Re: [pygame] Creating a stereo sound from an array doubles frequency

[Mark Wexler <mark.wexler@xxxxxxxxx>]

Hello. On my system (Win 7 Pro, 64 bits, Python 2.7, pygame 1.9.2pre),
your program doesn't produce any sound at all. For it work, I have to
replace pg.mixer.pre_init(...); pg.init() by pg.mixer.init(...). (I
discovered this a while back, and put a comment about it in the
documentation. If you open a window, then pg.mixer.pre_init(...);
pg.init() works.)

If I put nchannels = 2, then make_sound() gives the following error:
ValueError: ndarray is not C-contiguous

Here's a simple script to synthesize stereo sound, that does work on
my system (and that doesn't double frequency, AFAICT):

import pygame, numpy, math

duration = 1.0          # in seconds
sample_rate = 44100
bits = 16

try:
    pygame.mixer.init(frequency = sample_rate, size = -bits, channels = 2)
    n_samples = int(round(duration*sample_rate))
    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
        buf[s][0] = int(round(max_sample*math.sin(2*math.pi*440*t)))
     # left
        buf[s][1] =
int(round(max_sample*0.5*math.sin(2*math.pi*440*t)))    # right
    sound = pygame.sndarray.make_sound(buf)
    sound.play()
    pygame.time.wait(int(round(1000*duration)))

finally:
    pygame.mixer.quit()


Mark




On Fri, Dec 2, 2011 at 9:24 AM, Ian Mallett <geometrian@xxxxxxxxx> wrote:
> On Fri, Dec 2, 2011 at 12:59 AM, Marcel Stimberg
> <stimberg@xxxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> Hi list,
>>
>> I'm having trouble playing sounds (generated from arrays) as stereo
>> sounds. Creating a sound and copying it to two channels seems to
>> double the frequency.
>>
>> The following script (see also here: http://pastebin.com/qqZbJavf )
>> should make it more clear. It constructs a simple 400Hz sine wave and
>> plays it as a mono sound (nchannels = 1) or duplicates the array and
>> plays it as a stereo sound (nchannels = 2). In the second case,
>> however, the tone that is played is an 800Hz tone instead of a 400Hz
>> one... This seems not to happen on Windows, I encountered the problem
>> on ArchLinux and Ubuntu 11.10 (both having pygame 1.9.1).
>>
>> import numpy as np
>> import pygame as pg
>> import time
>>
>> frequency, samplerate, duration = 400, 44100, 20000
>> nchannels = 1 # change to 2 for stereo
>> pg.mixer.pre_init(channels=nchannels, frequency=samplerate)
>> pg.init()
>>
>> sinusoid = (2**15 - 1) * np.sin(2.0 * np.pi * frequency * \
>>                   np.arange(0, duration) / float(samplerate))
>> samples = np.array(sinusoid, dtype=np.int16)
>> if nchannels > 1: #copy mono signal to two channels
>>       samples = np.tile(samples, (nchannels, 1)).T
>> sound = pg.sndarray.make_sound(samples)
>> sound.play()
>>
>> time.sleep(duration/float(samplerate))
>
> Hi,
>
> I can reproduce the sound doubling effect on Windows 7 Professional, 32 bit.
>
> Thanks,
> Ian



-- 
Mark Wexler
Laboratoire Psychologie de la Perception
CNRS - Université Paris Descartes
45, rue des Saints-Pères
75006 Paris, France
http://wexler.free.fr