sine wave glissando from one pitch to another in Numpy

Question

I have been working on a program where I need to slowly and smoothly change the pitch of a sine wave from one pitch to another. I am able to get an array of the frequency the p

Answer 1

I like to think of frequency as the rate at which you are stepping through your sound sample - in this case a sine wave. Here's an attempt at some Python code to do what you want. We assume that the freq() method gives frequency as a function of time. For your purposes, it will be some kind of exponential. We are trying to fill a pre-allocated list called wave.

index = 0
t = 0
while t < len(wave):
  wave[t] = math.sin(2*math.pi*index/sample_rate)
  t = t+1
  index = index + freq(t/sample_rate)

Excuse my Python, I'm still learning the language.

Answer 2

The issue is that as you ramp through the frequencies, each frequency effectively has a different phase for the given time. When you scroll through these phases quickly and continuously, they drive the sine wave at higher frequency (or lower is also possible).

Imagine, for example, that you changed the frequency instantaneously -- to do this you'd have to supply the phase correction p_1 = p_0 + 2*pi*t*(f_0-f_1) to make the phases match up at time t. As you do this is little steps, you also have to make a similar phase correction, with each phase correction adding to the previous.

Here's the resulting figure, with the code below. The top figure is the frequency the middle is without the phase correction, and the bottom has the continuously corrected phase.

from pylab import *

sample_rate = .001
f0, f1 = 10, 20
t_change = 2

times = arange(0, 4, sample_rate)

ramp = 1./(1+exp(-6.*(times-t_change)))
freq = f0*(1-ramp)+f1*ramp
phase_correction = add.accumulate(times*concatenate((zeros(1), 2*pi*(freq[:-1]-freq[1:]))))

figure()
subplot(311)
plot(times, freq)
subplot(312)
plot(times, sin(2*pi*freq*times))
subplot(313)
plot(times, sin(2*pi*freq*times+phase_correction))

show()