How to interpret this PCM data?

Question

I am attempting to write a program to get the ATmega328P to play audio. I first tried to use this library , but I could not get it to work, so I am now attempting to program my own. The problem is that I don't know how to interpret the audio data that I have:

const unsigned char samples[] PROGMEM = {
  128, 128, 128, 130, 129, 127, 127, 128, 
  127, 127, 128, 130, 128, 126, 125, 125, /* ... additional 26 kilobytes of numbers ... */
};

This is the same data used by the library, and it came from an audio file that had a bitrate of 8 kilobits per second. I then used an executable from a tutorial to extract the data from an MP3.

I first tried to see if these were the frequencies that needed to be played each for 1 thousandth of a second (calculated from the bitrate), but it sounded like static and didn't seem to work.

The wiring is pretty simple, just a transistor with the base going to the output pin and the speaker being controlled by this transistor. I know that this part works because I can output a PWM signal on the pin and it sounds as you would expect.

What is this data and how would I write a program to play it?

Answer 1

Your data have relation neither to bitrate, nor to frequencies. The name samples[] suggests that they are samples, i.e. amplitude values in particular time points:

                 

These time points are determined by sample rate — e.g., if the sample rate is 48,000 Hz (48,000 samples per second), time points are 0 s, (1/48000) s, (2/48000) s, etc.

As your numbers are defined as unsigned char, the bit depth is 8 bits (1 byte) and their values are in the range from 0 to 255. Because the original audio oscillates between negative and positive values — for your 8-bit audio between -128 and +127 — you will eventually need to convert them into this range.