Shepard Tones for Unity3D

ShepardTones.cs

namespace Zooming
{
	using System.Collections;
	using System.Collections.Generic;
	using UnityEngine;

	// @kurtdekker - Shepard Tones for Unity3D.
	//
	// https://en.wikipedia.org/wiki/Shepard_tone
	//

	public class ShepardTones : MonoBehaviour
	{
		public class Tone
		{
			public float theta;

			public float frequency;

			public float amplitude;
		}

		List<Tone> AllTones = new List<Tone>();

		// one over sample frequency
		float sampleK;

		// base frequency
		const float F0 = 100;

		const int octaveCount = 3;
		const int octaveSpan = 4;

		AudioSource azz;

		public static ShepardTones Create()
		{
			GameObject go = new GameObject("ShepherdTones.Create();");

			var st = go.AddComponent<ShepardTones>();

			var azz = go.AddComponent<AudioSource>();
			azz.bypassListenerEffects = true;
			azz.Play();

			st.azz = azz;

			return st;
		}

		const float VolumeSnappiness = 2.0f;
		float currentVolume;
		float desiredVolume;
		float actualVolume;

		public void SetVolume( float v)
		{
			desiredVolume = Mathf.Clamp01(v);
		}

		bool dead;
		public void SetDead()
		{
			dead = true;
		}

		void Start()
		{
			sampleK = 1.0f / 22050;

			// this detunes the pair frequency by a linear amount, making
			// for an ever-evolving beat frequency
			float beatFrequencyOffset = Random.Range(1.0f, 3.0f);

			// make all the octaves
			for (int octave = 0; octave < octaveCount; octave++)
			{
				float f = F0 * Mathf.Pow(2, octave);

				// the core frequency
				AllTones.Add(new Tone
				{
					frequency = f,
				});
				// the linear detuned frequency
				AllTones.Add(new Tone
				{
					frequency = f + beatFrequencyOffset,
				});
			}
		}

		void Update()
		{
			currentVolume = Mathf.Lerp(currentVolume, desiredVolume, VolumeSnappiness * Time.deltaTime);

			actualVolume = currentVolume;
			if (Time.timeScale == 0) actualVolume = 0;

			// all sounds fall to zero
			if (dead)
			{
				foreach (var tone in AllTones)
				{
					float f = tone.frequency;

					// linear - sounds intentionally awful
					float fx = f / 2 + 250;

					f -= fx * 0.5f * Time.deltaTime;

					tone.frequency = f;

					tone.amplitude -= Time.deltaTime * 0.05f;

					if (f < 100)
					{
						tone.amplitude = 0;
					}
				}
				return;
			}

			// frequency bands:
			// fade in between these frequencies
			float F1 = F0;
			float F2 = F1 * 2;
			// fade out between these frequencies
			float F3 = F1 * Mathf.Pow( 2, octaveSpan - 1);
			float F4 = F3 * 2;

			foreach (var tone in AllTones)
			{
				float f = tone.frequency;

				f += f * 0.1f * Time.deltaTime;

				if (f >= F4)
				{
					f /= (F4 / F1);
				}

				tone.frequency = f;

				float amplitude = 1.0f;

				// fading in?
				if (f < F2)
				{
					amplitude = (f - F1) / (F2 - F1);
				}
				// fading out?
				if (f > F3)
				{
					amplitude = 1.0f - (f - F3) / (F4 - F3);
				}

				tone.amplitude = amplitude;
			}
		}

		private void OnAudioFilterRead(float[] data, int channels)
		{
			float volume = actualVolume;

			for (int i = 0; i < data.Length; i += channels)
			{
				for (int channel = 0; channel < channels; channel++)
				{
					int n = i + channel;

					float d0 = data[n];

					foreach (var tone in AllTones)
					{
						float theta = tone.theta;

						theta += sampleK * tone.frequency;

						if (theta >= Mathf.PI * 2) theta -= Mathf.PI * 2;

						tone.theta = theta;

						float sample = Mathf.Sin(theta);

						sample *= tone.amplitude;

						d0 += sample;
					}

					data[n] = d0 * volume;
				}
			}
		}
	}
}