Sequencing
A Sequencer is either a software application installed on a computer, or an application residing in a stand-alone hardware sequencer, that will process and organize the MIDI instructions into various instruments and notes and replay the performance. The Sequencer also allows one to edit the performance after the data has been stored on the hard drive and timing and off-note mistakes can be revised. Some of the Sequencer applications have become so sophisticated that they really now function as Multi-Track Software Studio applications that combine the ability to process MIDI tracks, sequence digital audio sound files, add effects processing plug-ins, add synthesizer and virtual instrument plug-ins, and handle live microphone or instrument line-in performances.
The Sequencer, both hardware and software, is where the MIDI information (and simultaneously digital audio) is temporarily stored. It can either be an integrated component of an instrument (usually a keyboard) or stand alone device (known as an expander) or a software application (they may also be capable of managing analog recording and sampled digital audio files. The Sequencer itself does not produce sound. Rather it assembles data, provides an opportunity to process the data, and then passes it on to another MIDI compatible device (a Synthesizer that is either part of the Sequencer or is connected to the Sequencer, soundcard or sound module) that will actually produce the sound. The MIDI signal can be sent and processed in real time or by single note (step time) and then replayed in sequence in real time. This allows the musician the opportunity to actually “hunt and play” notes on an instrument that one may not be proficient in and then assemble the sequenced notes into a “performance.”
A MIDI controller instrument can be used to enter MIDI data into the Sequencer. Or, the performance can be entered into the Sequencer itself through a computer screen or on-board screen graphic interface. If entered into the Sequencer first, once completed it can then be routed back to the MIDI compliant instrument to play the sounds of the performance. The complete, edited performance than can be saved within the Sequencer (either the software application saves it to the hard disk under a retrievable file name or the hardware Sequencer has a memory storage capacity). This makes it possible to edit a performance several different times, add or subtract instruments, control many different instrument sounds at once, control tempos, and create different versions of a basic performance for comparison.
Each Sequencer has a clock that uses the first signal as a reference point. Each subsequent signal is timed in relation to the first signal. In this manner the Sequencer assembles the information about each signal (pitch) and a sequence of signals (time or rhythm). The arriving signal information is timed by a pre-determined clock rate which is a fixed internal of pulses (24 pulses per quarter note) that commence after the first signal is sequenced. Each subsequent signal is synchronized to each subsequent clock pulse. The Sequencer then allows for the manipulation of the clock pulse rate in order to get the rhythm the musician desires.
Thus, to combine MIDI and Sequencing, typically what one would do with a MIDI keyboard is assign a specific instrument sound or selection to any one of the 16 channels. This is done by sending a MIDI Program Change signal on a specific channel. The signal has the value that identifies and assigns the instrument to that channel or is already assigned under GM. One then sends MIDI note signals on each specific channel for that specific instrument. Information for volume, panning and other effects can also be sent at the same time. The complete information for each channel can then be entered into the Sequencer, again for each channel. The data is saved in the RAM of the computer or stand-alone sequencer, and then can be saved to a hard drive. The MIDI (.mid) file will then play all the correct instruments and respective notes of the instrument automatically when called up and played by a MIDI compliant device.
Similarly, digitized audio in .wav format can also be cued into a Sequencer. Each file or loop can be assigned a track in the Sequencer. However, this is normally done in the context of a audio software package. The application may also allow for the simultaneous sequencing of MIDI data.
There are also digital audio sound file sequencers that allow one to map out on a time-line either single digital audio files (in .wav or a proprietary “native” format) or “loop” a file over-and-over in order to create a composition. Thus, from a single sample one can create a repetative percussion pattern or a sustained instrument note.