Reading raw audio values from ADC chip on raspberry pi

Question

I wired up the MCP3008 ADC chip to an Electret Microphone and to my pi. I'm reading the input using bit-banging in python, and I'm getting an integer from 0-1024.

I followed this tutorial to do the bit-banging: https://learn.adafruit.com/reading-a-analog-in-and-controlling-audio-volume-with-the-raspberry-pi/connecting-the-cobbler-to-a-mcp3008

My question is how do I take this integer and convert it to something meaningful? Can I somehow write these bytes to a file in python to get the raw audio data that Audacity can play? Right now when I try to write the values they just show up as the integer instead of binary. I'm really new to python, and I've found this link for converting the raw data, but I'm having trouble generating the raw data first:Python open raw audio data file

I'm not even sure what these values represent, are they PCM data that I have to do math with related to time?

Answer 1

What you are doing here is sampling a time-varying analogue signal. so yes, the values you obtain are PCM - but with a huge caveat (see below). If you write them as a WAV file (possibly using this to help you), you will be able to open them in Audacity. You could either convert the values to unsigned 8-bit (by truncation and) or to 16-bit signed with a shift and subtraction.

The caveat is that PCM is the modulation of a sample clock with the signal. The clock signal in your case is the frequency with which you bit-bang the ADC. Practically, it is very difficult to arrange for this to be regular in software - and particularly when bit-banging the device from a high-level language such as Python. You need to sample at twice the bandwidth of the signal (Nyquist's law) - so realistically, 8kHz for telephone speech quality.

An irregular sample clock will also result in significant artefacts - which you will hear as distortion.