AppDelegate.cs

using System;

using Foundation;
using UIKit;
using AudioToolbox;
using AVFoundation;

namespace tone {
	[Register ("AppDelegate")]
	unsafe public partial class AppDelegate : UIApplicationDelegate {
		double sampleRate;
		const int numBuffers = 3;
		bool alternate = false;
		NSError error;

		public override bool FinishedLaunching (UIApplication app, NSDictionary options)
		{
			//
			// Setup audio system
			//
			var session = AVAudioSession.SharedInstance ();
			session.SetCategory (new NSString ("AVAudioSessionCategoryPlayback"), AVAudioSessionCategoryOptions.DefaultToSpeaker, out error);
			if (error != null) {
				Console.WriteLine (error);
			}
			//
			// Format description, we generate LinearPCM as short integers
			//
			sampleRate = session.SampleRate;
			var format = new AudioStreamBasicDescription {
				SampleRate = sampleRate,
				Format = AudioFormatType.LinearPCM,
				FormatFlags = AudioFormatFlags.LinearPCMIsSignedInteger | AudioFormatFlags.LinearPCMIsPacked,
				BitsPerChannel = 16,
				ChannelsPerFrame = 1,
				BytesPerFrame = 2,
				BytesPerPacket = 2,
				FramesPerPacket = 1,
			};

			//
			// Create an output queue
			//
			var queue = new OutputAudioQueue (format);
			var bufferByteSize = (sampleRate > 16000) ? 2176 : 512; // 40.5 Hz : 31.25 Hz

			//
			// Create three buffers, generate a tone, and output the tones
			//
			var buffers = new AudioQueueBuffer* [numBuffers];
			for (int i = 0; i < numBuffers; i++) {
				queue.AllocateBuffer (bufferByteSize, out buffers [i]);
				GenerateTone (buffers [i]);
				queue.EnqueueBuffer (buffers [i], null);
			}

			//
			// Output callback: invoked when the audio system is done with the
			// buffer, this implementation merely recycles it.
			//
			queue.BufferCompleted += (object sender, BufferCompletedEventArgs e) => {
				if (alternate) {
					outputWaveForm += 1;
					if (outputWaveForm > WaveForm.Square)
						outputWaveForm = WaveForm.Sine;
					GenerateTone (e.UnsafeBuffer);
				}
				queue.EnqueueBuffer (e.UnsafeBuffer, null);
			};

			queue.Start ();
			return true;
		}

		// Configuration options for the audio output
		const float outputFrequency = 220;

		enum WaveForm {
			Sine,
			Triangle,
			Sawtooth,
			Square
		}

		WaveForm outputWaveForm;

		//
		// Simple tone generator
		//
		void GenerateTone (AudioQueueBuffer* buffer)
		{
			// Make the buffer length a multiple of the wavelength for the output frequency.
			uint sampleCount = buffer->AudioDataBytesCapacity / 2;
			double bufferLength = sampleCount;
			double wavelength = sampleRate / outputFrequency;
			double repetitions = Math.Floor (bufferLength / wavelength);
			if (repetitions > 0)
				sampleCount = (uint) Math.Round (wavelength * repetitions);

			double x, y;
			double sd = 1.0 / sampleRate;
			double amp = 0.9;
			double max16bit = Int16.MaxValue;
			int i;
			short* p = (short*) buffer->AudioData;

			for (i = 0; i < sampleCount; i++) {
				x = i * sd * outputFrequency;
				switch (outputWaveForm) {
				case WaveForm.Sine:
					y = Math.Sin (x * 2.0 * Math.PI);
					break;
				case WaveForm.Triangle:
					x = x % 1.0;
					if (x < 0.25)
						y = x * 4.0; // up 0.0 to 1.0
					else if (x < 0.75)
						y = (1.0 - x) * 4.0 - 2.0; // down 1.0 to -1.0
					else
						y = (x - 1.0) * 4.0; // up -1.0 to 0.0
					break;
				case WaveForm.Sawtooth:
					y = 0.8 - (x % 1.0) * 1.8;
					break;
				case WaveForm.Square:
					y = ((x % 1.0) < 0.5) ? 0.7 : -0.7;
					break;
				default:
					y = 0;
					break;
				}
				p [i] = (short) (y * max16bit * amp);
			}

			buffer->AudioDataByteSize = sampleCount * 2;
		}

		static void Main (string [] args)
		{
			UIApplication.Main (args, null, "AppDelegate");
		}
	}
}