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Picture display for oscilloscopes

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# Picture oscillator

You can download binaries from [release page](https://github.com/boochow/picture/releases).

[![top-page](https://raw.githubusercontent.com/boochow/picture/images/picture_osc.gif)](https://www.youtube.com/watch?v=RcrHFz_dZ6o)

## About
Picture is an oscillator module that can draw pictures on an XY-mode oscilloscope using [lissa delay](https://github.com/boochow/lissa). It is a kind of wavetable synthesizer and the picture to be drawn is encoded in a wavetable.

Editing `figure.h` enables you to make your own oscillator which shows your own pictures on an oscilloscope. 
Since the module can hold multipe waveforms, it is possible to include a set of pictures in a single oscillator module.

You can select waveform with the Shift-shape knob.
The shape knob squashes the picture. It affects the sound in a way similar to PWM.

Picture works on any logue-sdk compatible hardware.

## Drawing pictures

It is known that the pair of two synchronized waveforms can draw a picture on an XY-mode oscilloscope screen. However, the logue-sdk oscillator is single-channel and is not able to generate two waveforms at once.

I use Lissa delay, a 90 degrees delay, to generate two signals from a single signal.

![top-page](https://raw.githubusercontent.com/boochow/picture/images/fig1.png)

The main idea of Picture OSC is to place the X signal and the Y signal in a different phase of a single waveform. 
The Y signal is placed at the first quarter of waveform. The X signal is placed at the fourth quarter.
Then Lissa delay aligns them to the same phase by delaying the signal for a quarter of the wavelength.

![top-page](https://raw.githubusercontent.com/boochow/picture/images/fig2.png)

The first quarter of the Y signal is copied to the X signal of the second quarter. 
The Y signal of the second quarter (which is also the X signal of the third quarter) could be -X of the first quarter to draw the same picture of the first quarter with 90 degrees rotation.

![top-page](https://raw.githubusercontent.com/boochow/picture/images/fig3.png)

The result would be four same pictures rotated by 90 degrees.

## Synchronizing the tone pitch and the delay time

Although playing music with the sound of Picture OSC is possible, there is an issue for drawing pictures while playing music.

To draw a stable figure, the delay time must always be precisely a quarter of the wavelength. However, there is no way for the delay module to get the current pitch value of the oscillator module.

To solve this, I used a MIDI signal processor plugin software on PC, which converts MIDI note number to MIDI control change for delay time (CC#30).
The MIDI filter I used is midiNotesToCC in [Piz MIDI Plugins](https://www.kvraudio.com/forum/viewtopic.php?t=300566).
![top-page](https://raw.githubusercontent.com/boochow/picture/images/fig4.png)

## Defining your own picture

The actual wavetable is stored in the file `figure.h`. There are two arrays of float, `figure_x`, and `figure_y`. They hold x-coordinates and y-coordinates of  vertices of a polygon that is drawn on the screen.

The coordinates must be between 0.0 and 1.0. If you want to draw a closed polygon, you have to place the same coordinates at the beginning of the array and the end of the arrays.

## Others

My other user oscillators / modules are [here](https://blog.boochow.com/logue).

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