What is MMC (Midi Machine Control)?

MIDI Machine Control (MMC) is a protocol specifically designed to remotely control hard disk recording systems, and other machines used for record or playback, over a MIDI cable. The only way to do this is with System Exclusive messages, and so several specific SysEx messages were defined in order to implement MIDI Machine Control.  MMC is transmitted and received over standard midi cables and ports.

What does this mean?  Basically, when you hit start, stop, record, fast-forwarded, punch-in, and other 'tape-like' transport controls on one piece of equipment the other responds in the same manner.    Remember it's not a synchronizing signal  and you will need something else to synchronize between the two (or more) devices like midi clock, MTC or SMPTE.

This allows you to use master set of controls to control two (or more) pieces of gear at the same time, one of them usually being some sort of recorder/playback device like a multitrack tape deck or even HD recording software.  With the MPC set to receive MMC and your HD recording system set to transmit, you can use the 'transport' controls in the software to start, stop, FF the MPC. The same applies the other way around, by using the controls of the MPC to start, stop, etc. the HD disk recorder.

Not all commands used with MMC are compatible with every piece of gear or software.   The best way to get to know and learn this is trial and error.