3. A little MIDI theory
But how does it work, this connection to a computer, e.g. using a plug-in synthesizer? Well, it is enabled by the use of system exclusive data, for example in MIDI format and of the MIDI interface on the audio interface. While the first "midified" hardware synthesizers of the early 1980s were able to receive only the pitch, volume, and duration of a sound through MIDI, it did not take long for the range of representable parameters to be significantly expanded. Low-pass base frequencies, resonance, envelope, all these can be controlled today by MIDI system exclusive messages.
The problem: Every instument manufacturer has cobbled together their own MIDI communication structure, so that the data packages for one and the same parameter can look very different indeed, and they may even differ from one type of device to another. This is why the "mapping" intelligence of a controller is of such decisive importance.
MIDI and system exclusive messages
The length and structure of a system exclusive message is not subject to standards and differs from manufacturer to manufacturer. They therefore deviate from the usual standardised MIDI data for note-on, note-off, velocity etc.
| Representation of system exclusive messages employs a hexadecimal system, i.e. a system of numerals based on 16. Its symbols are: 0 - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - A - B - C - D - E - F |
Application is not that simple, however, and handling the system requires some getting used to.
The starting point is a status byte (F0H) which opens the message. At the end, the "end of exclusive" combination F7H closes the message. The first data package contains information on the manufacturer (manufacturer ID between 0 and 127, e.g. 41H for Roland), which may occupy from one to three digits. It is folloowed by the device identification (e.g. 7GH for VS-880, etc) and the MIDI device ID (Roland factory pre-defined 10H) which allows several identical devices to be distinguished from one another.
Next, the actual data packages for the device parameters are transmitted, which may differ, as described above, even within the devices offered by a single manufacturer. Suitable for transmission are pre-sets, individual sound and system paramters for the device, sound banks, MIDI sequences and samples. The latter have achieved standard status in MIDI history: Standard MIDI File (SMF) for MIDI sequences, and MIDI Sample Dump for samples.
You may know this problem from working with controllers: we do not enjoy the crude gradation on a single control element, e.g. for low-pass control. It leaves audible jumps in the sound. One solution is using two controllers internally for a single control parameter. This turns an 8-bit controller into a 16-bit controller, and 256 grades become an inaudible 64,000.
