CHIP, guitar, music, pedals, pop, sound

POP: Programmable Open Pedal

What is POP?


This is my most recent project. Based upon the C.H.I.P. board form Next Thing Co, I have built a stompbox guitar digital pedal. These are its main features:

  • stereo output
  • true bypass
  • standard 9v input
  • three modes of operation
  • reset/shutdown  button (the red one)
  • additional programmable button (the black one)
  • wifi, enabled inly in one of the modes
  • microusb input, for serial connection
  • USB  input, where you can connect a MIDI controller, so effect parameters can be modified on the run
  • backup battery, so if you unplug the unit, you have the chance to shutdown gracefully

C.H.I.P board offers a good opportunity to build this kind of units because it has audio capture capabilities natively, without the need to plug an external (usually slow) USB sound card.

At this moment I am still doing tests, trying to improve sound quality. Some software stacks I have tested up to now:

  • Rakarrack on jack: unstable in this platform
  • Guitarix on jack:  quite stable and very good sound quality, but a lot of CPU consumption. The good point is that you can configure and control the unit graphically, doing a SSH -X from a computer.
  • Supercollider on jack. Super-flexible, Supercollider is a programming language, so you can create any effect your imagination dictates. The problem is that I haven’t reached the sound quality I am looking for, maybe a SC expert can do.
  • LV2 plugins on jack, running with mod-host. With this combination I have got the best results, in a next post I will upload some sound clips. You can control parameters with a MIDI controller, and preset parameters. You can choose from a huge list of already programmed effects, and combine them in any order.

My current software configuration is based on LV2 on jack. I have programmed three modes of operation using the switch you can see at the right in the above picture:

  • switch up: double delay, controlling feedback and delay time of the two delays with a Korg Nanokontrol2 MIDI controller
  • switch middle: compressor, controlling attack, release and gain
  • switch down: reverb, controlling room dimensions and warmth. In this position, I activate wi-fi interface and sshd, so I can connect my computer and make changes, update the software, fix issues, etc.

Some advices for anyone looking for something similar:


Wireless interface can be a problem if you enclose a computer inside an aluminium box, as is the case in this project. If you use a plastic box, you will have too much electromagnetic noise. For a compromise, I decided to drill a hole in the case, right above the wireless antenna. With an eight millimiters hole, and putting the unit not far from an access point, I am getting a quite good network access.


Another problem derived from the case is temperature. Without a fan, temperature was reaching 70 Celsius degrees. With a small 12V 25 mm fan, powered at 9V, temperature keeps stable at 51 degrees, even lower with wireless deactivated. I put the fun below the hole I had already done for wifi waves to escape the enclosure, as you can see in the pictures just below the switch.


I mean jack software. You have to compile it for no GUI, as they describe here:

For the rest of the software (LV2, mod-host) I will write a dedicated article.


I have got the best results using the minijack input for audio capture, and board connectors for output. I have grounded the audio capture and output with the board connectors and at this moment there is almost no computer noise.


My advice is to power the board using pin U13-2 CHG-IN, instead of powering it using the microUSB connector: it is discussed here:

I built a 5v/9v converter using a tipical 7805 IC, and fixed to the case with a screw. Don’t put the thing too close to the board.


With kernel 4.4, I got errors when capturing audio, so I sticked with kernel 4.3, until I can do some new tests or someone tells me it is fixed:


So I am very excited now with this project. My main concern now is sound quality and usability.  I’ll write some additional posts with audio clips and new notes and advices. I would like to upload some schematics with the internals of the unit.

If someone is interested in more details about the project, or have some questions, please write some comments,  or send me an email to


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s