Carpc project - digital audio

I recently uploaded a new video to show my custom-made carpc based on a Raspberry Pi. In the video, I had to leave a lot of information out for the sake of runtime, so, I decided to write a little series of posts to explore thoughts and ideas about this project.

This episode is about digital audio.

Disclaimer (sort of):
Be aware that this post is not self-contained. If you haven't seen the video, you probably won't get much out of this reading. Also, this is not a tutorial on how to build a carpc like the one I made. For that, I'd need much more time and space. I just want to document the thought process behind a few hardware and software choices, share some techno-ramblings, and provide you with some links to get you started using the components I used.

Have you ever modified your car with custom electronics? Are you planning to hack a Raspberry Pi into a device that was not intended to be modified? Have you ever started a simple project (such as adding music to your car), and somehow turned it into a huge, titanic undertaking?

Digital audio

 

The name of the game here is "signal integrity".

In its perilous journey from the Raspberry Pi to my car radio, the audio signal will suffer quite some degradation due to the conversions performed by the FM transmitter, losses in the transmission medium, interferences with other signals, imperfections in the FM modulation and demodulation, and more. Ah, that poor signal....

It is then understandable why I don't want to use the standard audio jack on the Raspberry Pi, isn't it? No? Well the Pi produces audio using Pulse Width Modulation, or PWM for short. PWM is a ridiculously cheap way to produce an arbitrary waveform, and it is often used to produce audio. Unfortunately, "ridiculously cheap" rarely comes with "good quality"... And PWM is no exception.

Generating analog signals (red) with PWM (blue).

PWM audio is affected by sound artefacts, and by a constant humming noise that cannot be removed. I already expect sound quality degradation from the FM transmission, and I really do not want to add another source of noise. The solution turned out to be quite simple. The Raspberry Pi supports digital audio via I2S, which is part of the GPIOs of the Pi. The only thing I had to do is to find a Raspberry Pi "sound card" capable of receiving digital audio, and producing a faithful analog representation of it.

iQaudIO Pi-DAC+.

I found the Pi-DAC+ on iQaudIO, where I also learned how to install the I2S kernel module and tell the Pi to output digital audio instead of PWM by default. The core of the Pi-DAC+ is a Digital to Analog Converter, whose job is to take a digital audio signal on the I2S input port, and produce a high quality analog audio signal ready to be amplified (or, in my case, transmitted).

I'm not an audiophile, but I can testify the difference is really noticeable! I'm really glad I got this device for my carpc.


Have fun,
Kradion.