Welcome to MIDI Misconceptions - Pt II. This time we'll look at MIDI Ports vs. MIDI Channels, How you can have multiple devices all connected by one cable, and explain the difference between MIDI Merge and MIDI Thru. For more, general MIDI misconceptions check out Part 1 of this series.
MIDI Ports vs MIDI Channels
Each time you plug a MIDI-to-USB interface into a computer, a MIDI port is created in the computer's operating system. Each port has 16 MIDI channels in it and they are numbered 1-16. Think of a port like a small apartment building with 16 apartments. The apartment building has it's own address which is different from any other apartment buildings that might also be on that street (i.e connected to the same computer).
Each apartment in the apartment building represents 1 of 16 MIDI channels. Each port has 16 different MIDI channels as part of it.
Different Strokes For Different Folks
There are different MIDI interfaces out there. Some have a single MIDI port in them. Some have multiples. The easiest, most practical way to determine how many ports are in an interface is to count the pairs of MIDI in and MIDI out jacks that are on the unit. Each set of MIDI in and out generally corresponds to 1 MIDI port and each port has 16 MIDI channels.
Usually the devices are named for how many INs and OUTs there are on the device. So, for instance, a MIDISport 1x1 has one MIDI IN and one MIDI OUT. That's one pair of INs and OUTs. So it has 1 MIDI port.
A MIDISport 4x4 will have 4 MIDI INs and 4 MIDI OUTs. So, 4 pairs of INs and OUTs and therefore 4 ports.
How can you have multiple devices (keyboards) all connected by one MIDI cable to the computer?
Communications between MIDI devices and the computer is all about addressing. Each device has it's own address and any time it sends a message to the computer, that message has the address attached to it.
So, say you have your 3 keyboards attached together and plugged into one MIDI-to-USB interface.
The MIDI-to-USB interface is like the aforementioned apartment building. Lets call it #100 Hauptwerk Street.
Keyboard 1 is on channel 1 or apartment 1 in this case. Keyboard 2 is on Channel 2/apartment 2. Keyboard 3 is on channel 3/apartment 3.
When keyboard 1 plays, the computer sees messages coming from #100 Hauptwerk St. Apt 1. When Keyboard 2 plays, the computer sees that the messages are coming from #100 Hauptwerk St. Apt. 2. Keyboard 3 follows the same pattern.
This is why the computer is able to have 3 keyboards all plugged into the same MIDI interface and still able to have individual keyboards mapped to each of the onscreen keyboards.
This is also why each keyboard (or MIDI device) needs to be on it's own, distinct channel. If 2 keyboards are both on channel 1, the messages appear to be coming from the same apartment. For those who have seen 2 physical keyboards playing the same onscreen keyboard, this is likely the problem. Once each keyboard has their own apartment, the problem is solved.
You could also have 2 MIDI interfaces and 2 keyboards on the same channel without creating confusion. MIDI interface 1 is #100 Hauptwerk St. and MIDI interface 2 is #200 Hauptwerk St. So, if each keyboard is on channel 1, keyboard 1's address is #100 Hauptwerk St. Apt. 1 and the second keyboard's address is #200 Hauptwerk St. Apt 1.
MIDI Merge vs. MIDI Thru
MIDI Thru simply sends a copy of what it gets from the IN port out of the OUT port . Merge accumulates data as it goes along. It allows for multiple devices to all send to one IN port.
Thrus are great for MIDI destinations (e.g synth modules) because you can daisy chain devices. You can have 1 controller whose data goes to multiple receivers (i.e synth modules) The modules do not add any information to the stream. So, a Thru works fine.
Merges are better for when you have multiple controllers that go to one destination (e.g Hauptwerk) because each device does add data along the way. The entire bunch of controllers can be chained together and result in a single MIDI cable that plugs into your MIDI port.