Copyright © 2011 by Denny Lin1 Simple Synthesizer Part 4 Based on Floss Manuals (Pure Data) “Building a Simple Synthesizer” By Derek Holzer Slides by Denny Lin

Copyright © 2011 by Denny Lin2 Controlling the Synthesizer Two ways for synthesizer and user interface: –Input from computer keyboard. The ASCII value of the key pressed is gathered by the key object, which converts its into frequency values using the mtof object –Input from MIDI keyboard. Reads the note number and velocity from channel 1 using the notein 1 object.

Copyright © 2011 by Denny Lin3 Building a 16-Step Sequencer Consists of a: –Constrained counter –MIDI note storage and retrieval –Simple subtractive synthesizer with ADSR envelope to play back notes

Copyright © 2011 by Denny Lin4 A Four Stage Filtered Additive Synthesizer Consists of the following stages: –Input –Oscillator –Filter –Amplifier These four stages are contained in sub- patches

Copyright © 2011 by Denny Lin5 Input Stage The key object gets the ASCII value of the key pressed on the computer keyboard ASCII value is sent to left most outlet Center outlet sends the same ASCII value as the left most outlet Right most outlet sends a bang used by the ADSR generator

Copyright © 2011 by Denny Lin6 Oscillator Stage Takes the midi number and converts it to a frequency used by the two phasor~ objects Second phasor~ object has slightly de-tuned pitch (input is multiplied by 0.99 –almost one--) Output is added, and scaled down to 0.5 before delivering to the audio outlet

Copyright © 2011 by Denny Lin7 Filter Stage Uses a Voltage Controlled Filter to perform subtractive synthesis. The VCO’s center frequency is set to 1.5 times (half an octave) above the incoming audio signal The center frequency is packed and sent to the line~ object, so it ramps to the center frequency in 300ms The ramp from the line~ object creates a filter sweeping effect

Copyright © 2011 by Denny Lin8 Amplifier Stage The amplitude of the audio signal is modified by the ADSR envelope Controls the overall volume of the signal coming in from the Filter Stage Output is sent to the audio outlet

Copyright © 2011 by Denny Lin9 The poly object The poly object assigns a voice number to a pitch and velocity pair poly object creation arguments: –First: Number of voices –Second: Voice stealing mode (what to do when input exceeds number of voices) 0 = no voice stealing; ignore new note event 1 = turns off oldest voice stored to make space for new note event

Copyright © 2011 by Denny Lin10 Building a polyphonic synthesizer Use the notein object to extract pitch and velocity pairs from the MIDI data stream Assign a voice number to pitch and velocity pairs with the poly object Pack the voice number, pitch, and velocity information into a list Route the list by voice number to multiple oscillators. Must have one oscillator for each voice. Can filter each voice individually, or as a group The next example filters voices individually, because each voice has a filter that sweeps to a unique pitch for that voice. To minimize control lines, the filter is built into the oscillator Polyphonic output amplitude can be modified by an ADSR envelope and amplified

Copyright © 2011 by Denny Lin11 A polyphonic MIDI synthesizer Based on Benoît Rouits’ “A Real Polyphonic MIDI Organ”