Algorithmic Musician

First Deadlines

12 Nov 2015

The first two deadlines for my project went hand-in-hand. The first major deadline was to write a class that would open waveform files, convert 2 channels to 1 channel, convert 16 bit samples to 8 bit samples, and save .wav files. The first minor deadline was to create a testing script for the waveform reader.

Waveform Reader Test

This was due on November 2nd. You can see the waveform reader test I commited on November 2nd here. However, I have since moved the test from the “tests” directory I had it in to the main directory. You can find the latest testing script on this commit.

There are three tests it runs:

1. Load data from wave files and write wave files
2. Changing the channel count
3. Changing the sample width

Waveform Reader

This is due tomorrow, November 13. The waveform reader must pass the tests set forth by the last deadline. I created a module to accomplish this task. The module has a class inside it called “WaveForm” to represent all the data of a waveform.

Solving the problem of changing the channel count

This problem was not terribly hard to solve. To go from two channels to one channel I just got rid of one of the channels.

new_samples = ''
for i in range(0, len(self._samples), 2 * self._sample_width):
    new_samples += self._samples[i+2:i+self._sample_width+2]

Solving the problem of changing the sample width

This was a little harder then the previous problem. I did a little research and found out that in the wav file format, 16 bit samples are signed, while 8 bit samples are unsigned. A little weird, right? So, to convert one sample from 16 bits to 8 bits, I do the following:

struct.pack("B", struct.unpack("h", self._samples[i:i+2])[0]/256 + 128)

I unpack the 16 bit sample using python’s struct module (The “h” stands for a signed short). I then divide by 256 to convert the 16 bit size to an 8 bit size. Then, because the 8 bit format should not be signed, I add 128. This pushes the entire sinusoid range into the positives. Finally, I use struct again to pack the altered sample into an 8 bit unsigned integer (The “B” stands for an unsigned char).

To convert 8 bit samples to 16 bits we do the opposite:

new_samples += struct.pack("h", (struct.unpack("B", self._samples[i])[0] - 128) * 256)

Putting it all together

Once I completed both of these deadlines, I ran the test:

Elliots-MacBook-Pro:musician-project elliot$ python waveform-test.py mary.wav
Testing successful!

There you have it! The waveform reader works!