WARBL doesn't make any sound on its own, so you'll need to plug it in to a MIDI host device. This is very easy, because most portable devices (and desktops) support MIDI, and there are lots of free or inexpensive MIDI apps. I highly recommend iOS (Apple) devices because they handle MIDI very well and are easiest to set up. Some suggestions for iOS apps include Appcordions Celtic Sounds, Universal Piper, and Roland Sound Canvas. Connecting WARBL to an iOS device also requires the Apple Camera Connection kit or equivalent adapter that converts USB to Lightning plug.

WARBL has very low latency itself, but latency can vary widely with host devices. I highly recommend iOS devices for their low latency. While WARBL will work with Android devices that support USB On-The-Go (OTG), Android devices typically have higher latency. This site has extensive information about audio latency on mobile devices, including an app that lets you test your device. I have had a report that the Fluidsynth app has some of the better latency performance on Android. Windows PCs can also suffer from MIDI latency. Here's some information and possible solutions.

If there's enough interest I'll consider making WARBL available with other hole spacing options. Please contact me to let me know your preference.

WARBL produces no sound on its own, and the 3.5 mm jack is only for the optional "bell" sensor for emulating uilleann bagpipes. If you use the bell sensor, be sure to also use the special right-angle micro USB cord with the slim plug (supplied with the sensor). Many micro USB cords have a wider plug that could put too much pressure on the sensor and cause damage to the device.

I'll consider adding more fingering patterns if they are ones that are close to real intruments and will have broad appeal. Because of limited space, I do need to be somewhat selective. Feel free to contact me with suggestions.

Yes! WARBL is open-source, so you are free to modify the software and use it as you wish, as long it you keep it open source. It is available on the Documentation page and uses the Arduino language. Information about uploading the software from the Arduino IDE is available on the WARBL GitHub page. If you aren't comfortable with coding but have ideas for software improvements, feel free to contact me with suggestions for future updates.

Here's a screenshot of an example setup for controlling drones and regulators. Byte 3 in the button configuration section is the velocity (volume) of the regulators, so you can adjust that to your preference. The drones will turn on and off if you click button 1 while covering only the right index-finger hole. There is a full list of possible drones and regulators here (click on "more" to see the full description of the app with command list).

Many connection problems with MIDI apps can be solved by forcing the app to close (on iOS, double-click the home button and swipe the app upward), unplugging/replugging WARBL, or a combination of those two things. Note: If you have been sending CC messages from WARBL to control the volume of the app with the pressure sensor, after every breath, the volume of the app may be turned down to zero. It is easy to forget about this after changing settings, so if your app isn't making sound, check to make sure the volume control in the app itself isn't turned all the way down.

Chrome sometimes freezes if a MIDI device that it's connected to is unplugged. If this happens on your system, I suggest closing the browser window before unplugging WARBL. You can also use Firefox or Safari with the Web MIDI plugin, which seems to be quite stable.

WARBL uses MIDI CC messages on channel 8 to communicate with the Configuration Tool. If it has been connected to the Configuration Tool, it will continue sending messages until it is unplugged. Most MIDI apps and modules will ignore CC messages in this range (CC 102-119), but if they are causing any problems you can just unplug WARBL and plug it in again to make it stop sending.

Unfortunately Apple has made it more difficult to use USB MIDI devices by removing the headphone jack on newer products, but you can use an adapter to add a headphone jack. A USB hub with an internal soundcard is a good option. I have been told that this one in particular works well with WARBL, and it also allows you to power both WARBL and your iPhone/iPad using a micro USB cable. You also still need the Apple Camera Connection Kit to plug the above adapter into your iPhone or iPad.