Simple Pygame Audio at a Frequency

Question

How can I create a 440 Hz sound that plays smoothly forever using audio with Pygame? I assume this should be easy enough, but I don't want to use any stupid files to accomplish the task. The final intent of this is to play a note while a key is being held down as I have asked in another question. Any help would be greatly appreciated as I have wasted great quantities of time trying to find an answer to this.

Answer 1

After getting entirely too many "ValueError: Array depth must match number of mixer channels", and other similar errors, I discovered this working example http://www.mail-archive.com/[email protected]/msg16140.html. It properly generates a multi-dimensional 16 bit integer array that works with a stereo mixer. Minimum working example below, mostly lifted from the previous link with the necessary pygame bits thrown in. Tested in Python 2.7.2 with pygame.ver '1.9.1release'.

This example will play a 440 Hz tone out of one speaker and a 550 Hz tone out of the other speaker in a stereo setup. After a bit of playing with the duration I discovered that audible clicks will pop up in the sound loop if you set the "duration" variable to anything other than a whole number.

import pygame
from pygame.locals import *

import math
import numpy

size = (1366, 720)

bits = 16
#the number of channels specified here is NOT 
#the channels talked about here http://www.pygame.org/docs/ref/mixer.html#pygame.mixer.get_num_channels

pygame.mixer.pre_init(44100, -bits, 2)
pygame.init()
_display_surf = pygame.display.set_mode(size, pygame.HWSURFACE | pygame.DOUBLEBUF)


duration = 1.0          # in seconds
#freqency for the left speaker
frequency_l = 440
#frequency for the right speaker
frequency_r = 550

#this sounds totally different coming out of a laptop versus coming out of headphones

sample_rate = 44100

n_samples = int(round(duration*sample_rate))

#setup our numpy array to handle 16 bit ints, which is what we set our mixer to expect with "bits" up above
buf = numpy.zeros((n_samples, 2), dtype = numpy.int16)
max_sample = 2**(bits - 1) - 1

for s in range(n_samples):
    t = float(s)/sample_rate    # time in seconds

    #grab the x-coordinate of the sine wave at a given time, while constraining the sample to what our mixer is set to with "bits"
    buf[s][0] = int(round(max_sample*math.sin(2*math.pi*frequency_l*t)))        # left
    buf[s][1] = int(round(max_sample*0.5*math.sin(2*math.pi*frequency_r*t)))    # right

sound = pygame.sndarray.make_sound(buf)
#play once, then loop forever
sound.play(loops = -1)


#This will keep the sound playing forever, the quit event handling allows the pygame window to close without crashing
_running = True
while _running:

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            _running = False
            break

pygame.quit()

Answer 2

What kind of 440 Hz sound? There are many types of waves at "440 Hz": sine, saw, square, etc. You could have a flute playing an A, and that might count too.

Assuming you want a sine wave, it looks like you can generate a Sound object with pygame.sndarray.samples. (I haven't tested this) You can create the samples with:

samples = [math.sin(2.0 * math.pi * frequency * t / sample_rate) for t in xrange(0, duration_in_samples)]

This is hopefully basic sine wave stuff. frequency is the desired frequency, in Hz. sample_rate is the number of samples / second in the generated audio: 44100 Hz is a typical value. duration_in_samples is the length of the audio. (5 * 44100 == 5 seconds, if your audio is at a 44100 Hz sample rate.)

It looks like you might have to convert samples to a numpy.array before passing to pygame.sndarray.samples. The docs indicate that the audio should match the format returned by pygame.mixer.get_init, so adjust samples appropriately, but that's the basic idea. (mixer.get_init will tell you the sample_rate variable above, and whether you need to account for stereo, and if you need to adjust the amplitude of the wave or shift it.)

Make samples a integral number of waves long, and it should loop.