*pi*t*6*15*cos(2*pi*6*t) the last term explains the effect you hear.
What you want is: f_inst = f0 + df*cos(2*pi*fm*t) Integrating this and multiplying by 2pi gives the phase: phi(t) = 2*pi*f0*t + sin(2*pi*fm*t)*df/fm So you can achieve the frequency modulation by using phase modulation (these two are related). You can do this with your own code by phi = oscillator(t, freq=6, amp=15/6) tone = oscillator(t, freq=1000, amp=0.1, phase=phi) cheers, Bas -- http://mail.python.org/mailman/listinfo/python-list
