Lpc.cs

//
// Copyright (C) 2003 Jean-Marc Valin
// Copyright (C) 1999-2003 Wimba S.A., All Rights Reserved.
// Copyright (C) 2008 Filip Navara
// Copyright (C) 2009-2010 Christoph Fröschl
//
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using System;

namespace NSpeex
{
	/// <summary>
	/// LPC - and Reflection Coefficients.
	/// 
	/// The next two functions calculate linear prediction coefficients and/or the
	/// related reflection coefficients from the first P_MAX+1 values of the
	/// autocorrelation function.
	/// 
	/// Invented by N. Levinson in 1947, modified by J. Durbin in 1959.
	/// </summary>
    internal class Lpc
	{
		/// <summary>
		/// Returns minimum mean square error.
		/// </summary>
		/// <returns>minimum mean square error.</returns>
		public static float Wld(float[] lpc, float[] ac, float[] xref, int p)
		{
			int i, j;
			float r, error = ac[0];
			if (ac[0] == 0)
			{
				for (i = 0; i < p; i++)
					xref[i] = 0;
				return 0;
			}
			for (i = 0; i < p; i++)
			{
				// Sum up this iteration's reflection coefficient.
				r = -ac[i + 1];
				for (j = 0; j < i; j++)
					r -= lpc[j] * ac[i - j];
				xref[i] = r /= error;
				// Update LPC coefficients and total error.
				lpc[i] = r;
				for (j = 0; j < i / 2; j++)
				{
					float tmp = lpc[j];
					lpc[j] += r * lpc[i - 1 - j];
					lpc[i - 1 - j] += r * tmp;
				}
				if ((i % 2) != 0)
					lpc[j] += lpc[j] * r;
				error *= ((Single?)1.0d).Value - r * r;
			}
			return error;
		}

		/// <summary>
		/// Compute the autocorrelation ,--, ac(i) = > x(n)/// x(n-i) for all n `--'
		/// for lags between 0 and lag-1, and x == 0 outside 0...n-1
		/// </summary>
		public static void Autocorr(float[] x, float[] ac, int lag, int n)
		{
			float d;
			int i;
			while (lag-- > 0)
			{
				for (i = lag, d = 0; i < n; i++)
					d += x[i] * x[i - lag];
				ac[lag] = d;
			}
		}
	}
}