CHIP, guitar, music, pedals, pop, Uncategorized

POP: Autoovation feature

Now POP has a new feature, that I call “autoovation”. You start playing with your favorite effect, and when you stop playing, sometimes, if the pedal likes your performance, you hear an ovation from a digital audience.

Autoovation is made basically by detecting audio activity in input, then waiting for silence and playing a file randomly from a list of mp3 recorded ovations. For audio detection, silentjack ( was used (thanks Nicholas J Humfrey for your work and for letting me add a feature to your code).

This feature is just a joke, of course, but gives an idea of the kind of things that can be done with a device like this one. Think about things like backing tracks, automated rythm boxes, etc.

CHIP, guitar, music, pedals, pop, sound

POP: Programmable Open Pedal, first test

First recordings of POP through a Fender Blues Deluxe 40W A/B tube amplifier. Camera is a GO Pro HERO. The amplifier is looking towards the wall in order to save our ears. Guitar is a Yamaha Revstar 420, I love it.


Sound quality looks promising, though still I can hear some computing noise, specially when configuring a compressor or a distortion/overdrive. When using delay type effects, or reverb, noise is quite bearable.

I am using a 512 bytes buffer, and getting around 20ms of latency, still too much.

Some more research is needed to get the best from this unit, keeping in mind the limitations: CHIP board works at 1GHz, and sound card is integrated in the SoC, as long as I know.

Stay alert for new tests.

I have also still to publish some documentation about the project. More details can be found here:

Please leave any feedback you consider relevant or funny.

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

math, music, sound

The ugly, the good and the minor third

Why does a melody sound good to us, to some of us but not to others? Why does a song sound sad while another one sounds glad or awfully dissonant? I don’t know, of course, but it seems to have to do with evocation of known melodies, cultural conventions and something in our brain that is still to be discovered.

Some time ago I was curious about what kind of sound resulted from the addition of two notes in a chord or a melody.

Let’s consider a root note, let’s say A440, known to have a frequency of 440Hz. The sound will be a sinusoidal wave,  something that would seem like a flute.


Now let’s add a perfect fifth. We’ll consider a perfect fifth to have a frequency = 3/2 of its tonic.


In black we have the tonic, in red perfect fifth and in blue the sum of both signals. What happened? The resulting sound is a signal with a frequency of 1/2 * 440Hz = 220 Hz.

Now let’s consider our A flute and an added major third. There are several standards for major and minor third frequencies related to tonic (see but for this exercise I will take the just intonation, in which the major third has a frequency of 5/4 multiplied by its tonic frequency.


In black the tonic, in red the major third and in green the sum. Now we have a resulting signal of  1/4 * 440Hz = 110 Hz.

Now let’s add a minor third to the tonic. In just intonation, minor third has a relationship of 6/5 with its tonic frequency.


In black the tonic, in red the minor third and in green the addition of both. Now the resulting frequency is 1/5 * 440Hz = 88Hz

The last plot will consider a very dissonant note, a minor second, with a relationship with the tonic of 16:15.


The frequency of the resulting signal is 1/14 * 440Hz = 31.42Hz

Obviously, as the second note’s frequency approaches the tonic’s frequency, the resulting signal has a lower frequency.

Now let’s take a chord. A major chord will be something like this:


In black the tonic and in red the addition of tonic, major third and perfect fifth. The resulting signal has a frequency of 1/4 * 440Hz=110Hz.

Now with A minor:


Now the resulting signal has a frequency of 1/10 * 440Hz.

Let’s summarize the results:

interval/chord frequency note
tonic 440Hz A4
perfect 5th 220Hz A3
major 3rd 110Hz A2
minor 3rd 88Hz ~F2-F#2
minor 2nd 31.42Hz ~B0-C1
A major 110Hz A2
A minor 44Hz ~F1-F#1

Perfect fifth and major 3rd have a curious property: when you add them to the tonic note, you produce a signal whose frequency has the same frequency of the same note in a different octave. However, when you add a minor 3rd or a minor second to the tonic, the resulting signal frequency has no relationship with any note.

I hope you have enjoyed this mathematical experiment and that it will make you think about intervals in a different way. But above all, please enjoy your favorite music, either thinking about intervals or not thinking at all.