I have a file that has 3 midi tracks on it. On one of the tracks I changed it from channel 0 to channel 3 and no audio. After I changed it back to channel 0 audio came back. Why is this?

I have found that changing it at the track level only changes it for new notes that you put on the track. If you actually want to change it to channel 3, then you need to use the MIDI Rechannel command to change the existing data on the track to the new channel.

Then, of course, make sure whatever synth you use supports or is set to the channel number you change to and set the instrument accordingly.

Btw, what is the difference between a port and a channel in a DAW? Thanks

A port is a hardware or virtual (aka software) interface (port) that supports all 16 MIDI channels to communicate with MIDI "devices" (a device can be an external MIDI module, synthesizer, FX box like a Lexicon reverb, etc.) or an internal VIRTUAL MIDI module (e.g., VSTi/DXi sampler/synths like Kontakt, sforzando, SampleTank, Garritan) or MIDI FX (various VST/DX things like Amplitube, or some compressors/limiters/eqs)

Since MIDI spec only allows for 16 MIDI channels on a PORT so lots of interfaces have multiple ports (e.g., a hardware interface like a MIDIMan 2x2 is a 2 port USB device - with it now you could access 32 total channels: 16 channels on Port 1 and 16 channels on Port 2; or a software multi-port interface like Loopbe1 - that you can get for free with 2 ports or the not free 30 port Loopbe30)

So if I am using less than 16 tracks ( which is almost always) I can use a single port with no issues, correct?

Yes. And in general, you'd only need one port as long as you only needed at most 16 channels.

But, and THIS IS JUST ONE WEIRD EXAMPLE:

let's say you wanted TWO different GM drum kits each on, obviously, different tracks. But becasue you are outputting this data to two different GM drum "modules" those modules might need to have the data on the standard & default Channel 10. So at that point you'd need two ports (or some clever routing using something like MIDIOx [free also] )


Finally, make a note: While PTPA and RB can use multiple ports - BIAB can ONLY USE ONE port for input and one port for output - they don't have to be the same port, however.