A MIDI controller is any piece of hardware that generates and transmits musical data to electronic instruments or applications. They are typically used to trigger sounds, similar to the way a keyboard triggers notes, or to control the parameters of a sound effect, like moving a knob to filter or process the sounds on a synthesizer.
There are many different mass-produced MIDI controllers. But most of the ones you will find on the market will look like keyboards, drum pads, or a set of faders and knobs commonly used for mixing.
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They are usually universal and almost all of them can be used with audio/music apps on iOS iPads and iPhones (through a special adapter or via Bluetooth). Their versatility is their main advantage. Any iOS app that has a MIDI learn option or MIDI settings can use these controllers.
However, their versatility is also their downside. Most apps have far more parameters than the number of knobs and buttons on a MIDI controller. At the same time, not all apps can assign the same MIDI control to the different parameters located in different panels/screens and change the parameter assigned for control when switching between app sections. Often, many parameters designed for touchscreen are difficult to associate with a knob or button on a MIDI controller (such as on-screen touchpad or other non-standard controls).
So the use of most MIDI controllers is limited to the commonly used parameters. The rest of the actions in the app are still performed through gestures on the device screen. Usually this is not a problem in those apps where you can choose a couple of key parameters for assigning them on MIDI. But this could be not so comfortable for apps which assume more active interaction in the creative process. In many apps for alternative scene such as ambient, experimental, sample mangling, noise, sound art, etc., quite a lot of parameters which would be more comfortable to have on hand.
It is also worth noting that most MIDI controllers do not have an audio interface. But to use iOS music apps live, you need an external audio interface, and this assumes additional devices, adapters and connections that surely do not add reliability to the setup.
Dedicated MIDI controllers
When a MIDI controller is specially designed for some specific app, working with such app using that controller is much more convenient. In this case, the controller usually allows you to control most of the app parameters. Or the app is designed to make the most of the controller. There are also enough of that kind of controllers and they are also mass-produced.
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The app screen is much more informative, and becomes more of a display showing the current state of scene, than one of the control element. The controller and app do not require additional configuration and are always ready to go. Usually no additional devices are required, since the controller also has an audio interface. This is a very big deal for live performance where the screen controls may not be very reliable and convenient to quickly change parameters during a performance. It is unlikely that someone will decide to use the on-screen piano keyboard during live :)
However, the flip side of this convenience is the hard binding of the controller to the specific app. Although it is quite possible that such a controller could work with other apps with similar functionality, it is unlikely that it will be as convenient to work with as with its native app.
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It seems to make sense to take a slightly different way. Let's imagine that it will be a special designed controller, but it will be focused on working with several apps in some general direction of creative ways to make music. Not as versatile as usual controllers for a variety of DAWs and synths, but not strictly focused on specific app.
A good area for that kind of apps could be an alternative scene with the different directions of experimental electronic music in which actions are performed live with several samples (sample mangling) or with other signal sources, or even for video art. Several different iOS apps along these directions can use a dedicated MIDI controller which is designed to give maximum control over these apps.
The user interface and functionality of these apps are designed for comfortable work with that controller. The screen of an iPad (or possible even an iPhone) with that apps becomes a convenient means of displaying the parameters and state of the current scene, rather than a control surface. This also seems to be significant, since often small on-screen controls or complex gestures are very inconvenient not only for live performance, but even when actively changing parameters during playing.
Typically, such apps have several sample or signal processing sections with the same functions, outputs from which are eventually mixed. The controller also consist of several sections with the same set of controls. But unlike controllers for DAWs, it contains one universal encoder with an indicator of the current value, with a nearby navigation buttons that select the parameter for it. Each section contains an output level knob for mixing the signals of each section. Also there is one (or even two) "hot” knob or encoder, which can be quickly assigned to any parameter just by click to one button. Such a knob is used for the "most required" parameters at the moment.
Users of these controllers are not limited to one app, but have the ability to choose from several different apps and obtain new apps for their controller in the future. At the same time, the main advantage of dedicated MIDI controllers remain - the maximum ability to control of app and work right out of the box, which does not require additional settings for the controller and app. Also it does not require additional devices and connections, as it already contains an audio interface and power supply (charger) for iOS device.
A fairly close idea is implemented in controllers for guitar apps. They are also dedicated, but can be used with several different apps, and also usually contain an audio interface and a power supply for the iOS device. However, they have a limited number of controls mainly designed to quickly change presets / scenes or switching effects. That actually is the most relevant for guitar effects.
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