MIDI is short for Musical Instrument Digital Interface and is a standard that was created to connect electronic musical equipment like synthesizers, samplers, drum computers and MIDI sequencers with each other and have a way to communicate between them. Later on, computers followed with MIDI interfaces and MIDI events could be recorded on a standard PC.
MIDI events do not contain audio at all and do not produce sound in itself. They contain information on key, pitch and velocity for notes or certain control signals to modify volume, panning, etc. or send start/stop signals. All MIDI events have a certain channel designated, from 1 to 16. MIDI is therefore often limited to 16 MIDI channels.
In multi-timbral synthesizers or sound modules, one can assign a certain instrument or sound to a certain channel: so you could have a piano on MIDI channel 1, a bass guitar on channel 2, strings on 3, and so on. Drums are always assigned to channel 10. A device's polyphony determines how many notes the device can play at the same time.
Before desktop computers became powerful enough, MIDI was always used in combination with outboard MIDI gear such as synthesizers to produce any sound.
At some point, computers were able to simulate these dedicated devices with low enough latency so one could let the computer produce the audio rendered from the MIDI events: the virtual instrument was born. Virtual instruments act just as a physical sound module, but this sound module is now running on your Android device's processor, calculating the wave forms that result from feeding MIDI notes into it.
We talk to many people who think that MIDI equals virtual instruments, but that is not the case. This distinction will also become apparent in Audio Evolution Mobile when creating tracks: you either create a USB MIDI track for use with external audio producing gear, or you create a MIDI instrument track for use with virtual instruments running on your Android device. So, depending on if you have and want to use external MIDI equipment that produces sound or not, you will select either method.
In Audio Evolution Mobile, you can play virtual instruments based on the SoundFont format. SoundFonts have been a popular format in the past 20 years on desktop computers and originated from E-mu Systems and Creative Labs. One SoundFont can contain many different instrument sounds: from piano to strings, synth pads, you name it.
To use SoundFonts, you start by adding either a MIDI instrument or drum pattern track. The app will then ask you to choose a soundfont, a bank and a preset. You can multiple soundfont files (.sf2 files) installed, containing many different banks and presets. A bank is a group of presets and a preset equals a 'sound', like a piano or violin. Note that the app will ask you to download a free soundfont when none has been installed yet. You can also purchase soundfonts by using the 'Shop…' button.
To facilitate above distinction between using external MIDI gear or virtual instruments to produce audio from MIDI events, you will have the option to choose between two MIDI track types:
In order to use external MIDI gear you will need some way to hook it up to your Android device. There are basically two ways:
Android device -> micro-USB to USB OTG converter cable -> USB cable -> MIDI hardware
Android device -> micro-USB to USB OTG converter cable -> USB cable -> USB audio/MIDI interface -> 5-pole MIDI cable -> MIDI hardware
You will also need an Android device that is 'compatible', meaning that it needs at least to support USB host and runs Android 3.1 or higher. Fore more information on compatibility, please see Support for USB MIDI interfaces.
When you will be using virtual instruments, hooking up an external MIDI keyboard (controller) is also very useful, because you could record MIDI parts by playing the MIDI keyboard and letting the MIDI events be recorded by Audio Evolution Mobile and the sound be produced by the virtual instrument. Note that the ability to play virtual instruments live depends on your hardware and the use of a USB audio interface. For more information on that, please also visit Support for USB MIDI interfaces.
Without any external MIDI gear, you can still enjoy the virtual instruments. If you simply want to play existing MIDI files (.mid) that you created elsewhere or downloaded from Internet, you can just import the MIDI file and then have it played by your Android device. This way, you could use that as orchestration or backing and then record your guitar or vocals on additional audio tracks and play them back at the same time.
You can also create MIDI instrument tracks from scratch by using a piano roll editor or use the virtual keyboard. To access the virtual keyboard, you need to create an instrument track first and then Arm it. If you don't have any external MIDI equipment connected, the virtual keyboard will appear. You can then press Record and play the virtual instrument by tapping on the keys of the keyboard. Note that you can only get low latency by using an USB audio device or a select number of Android devices that support the 'low latency mode' (mostly Nexus models).