Building an advanced audio effects unit for the Red Tin using the BeagleBone Black SBC and the Bela software with an audio processor cape.
Having built a simple sample-based sound effects unit and a Fuzz Box guitar pedal, I thought it would be fun to try to build something more advanced that could be incorporated into the Red Tin.
, but for the BeagleBone Black development board — and decided to give this a go with a BeagleBone Black Wireless (125-2412) that we had.
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Bela was developed at the Queen Mary University of London as a project of the Augmented Instruments Laboratory, who needed a versatile, low latency audio platform for a project they were working on and could not find what they wanted “off the shelf”, so built their own. You can read the story in more detail here.
The first job was to fit the Bela cape on to the BeagleBone Black, I could then set up the Bela software. I use Ubuntu for my work computer and Windows in the Red Tin, so I will outline the procedure for both operating systems here.
I downloaded the Bela image from here and un-zipped it. My copy of Ubuntu already had software that un-ziped the .xz format that Bela uses, but if you are using Windows you may need to use something like 7Zip to extract the .img file.
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Then, making sure you have the SD card selected in the “Destination” menu, click “Start Restoring...”, confirm you really want to do it, enter your password and then just wait for the image to be written to the card. This will erase any existing data on your SD Card!
If you would rather do it from the command line the instructions are on the “Flashing the Bela Image” section of the Bela github pages. If you are using Windows it is recommended to use the Win32 Disk Imager.
Having done the above and inserted the Micro SD Card with the newly written Bela image into its slot on the BeagleBone Black, I connected the Bela/Beaglebone to my computer and waited for it connect. It takes a little while, but the blue flashing LEDs reassuringly indicated stuff was happening. It is then just a case of opening a web browser and going to http://192.168.7.2, the IP address of the BeagleBone Black.
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You should then see the Bela IDE in your browser Window. Only what I saw was the Beaglebone page with no sign of Bela. After some investigation I discovered that the BeagleBone Black boots from its built in eMMC memory, which contains a default O/S image. You can force it boot via Micro SD card by powering it down, holding the User button down and then re-connecting it, keeping the button held until the group of 4 LEDs by the Micro USB connector start to show activity.
The little button is quite hard to get at with the Bela cape installed, so it is actually easier to remove it first.
Once I could get into the Bela IDE I could run the following command by entering it into the console section at the bottom of the IDE:
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With the Bela image now on the eMMC it would automatically boot from that, so I powered it down, removed the Micro SD and replaced the Bela cape before restarting it. As expected, this time it booted from the Bela image.
The cape comes with cables to connect 3.5mm stereo jack sockets to the audio in and out connectors, which makes life easier.
I decided the best thing to do now would be to run through some of the examples which can be found on the Examples and Projects page.
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This would be a good way to familiarise myself with the Bela IDE and, as I discovered, there were links to the code for all the Examples in the IDE’s menu.
I started with the sine wave generator. It was simplicity itself to load the code into the Bela IDE and then I just clicked the big green run button. I connected my headphones using the adaptor cable provided and sure enough, I could hear a sine wave.
As you can see in the code above, each of the examples has a comprehensive explanation of what the code is doing as comments at the bottom. As it explains “The frequency of the sine tone is determined by a global variable, `gFrequency` — so having found that variable in the code I could easily alter the value, re-run the code and hear the difference.
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Once I had the components it was straight forward enough to put together and I soon had it wired up and connected to the cape, with the code loaded and running in the IDE. This is when I discovered the LEDs were back to front in the example. I could just swap the yellow and red LEDs with the three green ones at the bottom and turn the whole thing the other way up, but that was going to make the wiring messy, so it was just as easy to reverse the order of the connecting wires and then it worked as expected.
I was now eager to try some of the effects, as my ultimate aim is to use this for an effects module.
As with the other examples, I found the code in the right-hand section of the IDE, listed in the Code Examples Section accessed by clicking the Light Bulb icon.
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I thought now would be a good time to learn how to load a sketch on boot, without being connected to a computer, as this would be vital for a standalone effects generator.
First I saved the delay code as a Project by clicking the folder icon in the IDE, cicking the “Save project as” button, calling it “Delay” and clicking Save. Then I could go to “Project Settings” (the Cog Wheel icon), click the drop down next to “Run project on boot” and select my newly saved Delay project. I shut down the BeagleBone and disconnected it from the computer. I then powered it up with a separate power supply and once it had started I could hear the delay in full effect.
The speed of the Tremolo is controlled by a 10K potentiometer (249-9200) that is connected to 3.3V and GND on the cape 1st and 3rd pins, with the 2nd middle pin to analogIn 0.
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I ran a sine wave in a loop on my DJ software, to give a clear illustration of how the effect sounds.
My plan now is to build the BeagleBone Black and Bela cap into a module that will fit in the Red Tin, similar to the sound effects module I built. I will also need to work out how to load multiple effects and switch between them with buttons mounted on the case. I know this should be possible, as there are examples of guitar effects boxes built with Bela that do just that.
I have a background in the arts, environmental conservation and IT support. In my spare time I do a bit of DJing and I like making things.Advanced Audio Effects with a BeagleBone Black and Bela Part 2: Building a Multi-Effects Module for The Red Tin star_border Follow article
Scriptable Guitar Pedal/boot/uenv Alsa.txt At Master · Thomasfaingnaert/scriptable Guitar Pedal · Github
In the first part of this post, I explored the potential of the Bela cape for the BeagleBone Black (125-2412) , trying out some different sketches (code projects) and getting a feel for what it was capable of. Now it was time to put it to use in the Red Tin.
I found an example of a guitar effects pedal based on the BeagleBone Black and Bela, that I thought would be a good place to start with my DJ effects unit. It featured a number of sound effects that could be selected with push buttons and then their parameters adjusted.
Being designed for use with a guitar, some of the effects were not exactly what I wanted, but I was hoping I could edit the Bela sketch to substitute others.
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I contacted the maker of the pedal on the Bela user forum and he was very helpful. Throughout my recent adventures in electronics, I have almost invariably found people to be very generous with their time, expertise and experience, for which I am very grateful.
I am basing the enclosure design on the Sound Effects module I built earlier. I know it fits well in my toolbox, but it does mean things might get a bit cramped as the combined BeagleBone Black and Bela take up a lot more space than the Adafruit Sound FX Processor board — and there are 6 potentiometers and 3 buttons to fit in. I would also in future like to include an LED light bar that could be used to indicate which effect was running or as a level meter, so I needed to leave room for that.
I made a prototype using MDF for the base and a scrap piece of clear acrylic for the top. Both were laser cut and the sides of the top were then formed by heating and bending them using an acrylic bender.
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