MIDI Interfaces for Home-Recording and Music Studio

MIDI is a communication protocol, which was established in the 80ies. (For those, who want to know: it means "musical instrument digital interface "). The new era of computers, home-recording studios and electronic music instruments, like keyboards or samplers, made it necessary to find a standard for communication.
Note: MIDI does not transmit sound or audio data. In a simple way it only says: "music instrument A, play this note" or: "computer, record that note".

To enable midi communication between 2 electronic music instruments (a computer is always one!), the devices must be connected with a special MIDI cable. The direction is always from the "MIDI out" port to the "MIDI in" port of the next device, and it depends, whether a device sends midi data, or receives them, or does both. For example: A PC with a sequencer software always sends Midi data; an extern hardware sampler might only receive them. Vice versa, an extern master keyboard sends data, too (you want to record on your Computer what you play!), so there must be a connection back from the "MIDI out" from the master keyboard to the "MIDI in" of the Computer.
Some music devices also have a "MIDI through" port. Incoming data, which arrive there, will be sent "through" that device to the next one. So you don´t need a free midi port for each instrument, when chaining devices in this way.

Some computer soundcards or audio-interfaces may have already built in MIDI ports. External Midi-Interfaces usually are connected via USB or firewire; they vary from small desktop boxes with just 1 "MIDI in" and 1 "MIDI out" port, up to big ones for 19´´ racks with many ports and a lot of further functions, like synchronization.
As each MIDI port provides 16 MIDI channnels, it is up to you, what you need for your music studio. However, 2 or 4 ports ("ins" and "outs" each) should be enough to start a small homerecording studio.
In any case, be sure to make correct MIDI settings on each of your instruments and choose the corresponding channel(s), on which your music devices will send or receive Midi.