Hi Ron,
"OTG" (USB On-The-Go) is a specification for Android devices only, so it has nothing to do with iPads. For details regarding OTG see here:
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https://en.wikipedia.org/wiki/USB_On-The-GoAs described in the thread Allan linked above, a Bluetooth (BT) connection via Yamaha keyboards built-in BT adapter can only be used to transmit the
Audio playback (instead of a wired connection to the AUX IN jack of the keyboard in question) so you can listen through the instrument's speakers.
In connection with the Chord Tracker app, however, you primarily need a
MIDI connection between the smart device (e.g. iPad) and the keyboard. To establish a USB-MIDI cable connection with the iPad, you need a USB type "AB" cable ("printer cable"). Connect the USB B connector of the cable to the USB-to-HOST terminal of the keyboard. You connect the USB A plug to the iPad using a suitable Apple "Camera Adapter" (Lightning or USB-C plug to USB-A socket, depending on the iPad used).
The various options for establishing a MIDI connection between the iPad and the keyboard can be found in the "iPhone/iPad Connection Manual", which you can download here:
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https://uk.yamaha.com/en/products/musical_instruments/pianos/portable_grand/dgx-670/downloads.htmlBy the way, I use (with my good old iPad4) an Apple "Lightning to USB3" camera adapter. In addition to the USB-A socket, this also has a Lightning socket, so that the iPad can be connected to the charging cable at the same time.
Best regards,
Chris