«Visual demonstration of aliasing/foldover (modified from James Harkins)» by jnarveson

on 15 Jan'14 18:30 in informativeeducationalnyquist

James Harkins already posted already excellent little visualizer for illustrating the importance of the Nyquist frequency - I just wanted to add a slider for variable sample rate, and some grid lines that makes it easier for me to see the sampling rate.

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(
// a slightly modified version of the excellent example patch by James Harkins.
// I added some grid lines for the samples, an adjustable sampling rate, and changed some colors.
// Jascha Narveson

var screen = Window.screenBounds,
	height = (screen.height - 120).asInteger,
	width = (screen.width - 120).asInteger,
win = Window(\aliasing, Rect.aboutPoint(screen.center, width / 2, height / 2 + 60)).background_(Color.white),
	sinPts = 400, sampPts = 20, sr,
	freq = 1, fsl,
	sinColor = Color.red, sampColor = Color.blue, lineColor = Color.grey;

win.drawHook = {
	var pt;
	pt = Point(0, height/2);
	Pen.color_(sampColor)
		.moveTo(pt);
	(1..sampPts).do { |x|
		Pen.moveTo(pt);
		pt = Point(x * (width / sampPts), sin(x * freq / sampPts * 2pi).linlin(-1, 1, height, 0));
		Pen.lineTo(pt).stroke
			.fillRect(Rect.aboutPoint(pt, 3, 3));
	};

	Pen.color_(sinColor)
		.moveTo(Point(0, height/2));
	(1..sinPts).do { |x|
		Pen.lineTo(Point(
			x * (width / sinPts),
			sin(x * freq / sinPts * 2pi).linlin(-1, 1, height, 0)
		));
	};

	Pen.stroke;

	Pen.color_(lineColor);
	(1..sinPts).do({|x|
		var xpos = x * (width / sampPts);
		Pen.moveTo(Point(xpos, 0));
		Pen.lineTo(Point(xpos, height));
		Pen.stroke;
	});

};

fsl = EZSlider(
	win,
	Rect(5, height + 10, width-10, 20),
	"freq of input wave (red):",
	ControlSpec(1, 40, 'lin', 0, 20),
	{|slider| freq = slider.value; win.refresh },
	1,
	labelWidth: 200,
	initAction: true
);

sr = EZSlider(
	win,
	Rect(5, height + 30, width-10, 20),
	"samp rate (output wave in blue):",
	ControlSpec(1, 44, 'lin', 1, 20),
	{|slider| sampPts = slider.value; win.refresh },
	1,
	labelWidth: 200,
	initAction: true
);
win.front;
)
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