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spxlAudioToMidi v11.3

by subpixel,


A small application made with Processing to produce MIDI control messages from a live audio feed. Auto-levelling frequency band meters are used to obtain "peak" output values across the entire audio spectrum analysed (whereas it is usual to have strong bass response with weaker treble response), and easing is applied to make the levels less erratic.

The auto-levelling algorithm tracks high and low values output by the audio analysis (FFT) for each frequency band:

10 separate monitor outputs are supplied, where each monitor is attached to one of the frequency band meters and that meter's output is scaled to the particular output range for the monitor, allowing for a reduced final output range as well as inverted ranges. Drop-down menus are provided to select the MIDI input and output devices, as well as the MIDI channels to use.

This app has been used to control VJ applications Resolume and Neon v2, allowing simultaneous control from an external MIDI controller on the same channel (using loopMIDI as a virtual MIDI driver and MIDI-OX for routing).

Monitoring audio on Windows 10

The "Stereo Mix" device only exists for some audio hardware. To do monitoring in Windows 10 when there is no "Stereo Mix" requires using a 3rd party driver. See How to Get Stereo Mix in Windows 10.

As of v11.3, spxlAudioToMidi searches for a mixer name starting with strings listed in a text file in the data directory called "mixer-prefs.txt", listed in order of preference., for example:

VoiceMeeter Output
CABLE Output
Stereo Mix

which should match the primary output of Voicemeeter from VB-Audio Software (tested with Voicemeeter Potato), VB-CABLE Virtual Audio Device, also from VB-Audio Software, or the Stereo Mix, if any of those are available. If you are using a different audio device, or want to change the order of preference, simply edit the text file and restart the app.


spxlAudioToMidi v11.3 (2020-10-10): developed and tested with Processing v3 (3.5.4 Win 64) on Windows 10 Home (64-bit).

NOTE: MIDI input has been disabled. It needs to be rewritten for The MidiBus library.

You can also check the file list.

Libraries used

LibraryPurposeLocal copy
The MidiBus MIDI input/output (132KB)
Minim Audio analysis (FFT) (18MB)






Demo videos


The text at the top of the window controls the MIDI output. The left column of 30 bars are audio meters; each meter representing an average for a frequency band produced by FFT analysis with the low frequencies at the top of the display and the high frequencies at the bottom of the display. The meters are auto-levelling and provide normalised output values between 0 and 1 (inclusive). The right column of 10 bars are meter monitors, with each one connected to a particular meter. A monitor's output value is determiend by scaling the input meter value over the monitor's output range between 0 and 127 (inclusive); the lower and upper limits of each range can be adjusted, including to scale the meter input inversely, and the meter it is connected to (thus the frequency band it monitors) can be changed. If a MIDI output device is selected, each time a monitor output value changes, the value is emitted as a MIDI control message on the selected output channel. The control number is in the range 1 to 10, with the top monitor being control 1 and the bottom one being control 10.

Selecting MIDI output device

Click on the "SELECT OUTPUT" text to pop up the "SELECT DEVICE:" menu. Click a displayed device to select. Subsequently, clicking a displayed device name presents the "SELECT DEVICE:" menu again, and clicking the displayed channel number presents a "SELECT CHANNEL:" menu with channels 1 to 16.

Adjust monitor output ranges

Each monitor ouutputs values in the range 0 to 127 (inclusive) by default. The "minimum" and "maximum" limits can be adjusted by clicking (and dragging) the monitor output bar. Clicking in the top 3/4 of the bar adjusts the maximum; clicking in the top 1/4 of the bar adjusts the minimum. When dragging, if the mouse is dragged above or below the bar, the limit being dragged (min or max) snaps back to the previous value. Right-click the bar to reset to the default full range (0 to 127).

Changing monitor links

When the mouse is just off the left edge of a monitor bar the link to the associated meter is hilighted and "Drag to meter" is displayed. Click and drag the mouse pointer, releasing over to the desired meter; releasing when not over a meter leaves the link as it was previously. Right-clicking when "Drag to meter" is shown resets the link to the meter assigned by default when the application was started.

Keyboard controls

The "selected monitor" refers to the monitor that the mouse is over.

Space / P
[P]ause (and unpause)
[M]ute meter inputs; Shift+M for a mute shot (a few values output while muted)
Reset [E]asing amount
Toggle [E]asing of meter values (on by default)
Up / Down
Increase / Decrease easing amount
[ / ]
Decrease / Increase input MIDI CC and note offsets
Reset ("[Z]ero") the meter levels
randomise links (to meters) for all monitors; Shift+Q for selected monitor
reset links (to default meters) for all monitors; Shift+A for selected monitor
[F]lip range (swap min and max output values) for all monitors; Shift+F for selected monitor
in[V]ert range (sets min to 127-min, max to 127-max) for all monitors; Shift+V for selected monitor
[R]andom range (sets min and max output values) for all monitors; Shift+R for selected monitor
re[S]et range to default (sets min to 0, max to 127) for all monitors; Shift+S for selected monitor
Random [U]pper range (sets max output values only) for all monitors; Shift+U for selected monitor
reset range upper value to default (sets max to 127) for all monitors; Shift+Y for selected monitor
Random [L]ower range (sets min output values only) for all monitors; Shift+L for selected monitor
reset range lower value to default (sets min to 0) for all monitors; Shift+K for selected monitor
1 2 3 4 5 6 7 8 9 0
Set MIDI output channel (channels 1 to 10 only, 0 for channel 10)
! @ # $ % ^ & * ( )
Set the the MIDI input channel (channels 1 to 10 only)
Toggle display of values in [H]exadecimal
Toggle display of meters
Toggle display of monitors
Analyse "Microphone" input
Analyse "Stereo Mix" / "CABLE Output"


Note: These screenshots are from an earlier version running on Windows 7. The appearance and operation is similar in this version.

Default state

Select MIDI device

Link to meter part 1

Link to meter part 2

Hover to show value

Set max output
Click (and drag)

Set min output
Click (and drag)

Random links,
random output ranges

Select MIDI channel

Pause + mute