While notes are playing, pressure can be applied to all of them. Many electronic keyboards have pressure sensing circuitry that can detect with how much force a musician is holding down keys. The musician can then vary this pressure, even while he continues to hold down the keys (and the notes continue sounding). The Channel Pressure message conveys the amount of overall (average) pressure on the keys at a given point. Since the musician can be continually varying his pressure, devices that generate Channel Pressure typically send out many such messages while the musician is varying his pressure. Upon receiving Channel Pressure, many devices typically use the message to vary all of the sounding notes' VCA and/or VCF envelope sustain levels, or control LFO amount and/or rate being applied to the notes' sound generation circuitry. But, it's up to the device how it chooses to respond to received Channel Pressure (if at all). If the device is a MultiTimbral unit, then each one of its Parts may respond differently (or not at all) to Channel Pressure. The Part affected by a particular Channel Pressure message is the one assigned to the message's MIDI channel.
It is recommended that Channel Pressure default to controlling the VCA level (ie, a volume swell/fade effect).
0xD0 to 0xDF where the low nibble is the MIDI channel.
One data byte follows the Status. It is the pressure amount, a value from 0 to 127 (where 127 is the most pressure).
What's the difference between AfterTouch and Channel Pressure? Well, AfterTouch messages are for individual keys (ie, an Aftertouch message only affects that one note whose number is in the message). Every key that you press down generates its own AfterTouch messages. If you press on one key harder than another, then the one key will generate AfterTouch messages with higher values than the other key. The net result is that some effect will be applied to the one key more than the other key. You have individual control over each key that you play. With Channel Pressure, one message is sent out for the entire keyboard. So, if you press one key harder than another, the module will average out the difference, and then just pretend that you're pressing both keys with the exact same pressure. The net result is that some effect gets applied to all sounding keys evenly. You don't have individual control per each key. A controller normally uses either Channel Pressure or AfterTouch, but usually not both. Most MIDI controllers don't generate AfterTouch because that requires a pressure sensor for each individual key on a MIDI keyboard, and this is an expensive feature to implement. For this reason, many cheaper units implement Channel Pressure instead of Aftertouch, as the former only requires one sensor for the entire keyboard's pressure. Of course, a device could implement both Aftertouch and Channel Pressure, in which case the Aftertouch messages for each individual key being held are generated, and then the average pressure is calculated and sent as Channel Pressure.