adpcm_decoder.m

用matlab编写的ADPCM编码和解码.里面包括了编码和解码的m文件.The algorithm is based on IMA ADPCM.

function raw_y = adpcm_decoder(adpcm_y)

% This m-file is based on the app note: AN643, Adaptive differential pulse
% code modulation using PICmicro microcontrollers, Microchip Technology
% Inc. The app note is avaialbe from www.microchip.com
% Example:  Y = wavread('test.wav');
%           y = adpcm_encoder(Y);
%           YY = adpcm_decode(y);
IndexTable = [-1, -1, -1, -1, 2, 4, 6, 8, -1, -1, -1, -1, 2, 4, 6, 8];
         
StepSizeTable = [7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 19, 21, 23, 25, 28, 31, 34, 37, 41, 45, 50, 55, 60, 66, 73, 80, 88, 97, 107, 118, 130, 143, 157, 173, 190, 209, 230, 253, 279, 307, 337, 371, 408, 449, 494, 544, 598, 658, 724, 796, 876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899, 15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767];

prevsample = 0;
previndex = 1;

Ns = length(adpcm_y);
n = 1;

while (n <= Ns)
    predsample = prevsample;
    index = previndex;
    step = StepSizeTable(index);
    code = adpcm_y(n);

    diffq = bitshift(step, -3);
    if (bitand(code, 4))
        diffq = diffq + step;
    end
    if (bitand(code, 2))
        diffq = diffq + bitshift(step, -1);
    end
    if (bitand(code, 1))
        diffq = diffq + bitshift(step, -2);
    end

    if (bitand(code, 8))
        predsample = predsample - diffq;
    else
        predsample = predsample + diffq;
    end

    if (predsample > 32767)
        predsample = 32767;
    elseif (predsample < -32768)
        predsample = -32768;
    end

    index = index + IndexTable(code+1);

    if (index < 1)
        index = 1;
    end
    if (index > 89)
        index = 89;
    end

    prevsample = predsample;
    previndex = index;

    raw_y(n) = predsample / 32767;
    n = n + 1;
end