If you're using a mixer with your DAW, you already have plenty of faders, buttons, and other controllers at hand. Wouldn't it be nice to use your mixer as a control surface, too? Most digital mixers on the market (and even a few analog mixers) allow you to do that to one degree or another through MIDI.
Several levels of control are possible. The most basic and common level simply lets you assign the hardware controllers (such as faders) to standard MIDI messages, usually continuous controller and switch messages. The second level of control involves MIDI Machine Control (MMC), which lets you control your DAW's transport from the mixer. The highest level of control incorporates more specialized implementations for more tightly integrated control.
Assigning faders and switches to standard MIDI messages is supported by every digital-mixer manufacturer I talked with. In all cases, faders can be mapped, but the number of mappable controls beyond the basics varies from mixer to mixer. Some, like Mackie's D8B, support virtually every knob and switch on the surface for MIDI transmission.
Generally, this assignment is accomplished by defining a custom layer that maps each fader or switch to a specific MIDI message. Most mixers allow the mapping to be done more or less arbitrarily, though they may impose some restrictions such as allowing a fader to send continuous controller messages but not Note On or Program Change messages.
Typically, however, assignments are made using one of two schemes: successive mixer channel strips all use the same controller numbers, with each strip having its own MIDI channel, or successive strips use different controller numbers for the same hardware controller, with all strips sent over the same MIDI channel. Tascam's DM24 has two kinds of layers (MIDI and controller layers) that match those two scenarios.
Mapping your mixer's controls to your DAW's parameters can be a time-consuming process, but the result can be stored as a snapshot once it's all done. Some manufacturers provide templates for particular DAWs, such as the Yamaha 01V's Cakewalk Pro Audio 9 template or the templates provided on Soundcraft's Web site for the Spirit 328. The 01V also has a Learn mode that is bidirectional; it can receive MIDI data from the DAW software and map the parameter to the selected controller.
Some mixers can also send MMC, though as you might expect, implementations vary. Roland's VM-3100 provides transport and track-arming controls, and the DM24 even transmits its Jog/Shuttle-wheel output as MMC. Panasonic's DA7, on the other hand, provides only basic transport functions.
Finally, some mixers incorporate more customized functions. The DA7 includes a mode that emulates Mackie's HUI control-surface protocol, instantly allowing control of basic fader, mute, solo, and pan functions in banks of eight channels at a time. Roland's VM-7200 is not able to transmit standard MIDI messages but does send SysEx messages that will be usable with some upcoming products. Tascam is also planning to add HUI emulation to the DM24 in its version 2 software.
Ultimately, however, use of a digital mixer as a control surface is limited by the DAW's capabilities to accept such messages. For example, Pro Tools no longer allows any MIDI controller to be mapped to its parameters; rather, it requires a special “profile” file for the controller. As always, check your owner's manual for details.