Your First Step to the Silverlight Voice/Video Chatting Client/Server

using System;
using System.Collections.Generic;
using System.Text;

namespace VoiceEncoder
{
    #region G.711 Encoder Classes

    namespace G711Audio
    {
        /// <summary>
        /// Turns 16-bit linear PCM values into 8-bit A-law bytes.
        /// </summary>
        public class ALawEncoder
        {
            public const int MAX = 0x7fff; //maximum that can be held in 15 bits

            /// <summary>
            /// An array where the index is the 16-bit PCM input, and the value is
            /// the a-law result.
            /// </summary>
            private static byte[] pcmToALawMap;

            static ALawEncoder()
            {
                pcmToALawMap = new byte[65536];
                for (int i = short.MinValue; i <= short.MaxValue; i++)
                    pcmToALawMap[(i & 0xffff)] = encode(i);
            }

            /// <summary>
            /// Encode one a-law byte from a 16-bit signed integer. protected use only.
            /// </summary>
            /// <param name="pcm">A 16-bit signed pcm value</param>
            /// <returns>A a-law encoded byte</returns>
            private static byte encode(int pcm)
            {
                //Get the sign bit.  Shift it for later use without further modification
                int sign = (pcm & 0x8000) >> 8;
                //If the number is negative, make it positive (now it's a magnitude)
                if (sign != 0)
                    pcm = -pcm;
                //The magnitude must fit in 15 bits to avoid overflow
                if (pcm > MAX) pcm = MAX;

                /* Finding the "exponent"
                 * Bits:
                 * 1 2 3 4 5 6 7 8 9 A B C D E F G
                 * S 7 6 5 4 3 2 1 0 0 0 0 0 0 0 0
                 * We want to find where the first 1 after the sign bit is.
                 * We take the corresponding value from the second row as the exponent value.
                 * (i.e. if first 1 at position 7 -> exponent = 2)
                 * The exponent is 0 if the 1 is not found in bits 2 through 8.
                 * This means the exponent is 0 even if the "first 1" doesn't exist.
                 */
                int exponent = 7;
                //Move to the right and decrement exponent until we hit the 1 or the exponent hits 0
                for (int expMask = 0x4000; (pcm & expMask) == 0 && exponent > 0; exponent--, expMask >>= 1) { }

                /* The last part - the "mantissa"
                 * We need to take the four bits after the 1 we just found.
                 * To get it, we shift 0x0f :
                 * 1 2 3 4 5 6 7 8 9 A B C D E F G
                 * S 0 0 0 0 0 1 . . . . . . . . . (say that exponent is 2)
                 * . . . . . . . . . . . . 1 1 1 1
                 * We shift it 5 times for an exponent of two, meaning
                 * we will shift our four bits (exponent + 3) bits.
                 * For convenience, we will actually just shift the number, then AND with 0x0f. 
                 * 
                 * NOTE: If the exponent is 0:
                 * 1 2 3 4 5 6 7 8 9 A B C D E F G
                 * S 0 0 0 0 0 0 0 Z Y X W V U T S (we know nothing about bit 9)
                 * . . . . . . . . . . . . 1 1 1 1
                 * We want to get ZYXW, which means a shift of 4 instead of 3
                 */
                int mantissa = (pcm >> ((exponent == 0) ? 4 : (exponent + 3))) & 0x0f;

                //The a-law byte bit arrangement is SEEEMMMM (Sign, Exponent, and Mantissa.)
                byte alaw = (byte)(sign | exponent << 4 | mantissa);

                //Last is to flip every other bit, and the sign bit (0xD5 = 1101 0101)
                return (byte)(alaw ^ 0xD5);
            }

            /// <summary>
            /// Encode a pcm value into a a-law byte
            /// </summary>
            /// <param name="pcm">A 16-bit pcm value</param>
            /// <returns>A a-law encoded byte</returns>
            public static byte ALawEncode(int pcm)
            {
                return pcmToALawMap[pcm & 0xffff];
            }

            /// <summary>
            /// Encode a pcm value into a a-law byte
            /// </summary>
            /// <param name="pcm">A 16-bit pcm value</param>
            /// <returns>A a-law encoded byte</returns>
            public static byte ALawEncode(short pcm)
            {
                return pcmToALawMap[pcm & 0xffff];
            }

            /// <summary>
            /// Encode an array of pcm values
            /// </summary>
            /// <param name="data">An array of 16-bit pcm values</param>
            /// <returns>An array of a-law bytes containing the results</returns>
            public static byte[] ALawEncode(int[] data)
            {
                int size = data.Length;
                byte[] encoded = new byte[size];
                for (int i = 0; i < size; i++)
                    encoded[i] = ALawEncode(data[i]);
                return encoded;
            }

            /// <summary>
            /// Encode an array of pcm values
            /// </summary>
            /// <param name="data">An array of 16-bit pcm values</param>
            /// <returns>An array of a-law bytes containing the results</returns>
            public static byte[] ALawEncode(short[] data)
            {
                int size = data.Length;
                byte[] encoded = new byte[size];
                for (int i = 0; i < size; i++)
                    encoded[i] = ALawEncode(data[i]);
                return encoded;
            }

