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Auto Chord (2.0)/What Chord (2.0)

Totally redesigned, Auto Chord automatically detects chord changes in live music and sends control signals (pitch, velocity, and volume) to attached MIDI devices such as synths and recording devices (e.g., Garage Band, Pro Tools, etc.). It can be used for live performance (e.g., to generate a "pad" accompaniment to an instrumental part such as a piano or guitar), for semi-automatic chord transcription (with external MIDI editing), and other applications. What Chord was an earlier version of a chord detection app that has since been improved. It uses the same polyphonic pitch detection algorithm as Auto Chord but does not generate an output MIDI stream.

Audiobus and Core MIDI compatible.

Sound Examples

The following examples were captured on the iOS simulator driven by an iPod. Auto Chord is shown running on the left. MIDI signals from the app drive Garage Band which is on the right.

The first example contains sections of Beethoven's Moonlight Sonata and a Bach Invention, both to string accompaniment.

The second example contains sections of music by Amy Winehouse, The Blues Brothers, Israel Kamakawiwo'ole, and Megawatt Blues Crushers.

How to Use

Adjusting the top slider changes the input level. Play music and adjust the slider until the pitch detector triggers. A row of lights above the keyboard shows the instantaneous output of the pitch detector bank collapsed into a single octave (chromagram).

Tap the gear icon to access the settings table. The first section of the settings table lets you specify the pitch range. A bank of filters continuously measures audio pitch in eight different ranges. When a chord is detected, MIDI pitch and velocity are transmitted for all chord notes in the selected pitch range. The velocity is estimated from the relative strength of a note.

The next section of the settings table sets the input signal to noise ratio (SNR). Auto Chord uses model-based techniques to match chords to harmonic intervals in the chromagram. SNR is the ratio of the response of the best matching chord model divided by the average response over the other models (discussed below). Increasing the SNR makes Auto Chord more selective causing it to fire less frequently (generate fewer chords).

The last section of the settings table specifies what chords to use. Thirteen types of chord are defined:

  • Single note (no chord)
  • 5th (no 3rd)
  • Major
  • Minor
  • Diminished
  • Augmented
  • Dominant 7th
  • Minor 7th
  • Major 7th
  • Sustained 4th
  • Diminished 7th
  • Minor 7th with flat 5th
  • Minor with #7th
Only those chord types that are selected are matched against the music being played.

Tap the gear to dismiss the settings.

The bottom left slider controls the amount of smoothing. Move it to the right (more smoothing) if you are playing a melody and want Auto Chord to extract chords from recent notes; otherwise move it to the left to minimize chord detection delay. The bottom right slider is the MIDI output level.

N.B. If the app is used to drive a synth, feedback will occur if the sound output from the synth is picked up by the device's microphone. This causes the app to lock in to the first chord played. To prevent this, the performer using Auto Chord should use a headphone to monitor the synth, and the synth's speaker should be directed away from the device.

Controlling Other Devices

Auto Chord continuously transmits MIDI pitch, velocity, and channel volume to networked devices Core MIDI. Midi Bridge or a similar app is required to route Midi signals between apps.

Follow the directions below for wireless control of networked computers.


Download a wireless MIDI interface such as DSMidiWifi and install on your computer. Launch Auto Chord on your iOS device. Connect your device's MIDI stream to the synthesizers you want to control. For example, on a Mac use Audio MIDI Setup to connect your iPhone/iPod Touch to Garage Band or other MIDI units networked to your computer. Turn on Garage Band or other MIDI synths. Launch Audio MIDI Setup, select MIDI, and click on the network icon. You should see this screen:

Select your device and click Connect. As you sing or play into the device the bar graph will show the network latency.


Be sure to install the latest version of iTunes, which contains Bonjour for Windows. Then install rtpMIDI, which is available free at: rtpMIDI on the PC looks and functions like Audio MIDI Setup on the Mac. rtpMIDI will detect your device when Auto Chord is running. Start a new session in rtpMIDI and then you will be able to select the session as a MIDI input device from within your DAW or other MIDI setup.

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