Using sound to draw on an oscilloscope

What/Why

Oscilloscopes are cool. What is even cooler is using them to display arbitrary pictures instead of boring waves! We also can use them to write text (emulate a terminal) or maybe even play pong! But for now, let's focus on the basics of dot drawing using sound...

sequential-2.png

Soundcard_Oscilloscope_2015-01-12_10-10-57.png

face.png

Sequential Drawing
with no Persistence

Sequential Drawing
with Persistence

Dumb Shortest Path

 

Oscilloscope

osc1[1]
An oscilloscope is a device that allows you to visualize signals (voltages) over time.
When dealing with multiple signals (channels), you usually have three viewing mode: Dual, Add, and Chop. We will work in Dual.

 

X-Y

For us the most interesting oscilloscope feature is called X-Y. In this mode you will view a single dot. The horizontal scale will no longer be time. It will be Channel 2's voltage. The Channel 1 will move the dot up and down according to its input voltage, the Channel 2 will move the dot left and right according to its input voltage. A bit like an Etch'n'Sketch.

 

Sound as a source

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A speaker creates sound by pulsing air at different frequencies. Sometimes it pushes air (positive voltage), sometimes it pulls air (negative voltage). In other words a speaker is driven by alternating current.

By controlling the amplitude of the alternating current (or sound), we will be able to move the dot on the screen.

Another key element to our project is stereo sound. Stereo sound means that we have two sound signals: Left and Right, X and Y.

 

Sound limitations

The oscilloscope has a sampling rate of millions of hertz. In other words it can measure the signals and move the dot accordingly millions of times per seconds, giving us impressive theoretical FPS for whatever we want to draw.

BUT, the sampling rate of most digital music player is limited to 48000 hertz (or at best 96000 Hz), greatly limiting our FPS for complex pictures.

Sound also have advantages like portability:
- You can generate the sound file and bring it with you on your cellphone to do your demo on any oscilloscope around or share it on the demoscene
- No extra hardware necessary, any sound card will work.

 

Drawing Basics

We will move the dot so fast that the eye will be tricked into seeing a full picture.
We set our scales at 1V and we center the dot (X-Y) on the oscilloscope screen. We will use a stereo sound source (two voltages) to move it. Left sound will move it horizontally and right sound will move it vertically. See first table up this page for an overview.

oscillo_face.png

Drawing Sequentially

This technique will draw a complete column (Y) pixel by pixel then advance the row (X), draw a new column, advance row, loop...

Thanks to Chris I realized that this technique only works if you have an oscilloscope with low display persistence and fair sampling rate.

If your scope meet these criteria the vertical lines in the image above should be fairly invisible.

If your oscilloscope doesn't meet these criteria then dots that shouldn't be connected together will be connected vertically(red lines in image), creating random lines hiding the picture's content.

My first script creates a wave file to draw sequentially. Find it at the end of this article.

Shortest Path Problem

A better approach is to use a shortest past or path finding algorithm.

Basically what we want to do is compute the best(shortest) route possible to connect all our pixels. The dot on screen will follow that route and hopefully minimize the noise (random lines) connecting isolated shapes. There will still be unwanted lines to connect our shapes, similar to a neon sign or cursive writing; but they will usually be thinner and dimmer than the rest of the shapes.

My second script implements a relatively dumb path finding algorithm. The result is correct for the pictures I tested, but I'm studying real algorithms and hopefully I'll do a proper path finding script.  Find the current script at the end of this article.

Vector Drawing

An elegant solution would be to generate vector graphics instead of raster. Using vectors would allow us to leverage the display persistence to form our shapes. I believe that is how Youscope was done. While it should be possible to transform a simple raster (gif/png) in vectors, it is way beyond my knowledge of image manipulation :).

 

 

Sound Sample

A digital sound sample is a number that represent the amplitude of the audio signal.
That number will represent one pixel coordinate (X or Y).

 

Bits per sample

The bits per sample, or bit depth, is the resolution of the sample. With 8 bits, we have 256 amplitudes possible (-127 to +127).

Assuming stereo sound at 8 bits, we could draw a picture of 65536 pixels (256x256). Because of output quality and oscilloscope precision, we could benefit of using 16 bit sound while keeping a low resolution and over sample it to make a sharper image.

 

Sampling rate

The sampling rate is how many samples are recorded or played in one second. Most digital devices have a sampling rate of 48000Hz (CD is 44100Hz). Because we use stereo sound, this means we can move our dot 24000 per second. To trick the eye we need at least 10fps, this means that we can only draw an image of 2400 pixels.

Considering the oscilloscope's dot luminosity and our FPS limitation at this resolution, we can skip pixels. One full pixel, one empty pixel. The dot size would fill the voids.
That brings us to 16384 pixels (128x128), or an ability to fill 15% of the screen and refresh it 10 times per second.

 

Wave

The wave format is perfect for us. We can use arbitrary sampling rate, bits per sample, and channels. Following the header, each channel samples are interlaced. The data look like this: left sample, right sample, left sample, right sample...

 

Converting pixels to samples

For simplicity we will assume a 128x128 image with a white background. Anything that is not white will be a pixel to draw.

 

/dev/dsp

- We can use /dev/dsp to stream the wave directly to the sound card by setting a very high sample numbers in the header

 

Considerations

- Using lines.
- Going back to 0V before moving to the next pixel
- Using closest pixel instead of doing line per line
- Remanence
- Refresh rate 44100hz

 

 

 

Script:

This PHP script allows you to transform an animated GIF or a directory of PNG files into a sound wave file.

It can be run locally:

php oscillo.php inputfile.gif outputfile.wav

Or from a web page:

http://alexou.net/pub/oscilloscope/oscillo.php

 

View the original script

View the new script with dumb path finding

Commentaires
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When will you post the scripts ?
avatar
well this look like a diversion of a well known software used to design laser shows, which communicates with the laser interface via the sound card...nothing really new since it creates a XY trace by appropriate coding of the audio signals.
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alex
Administrateur
I wrote a few scripts to make use of this technique. The reason they are not in the article is that while they are working great, they are drafts that are most likely useable only by me. The other reason is that this page itself is a draft that I need to complete.

Here's a script that can be used to do real time drawing using an image editor, transforming a gif file to a .wav, creating a .wav from a bunch of .png.

https://alexou.net/pub/?script-collection/oscillo.phps

I obviously didn't claim that the technique was new or that I was the first one to implement it, but I have not used another such software. Can you tell me more about the one you are talking about? I'm sure it can do a lot more than I can imagine!
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Hello, I'm very interested in your scripts and using them in a project, however if I follow the link you posted (https://alexou.net/pub/?script-collection/oscillo.phps) I'm missing the includes like 'GifFrameExtractor.php'... Could you please upload a zip file including all required files?
I'd greatly appreciate that!
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alex
Administrateur
Hello Guest, you are correct I forgot to include it. I will fix that ASAP thanks!
I'll translate the remaining french comments in english at the same time.

Meanwhile you can download the library from its github: https://github.com/Sybio/GifFrameExtractor

My code assumes no namespace, you might want to comment the "namespace GifFrameExtractor" line.
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Hi Alex, thank you very much for your quick reaction. I made some alterations to your script already and I sent you a mail (to the address mentioned in the code), so you can see what has changed.
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