After the unprecedented success of making sound in the last post, it’s now time to actually process the input in some way. I’ll look at two types: fixed length and variable length. The basic idea of a delay is that you write the output into a buffer and then read it out again after some delay, adding the delayed output to the main output. This gets saved again and the process repeats, thereby creating an echoed fade out sound.

I recently went back to my old bootcamp, Founders and Coders, to give an alumni talk on The Internet. Bootcamps have the difficult task of condensing an immense amount of information into a short space of time. The focus is on getting people to the stage where they can be productive developers. Inevitably this means that some topics get delegated to the “you’ll pick this up along the way” category. At Founders & Coders we use Heroku for deployment in order to abstract away the infrastructure details and allow students to focus on their development skills.

After four posts we still haven’t produced any sound. Let’s change that! I’m going to start with the aural equivalent of “Hello, world!”, which is changing the volume of the output. I’ll also go through a refactor I did to allow each effect component to be linked to multiple parameters. You can find the code for this post here. Setup So, I’m using the Audio Injector stereo hat to handle audio input and output on my Pi. There are plenty of Pi sound cards available but this one had good reviews at a reasonable price. Setup is pretty straightforward, requiring nothing more than connecting it and running a shell script. Straight away I could play tracks through my Pi using sox.

Having updated the Arduino code to output more complex messages, we now need to update the Pi app to be able to understand these messages. Remember that we’ll have a set of audio effect components running, each linked to a physical control made up of a potentiometer and button. The task that now befalls our Pi app is to parse the binary messages coming across on the serial and then dispatch the message to the correct effect component.

So far in the blog series we’ve set up the basic hardware structure and got everything talking. The next step is to expand the Arduino code so that it can handle multiple controls and send more useful messages. Remember that each virtual effect component running on the Pi will have a single potentiometer and button for control. I’m working on the basis that I’ll end up with six effects simply because my Arduino only has six analogue pins but the code will be able to handle more.

Continuing my theme of ’things to do with a Raspberry Pi involving music’, I have turned my Pi and external HDD into a little NAS. My iTunes library now sits on the HDD and is shared over Samba to my laptop. The cool thing about this is that it makes it possible to access my music library from anywhere over the internet. In the age of Spotify and other streaming services this is a lot less cool than it would have been, but it’s still pretty useful if you value having control over your music files. I have tried two approaches, each with different pros and cons.

In the previous post we looked at how to use an Arduino to read an analogue control and transmit it to a Raspberry Pi over serial. Now it’s time to look at how to handle those values on the Pi. We’ll also look briefly at how the C++ application uses the JUCE framework and its multi-threaded approach. You can look at the complete C++ code here (MainComponent.cpp will be of most interest). The structure of this code is roughly taken from the multi-threading example here.

In this series of posts I’m going to document building a digital DJ effects box using a Raspberry Pi and Arduino. The inspiration for this project came from the combination of me having a crappy mixer with no built-in effects, a Pi and Arduino (both in need of a purpose) and a desire to learn more about audio programming and digital signal processing (DSP). In terms of hardware I’ll be using the aforementioned Raspberry Pi 3 with this sound card to handle audio inputs and output and the Arduino Uno to handle the analogue control inputs. The Arduino side will be done in C, because it’s fun to program Arduinos in C, and the Pi app handling the actual audio processing will be in C++ because everyone seems to use it for audio DSP. I’m using the JUCE framework as an easy way in.