From what I understand, aftertouch in MIDI is a parameter (like a continuous controller) that a synthesizer can respond to. I have heard that some keyboards can send polyphonic aftertouch data, where the aftertouch information is sent per-note. But how does this work, if aftertouch is just one parameter? Is there a mechanism where the note data can be set alongside the aftertouch data, or is it just aftertouch per-channel, which would max out at 16 "notes" and intrude on the other channels? Or are these two sent as entirely different data in the first place?
Aftertouch and polyphonic aftertouch are two different event types in the MIDI standard:
1010nnnn 0kkkkkkk 0vvvvvvv Polyphonic Key Pressure (Aftertouch). This message is most often sent by pressing down on the key after it "bottoms out". (kkkkkkk) is the key (note) number. (vvvvvvv) is the pressure value. 1101nnnn 0vvvvvvv Channel Pressure (After-touch). This message is most often sent by pressing down on the key after it "bottoms out". This message is different from polyphonic after-touch. Use this message to send the single greatest pressure value (of all the current depressed keys). (vvvvvvv) is the pressure value.
Both kinds of aftertouch are tied to a specific MIDI channel. In addition, the polyphonic variant is also, as you noted yourself, tied to a specific note.
A synthesizer or MIDI controller can send an arbitrary amount of both polyphonic key pressure and channel pressure events (or at least as fast as the MIDI link allows).
Kim Burgaard pretty much hit the nail on the head, but I wanted to add just one small bit of explanation.
The reason that we have two different types of aftertouch is that the sensors used to measure aftertouch are somewhat expensive from a manufacturing perspective, and therefore most mid- to low-end MIDI keyboards have only one sensor for all keys. More expensive keyboards have per-key aftertouch sensors, but both types are capable of sending some type of aftertouch data.
Some better googling led me to the answer. They are indeed separate data and aren't the same thing.
Channel pressure is just a pressure value, global to the channel, while aftertouch sends the note number in addition to pressure as part of its data. Only one channel is required.
i guess this question is old, but it should be noted that VERY few keyboards can generate polyphonic aftertouch (a note specific control - also known as "note pressure"), but almost all synth keyboards can generate CHANNEL aftertouch (a control for affecting all the notes in a channel - "channel pressure").
ALSO, it should be noted that not all synthesizers even RESPOND to polyphonic aftertouch. I mean, what does monophonic aftertouch (channel pressure) =do= to the sound? Usually adds something like vibrato, same sort of thing a mod wheel would do.
What does polyphonic aftertouch (note pressure) =do= to the sound? Mmmm, whatever the synthesizer is programmed to do. But it's PER NOTE - so maybe the 3rd of a chord gets vibrato, but not the 1st and 5th (for example).
The reason not many synthesizers even respond to note pressure (poly aftertouch) is
it's rare to even SEE it from a keyboard.
response to it has to be programmed per the preset and that adds greatly to what a preset can be expected to do (and must be implemented and saved and so forth).
note pressure sends a FLOOD of messages over midi. And can clog up responding to NOTES fast enough causing hiccups in the sound - bad news.