CHIP, guitar, pop, sound

POP: noise reduced and schematics uploaded

After cabling audio internally with shielded wires, and solving a grounding problem in the input jack, noise has been reduced drastically when working from battery. There is still an annoying computing noise when connected to an external 9V power supply. I am going to put a removable ferrite bead, still waiting for the parcel to arrive.

I have also finished schematics for the project, that have been uploaded to



Next steps:

  • Trying with ferrite bead
  • Adjusting output volume, too high right now
  • Giving a function to second switch, maybe fast reset for changing mode quickly
  • Reducing latency
  • Creating new effects: some ideas:
    • dubbler
    • auto-ovation: just for fun, when you stop playing, simulate an audience ovation, just like in  “Blues Deluxe”, from Jeff Beck Group
    • tremolo, phaser, 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.

guitar, pedals, sound

Where to put a Fuzz Face

A lot has been written about where to put Fuzz Faces when being combined with other pedals, especially with Wahs. The fact is that Fuzz pedals are based on a very basic circuit, with a very low input impedance, and where the serial resistence of the 9v battery has a lot of influence. I have a Dunlop Fuzz Face that was living in ebay until I changed its alkaline battery for a very cheap carbon zinc one. The improvement in sound was awesome, and its price has increased  10 times (just in case you want to buy it). After I abandoned the idea of putting it in ebay, I had to decide where to put it in the pedals ecosystem.

Instead of explaining the principles behind the personality of this pedal, I will try to test it in different configurations and show the results as sound clips. These results are valid for *my* Fuzz Face, since each one has its own idiosyncrasy, but I expect them to be similar for any other pedal out there.

Guitar: Fender Stratocaster American Standard – 2012. Maple neck. Pickup: neck.
Wah: BOSS Fw3
Fuzz: Dunlop Fuzz Face germanium with carbon zinc battery.
Univibe: TC Electronic Viscous Vibe, chorus position, true bypass set (it can be configured as buffered, but I left it as true bypass on purpose, in order to illustrate how impedances influence sound).
OD: Ibanez TS-9
Amplifier: POD 2.0 for recording, bypass position and “modern class a” emulation

Recorded with Ardour.

Before listening to the clips, please keep in mind that I am not a guitar virtuoso, so please concentrate on sound 🙂

  1. Guitar -> Fuzz -> Wah -> OD -> Univibe
    1. Fuzz On, rest off – 
    2. Fuzz On, Wah On – 
    3. Fuzz On, Overdrive On –
    4. Fuzz On, Univibe On – 
  2. Guitar -> Wah -> Fuzz -> OD -> Univibe
    1. Fuzz On, rest off – 
    2. Fuzz On, Wah On – 
    3. Fuzz On, Overdrive On – 
    4. Fuzz On, Univibe On – 
  3. Guitar -> Wah -> Fuzz -> Univibe -> OD
    1. Fuzz On – 

I have tested only some logical configurations. Usually guitarrists have doubts about how to combine Fuzz with Wah or Univibe, but few people would put a Wah after a Univibe or a Fuzz after a Univibe. Some guys put Fuzz after overdrive, I prefer the opposite, but this is just a personal preference.

So I have ommited many configurations, like:
– Guitar -> Univibe -> Wah -> OD -> Fuzz
– Wah -> Guitar -> OD -> Fuzz -> Univibe 🙂
– etc.

Some comments about the configurations, from my point of view: In general, I prefer configurations 2.X over configurations 1.X. With Fuzz Face right after guitar you can control the Fuzz effect with guitar volume. Below 8, there is virtually no fuzz, and beyond that, fuzz comes and begins to increase until 10. I’ll show it in some other post, testing different pickups. But for my taste, the sound is worse than fuzz after Wah (config. 2). After wah the sound is much more full of frequencies, louder, more uniform in time, with better bass presence and more sustain, and what is more important, more Hendrix-like. Univibe in configuration 2 is more metallic and brilliant and again, more Hendrix-some. The difference has to do with the buffer in the Wah pedal, with very low output impedance, instead of the guitar pickup impedance, in the KOhms range. It has a very noticeable influence in the Fuzz Face circuit, that has a low input impedance, far from an ideal effects circuit. For my ears, when putting the fuzz face between two buffers (FW3 and TS-9) the sound is clearer and reacher in frequencies and has better sustain.

I tested configuration 3 in order to show the difference when Fuzz Face has a True Bypass pedal (Viscous Vibe with factory configuration) instead of a buffer. The influence in the output is not as noticeable as in the input, but for me it is somewhat darker than the other way around.

Viscous Vibe pedal is a super-versatile pedal. I have tested it in Chorus Position because I like the sound it gives by default when combined with fuzz. When I combine it with an overdrive, the volume drops too much when it is on, this will be discussed in another post. Actually, I’ve heard that the original Shin-ei Univibe has the same “problem”, so they at TC Electronic did a good job.

Combining Fuzz and Overdrive in my tests is producing too much noise. When combining them, it is better to keep the effect pot at middle position in both. Overdrive can tame the Fuzz effect and shape the frequencies response in some measure, as well as volume.

My comments are of course debatable, and if your opinion is different or you have a different experience, please leave a comment below, it will be appreciated.

To know more about Fuzz Faces, please visit these links:

You can buy Viscous Vibe here:



emulators, gamepad, sound

XBMC + ROM Collection Browser + retroarch: sound and joystick calibration

A new modification to, so you calibrate and map the joystick and force mixer before launching the emulator.
Don’t take the arguments as is, they are just an example. Well, the amixer arguments force audio output to HDMI.


# Check for arguments
if [ -z "$*" ]; then
echo "No arguments provided."
echo "Usage:"
echo " [/path/to/]executable [arguments]"

echo "Trying to stop xbmc "
sudo /etc/init.d/xbmc stop
# Deactivates HDMI output
sudo /opt/vc/bin/tvservice -o
# Activates HDMI output again
sudo /opt/vc/bin/tvservice -p

# Wait for the kill
sleep 1

# Joystick calibration and mapping
jscal -s 3,1,0,82,82,8388352,8134160,1,0,99,99,6882750,8388352,1,2,84,84,7254791,7063875 /dev/input/js0
jscal -u 3,0,1,6,4,288,289,290,291 /dev/input/js0
# Force audio output
amixer cset numid=3 2

echo "$@"

# Launch app - escaped!

echo "Emulator terminated"

# Done? Restart XBMC
sudo /etc/init.d/xbmc start