            /// <summary>
            /// Encode an array of pcm values
            /// </summary>
            /// <param name="data">An array of bytes in Little-Endian format</param>
            /// <returns>An array of a-law bytes containing the results</returns>
            public static byte[] ALawEncode(byte[] data)
            {
                int size = data.Length / 2;
                byte[] encoded = new byte[size];
                for (int i = 0; i < size; i++)
                    encoded[i] = ALawEncode((data[2 * i + 1] << 8) | data[2 * i]);
                return encoded;
            }

            /// <summary>
            /// Encode an array of pcm values into a pre-allocated target array
            /// </summary>
            /// <param name="data">An array of bytes in Little-Endian format</param>
            /// <param name="target">A pre-allocated array to receive the A-law bytes.  This array must be at least half the size of the source.</param>
            public static void ALawEncode(byte[] data, byte[] target)
            {
                int size = data.Length / 2;
                for (int i = 0; i < size; i++)
                    target[i] = ALawEncode((data[2 * i + 1] << 8) | data[2 * i]);
            }
        }


        /// <summary>
        /// Turns 8-bit A-law bytes back into 16-bit PCM values.
        /// </summary>
        public static class ALawDecoder
        {
            /// <summary>
            /// An array where the index is the a-law input, and the value is
            /// the 16-bit PCM result.
            /// </summary>
            private static short[] aLawToPcmMap;

            static ALawDecoder()
            {
                aLawToPcmMap = new short[256];
                for (byte i = 0; i < byte.MaxValue; i++)
                    aLawToPcmMap[i] = decode(i);
            }

            /// <summary>
            /// Decode one a-law byte. For protected use only.
            /// </summary>
            /// <param name="alaw">The encoded a-law byte</param>
            /// <returns>A short containing the 16-bit result</returns>
            private static short decode(byte alaw)
            {
                //Invert every other bit, and the sign bit (0xD5 = 1101 0101)
                alaw ^= 0xD5;

                //Pull out the value of the sign bit
                int sign = alaw & 0x80;
                //Pull out and shift over the value of the exponent
                int exponent = (alaw & 0x70) >> 4;
                //Pull out the four bits of data
                int data = alaw & 0x0f;

                //Shift the data four bits to the left
                data <<= 4;
                //Add 8 to put the result in the middle of the range (like adding a half)
                data += 8;

                //If the exponent is not 0, then we know the four bits followed a 1,
                //and can thus add this implicit 1 with 0x100.
                if (exponent != 0)
                    data += 0x100;
                /* Shift the bits to where they need to be: left (exponent - 1) places
                 * Why (exponent - 1) ?
                 * 1 2 3 4 5 6 7 8 9 A B C D E F G
                 * . 7 6 5 4 3 2 1 . . . . . . . . <-- starting bit (based on exponent)
                 * . . . . . . . Z x x x x 1 0 0 0 <-- our data (Z is 0 only when exponent is 0)
                 * We need to move the one under the value of the exponent,
                 * which means it must move (exponent - 1) times
                 * It also means shifting is unnecessary if exponent is 0 or 1.
                 */
                if (exponent > 1)
                    data <<= (exponent - 1);

                return (short)(sign == 0 ? data : -data);
            }

            /// <summary>
            /// Decode one a-law byte
            /// </summary>
            /// <param name="alaw">The encoded a-law byte</param>
            /// <returns>A short containing the 16-bit result</returns>
            public static short ALawDecode(byte alaw)
            {
                return aLawToPcmMap[alaw];
            }

            /// <summary>
            /// Decode an array of a-law encoded bytes
            /// </summary>
            /// <param name="data">An array of a-law encoded bytes</param>
            /// <returns>An array of shorts containing the results</returns>
            public static short[] ALawDecode(byte[] data)
            {
                int size = data.Length;
                short[] decoded = new short[size];
                for (int i = 0; i < size; i++)
                    decoded[i] = aLawToPcmMap[data[i]];
                return decoded;
            }

            /// <summary>
            /// Decode an array of a-law encoded bytes
            /// </summary>
            /// <param name="data">An array of a-law encoded bytes</param>
            /// <param name="decoded">An array of shorts containing the results</param>
            /// <remarks>Same as the other method that returns an array of shorts</remarks>
            public static void ALawDecode(byte[] data, out short[] decoded)
            {
                int size = data.Length;
                decoded = new short[size];
                for (int i = 0; i < size; i++)
                    decoded[i] = aLawToPcmMap[data[i]];
            }

            /// <summary>
            /// Decode an array of a-law encoded bytes
            /// </summary>
            /// <param name="data">An array of a-law encoded bytes</param>
            /// <param name="decoded">An array of bytes in Little-Endian format containing the results</param>
            public static void ALawDecode(byte[] data, out byte[] decoded)
            {
                int size = data.Length;
                decoded = new byte[size * 2];
                for (int i = 0; i < size; i++)
                {
                    //First byte is the less significant byte
                    decoded[2 * i] = (byte)(aLawToPcmMap[data[i]] & 0xff);
                    //Second byte is the more significant byte
                    decoded[2 * i + 1] = (byte)(aLawToPcmMap[data[i]] >> 8);
                }
            }
        }
    }
    #endregion G.711 Encoding
}