MIDI Synthesizer Test Patch

The example synth is loaded into pd like any plugin-wrapper. A manually written test patch (cpgrs-help.pd) is shown in Fig.K.16. Note that the standard MIDI-synth control parameters (freq, gain, gate) are handled behind the scenes and do not appear among the plugin GUI controls.

To drive our MIDI synth, we need a source of MIDI data. Perhaps the simplest resource for this purpose is the Virtual Keyboard (vkeybd), which is standard in Red Hat Fedora 6, and in the planetccrma-menus at ``Applications / Planet CCRMA / MIDI / Vkeybd''). Figure K.17 shows a screen shot of the Virtual Keyboard with its key-range and velocity controllers displayed (menu item ``View / Key/Velocity''). The velocity controller sets the gain parameter, mapping MIDI velocity (0-127) to the unit interval (0-1). The key-range controller transposes the keyboard by octaves. Pressing a key determines, together with the key-range, the freq parameter in our synth. Pressing a key also sets the gate parameter to 1, and releasing it sets gate to 0. The ADSR envelope is triggered when gate transitions to 1, and it begins its ``release'' phase when gate transitions to 0, as is standard for ADSR envelopes triggered by a keyboard. Note that the bottom two rows of ASCII keyboard keys are mapped to virtual-keyboard keys, enabling the playing of chords in real time on the regular computer keyboard.

Figure K.18 illustrates the MIDI tab of qjackctl's Connect window after connecting the Virtual Keyboard MIDI output to pd's MIDI input.^K.16

To play back a MIDI file (extension .mid), a nice way is to open it in Rosegarden (``Applications / Planet CCRMA / Sequencers / Rosegarden'') and connect Rosegarden's MIDI output to pd's MIDI input as above. (You can still play along on the Virtual Keyboard.)

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About the Author: Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.