A phase shifters gives the impression that the sound of a note is moving away and then coming back in again, like a swirling effect that might be heard if the speaker was rotating. Need to work out the maths, but this should be an effect we can create.

See musicdsp.com

Optionally, for each note played, mix the note and the same note one octave lower (i.e. freq / 2), to give a richer tone.

Can we add a graphic equaliser to the synthesizer? In the graphic equalizer, the input signal is sent to a bank of filters. Each filter passes the portion of the signal present in its own frequency range or band. The amplitude passed by each filter is adjusted using a slide control to boost or cut frequency components passed by that filter. The vertical position of each slider indicates the gain applied at that frequency band, so that the knobs resemble a graph of the equalizer's response plotted versus frequency.

Add a graphical waveform visualisation to the GUI.

Add the ability to record a loop, e.g. a chord progression, and play it back whilst playing notes from the synth over the top of the loop. As the name suggests, the loop should play continually until it is turned off.

Using the NAudio BiQuadFilter class, implement a low pass filter sample provider. This will allow low frequencies to pass through, which can be useful for filtering out harsh-sounding high frequencies. GUI will need a slider for "cut-off frequency" in Hz and "resonance a.k.a. Q" (a float), which is an emphasis added through feedback at the cutoff frequency. Basically, Q defines how steep the cut-off curve is.

The BiQuadFilter class looks useful for other types of filter too.

Add frequency modulation with a low frequency oscillator (LFO) (see #11). It should be possible to extend the SignalGenerator class to support this. We could use this to implement a Vibrato effect, for example.

Using wave tables limits what we can do, specifically around implementing a Low Frequency Oscillator to do some frequency modulation in the signal chain. Remove the use of wave tables and replace them with, initially, the NAudio SignalGenerator class.

Later, we can create a custom SignalGenerator to allow us to add LFOs as part of #9.

Find a way to create pre-defined instruments for the synthesizer. These would include pre-set:

• ADSR envelope
• combinations of different wave sources
• possibility of adding white noise as a source
• low frequency oscillator

Use one mixer per note? See #22.

Modulate volume of the VolumeSampleProvider in the signal chain with a low frequency oscillator to create a tremolo effect.

Need to be able to use the GUI to select different waveforms when not using a pre-set instrument/voice:

• sine wave
• sawtooth wave
• square wave
• triangle wave
• white noise
• pink noise

See also #11.

See #9 for option to feed in LFO frequency modulation.

Update or replace the SythWaveProvider class to allow user to create different Attack, Decay, Sustain and Release parameters via the GUI, so they can shape the sound envelope of notes. NAudio's AdsrSampleProvider only allows user control of Attack and Release, so we will not use this class directly, but bring its functionality into our SynthWaveProvider or its replacement.

Need to be able to control:

• Attack seconds
• Decay seconds
• Sustain volume level
• Release seconds

See also #11.

In SetSlide() method.

If the Tremolo Frequency is raised above 20 Hz, some interesting amplitude modulation effects occur - the appearance of sidebands. Add a Tremolo Frequency Multiplier so that frequencies of up to 1000 Hz can be set up, so that we can explore the sounds created from AM at these frequencies.

Create a Flanger effect for the synthesizer.

When many of the user-controlled parameters are changed rapidly, then clicks and other artefacts are evident in the synths output. We need to smooth the transition to new parameter values, using an equation such as the follows:

x = x - z (x - y)

where:

• x = intermediate "smoothed" value
• y = the target value
• z = the smoothing factor e.g. 0.001

A2 = 33; A3 = 45; A4 = 57.

Let’s enable playing using a mini MIDI keyboard controller! NAudio has a whole set of MIDI classes to support this feature.

As a step towards creating preset voices or instruments, add a feature that allows the user to set up various effects and configurations of the synthesizer, and save them as “patches’. Allow these to be recalled, to instantly restore the synth to the saved preset values. Save settings as JSON?

Create a chorus ISampleProvider for the synthesizer.

Have the low pass filters for each note use a relative cutoff frequency, e.g. 4 octaves above the frequency of the note being played.

Add 2 maybe 3 oscillators that can be mixed into a note, with settings for:

• Level (dB)
• Wave Type (Sine, SawTooth, Square, Triangle, White Noise, Pink Noise)
• Relative tuning

Add a graphical spectrum analyser visualisation to the GUI.

Add a configurable (via the GUI) Delay effect to the synthesizer, so that we can have echo. Need to find an open source algorithm for this!

See musicdsp.com

y = 10^(x/20)

x in range -60 to 0 dB => y from ~0.0 to 1.0