Question

The following code writes a simple sine at frequency 400Hz to a mono WAV file. How should this code be changed in order to produce a stereo WAV file. The second channel should be in a different frequency.

import math
import wave
import struct

freq = 440.0
data_size = 40000
fname = "WaveTest.wav"
frate = 11025.0  # framerate as a float
amp = 64000.0     # multiplier for amplitude

sine_list_x = []
for x in range(data_size):
    sine_list_x.append(math.sin(2*math.pi*freq*(x/frate)))

wav_file = wave.open(fname, "w")

nchannels = 1
sampwidth = 2
framerate = int(frate)
nframes = data_size
comptype = "NONE"
compname = "not compressed"

wav_file.setparams((nchannels, sampwidth, framerate, nframes,
    comptype, compname))

for s in sine_list_x:
    # write the audio frames to file
    wav_file.writeframes(struct.pack('h', int(s*amp/2)))

wav_file.close()

Answer 1

Build a parallel sine_list_y list with the other frequency / channel, set nchannels=2, and in the output loop use for s, t in zip(sine_list_x, sine_list_y): as the header clause, and a body with two writeframes calls — one for s, one for t. IOW, corresponding frames for the two channels “alternate” in the file.

See e.g. this page for a thorough description of all possible WAV file formats, and I quote:

Multi-channel digital audio samples
are stored as interlaced wave data
which simply means that the audio
samples of a multi-channel (such as
stereo and surround) wave file are
stored by cycling through the audio
samples for each channel before
advancing to the next sample time.
This is done so that the audio files
can be played or streamed before the
entire file can be read. This is handy
when playing a large file from disk
(that may not completely fit into
memory) or streaming a file over the
Internet. The values in the diagram
below would be stored in a Wave file
in the order they are listed in the
Value column (top to bottom).

and the following table clearly shows the channels’ samples going left, right, left, right, …