a-plugins

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a-plugins is a suite of plugins bundled with Ardour, from version 5.0 onwards. They include all the basic plugins you need to get started mixing. The suite currently consists of the following -

a-Amplifier A simple multichannel plugin to amplify or attenuate your signal by +/- 20dB
a-Compressor A powerful compressor, including a sidechain input. Comes in both mono and stereo versions
a-Delay A simple but capable delay plugin
a-EQ    4 band parametric EQ with high and low shelf filters
a-Fluid Synth SF2 Synthesizer using fluidsynth as its backend
a-High/Low Pass filter High and low pass filters with steepness and resonance controls
a-Inline Scope   Mixer strip inline waveform display
a-Inline Spectrogram Mixer strip inline spectrum display
a-MIDI Monitor Plugin to display recent MIDI events inline in the mixer strip
a-Pong Play pong in Ardours inline display (example plugin)
a-Reasonable Synth A simple synthesizer
a-Reverb A Schroeder reverberator

Many of the a-plugins take advantage of Ardour's inline display, eg. EQ curve and compressor gain reduction.