Speedster (Mac) — a time-stretching and pitch-shifting audio processor with some interesting twists — is the latest and most sophisticated of five free Audio Units (AU) plug-ins available from Airy Andre (http://airy.andre.online.fr/AU). Speedster provides independent control of pitch and playback speed, and it allows you to enter those values in real time in any of three ways: typing in numbers, using separate pitch and speed sliders, or with an onscreen x-y control. You can also control pitch using a MIDI keyboard if your AU host supports it, in which case incoming MIDI notes will set the pitch shift in semitones relative to middle C (MIDI Note Number 60).
What makes Speedster unique is its alternate control panel (accessed by clicking on the Time button) for independently processing the left, center, and right audio channels. You can separately shift the pitch of each channel and set their volumes and pan position. Zeroing the center channel's volume is useful for karaoke effects. All alternate-panel settings are subject to the global main-panel settings for speed and pitch shift. Used as a send effect with appropriate panning, Speedster can independently pitch-shift three separate audio tracks, reposition those tracks in the stereo field, and shift the pitch and speed of the whole output mix. Add real-time control, and as my online audio example illustrates, you can create quite a mess (see Web Clip 1).
The remaining four plug-ins include three synths and a filter effect. The most interesting of the synths is a virtual TB-303-style bass machine called AU303. It contains a 16-step sequencer for playing a built-in bass synth, consisting of a sawtooth- or square-wave oscillator followed by a resonant low-pass filter with a simple decay envelope. Individual steps can be turned off or can be linked to the previous step for a legato effect. Each step has its own Velocity slider and Glide button. AU303 stores 16 sequences, and you can select them from a MIDI keyboard using Note C5 and above. Notes below C5 serve to transpose the sequence.
AirySynth is a deceptively simple synth with an LFO and an attack-release amplitude envelope. Its oscillator has four waveforms: sine, pulse, triangle, and noise. The LFO controls the symmetry of all waveforms but has no effect on the noise source. What makes AirySynth interesting is its morphing Vowel filter, which lets you select two vowels and morph between them. Automating the morph setting and the two vowel selections and adding a little LFO modulation produces interesting vocal-like timbres reminiscent of the classic Theremin. The Vowel filter is available separately as an effect plug-in called Voweler.
The last synth, AUStk, showcases a set of developer tools for building AU instruments, which are also free. It has 21 preset sounds ranging from blown to plucked instruments. AUStk has no controls other than the sound selector, but many of the sounds — especially the plucked ones — are quite usable. Speedster is definitely worth the trip to Airy's Web site, and while you are there, pick up the rest of the plug-ins.