table_absolute_threshold.m

这是一个包含对MPEG视频各种变换处理的Matlab程序包

function [TH, Map, LTq] = Table_absolute_threshold(Layer, fs, bitrate)
%[TH, Map,  LTq] = Table_absolute_threshold(Layer, fs, bitrate)
%
%   Returns the frequencies, critical band rates and absolute threshold
%   in TH. Map contais a mapping beween the frequency line k and an index
%   number for the TH or LTq tables. LTq contains only the threshold in quiet
%   LT_q(k) defined in tables D.1x of the standard [1, pp. 117].
%   
%   These values depends on the Layer, the frequency rate fs (H)z and the
%   bit rate `bitrate' kbits/s.
%
%   Conversion of the frequency f into barks is done using (f * bitrate / fs)
   
%   Author: Fabien A. P. Petitcolas
%           Computer Laboratory
%           University of Cambridge
%
%   Copyright (c) 1998--2001 by Fabien A. P. Petitcolas
%   $Header: /Matlab MPEG/Table_absolute_threshold.m 3     7/07/01 1:27 Fabienpe $
%   $Id: Table_absolute_threshold.m,v 1.2 1998-06-22 17:47:56+01 fapp2 Exp $

%   References:
%    [1] Information technology -- Coding of moving pictures and associated
%        audio for digital storage media at up to 1,5 Mbits/s -- Part3: audio.
%        British standard. BSI, London. October 1993. Implementation of ISO/IEC
%        11172-3:1993. BSI, London. First edition 1993-08-01.
%
%   Legal notice:
%    This computer program is based on ISO/IEC 11172-3:1993, Information
%    technology -- Coding of moving pictures and associated audio for digital
%    storage media at up to about 1,5 Mbit/s -- Part 3: Audio, with the
%    permission of ISO. Copies of this standards can be purchased from the
%    British Standards Institution, 389 Chiswick High Road, GB-London W4 4AL, 
%    Telephone:+ 44 181 996 90 00, Telefax:+ 44 181 996 74 00 or from ISO,
%    postal box 56, CH-1211 Geneva 20, Telephone [phone], Telefax
%    [phone]. Copyright remains with ISO.
%-------------------------------------------------------------------------------
Common;

if (Layer == 1)
   if (fs == 44100)
      % Frequency | Crit Band rate | Absolute threshold
      TH = [
            86.13   0.850  25.87  ;   172.27   1.694  14.85  ;
           258.40   2.525  10.72  ;   344.53   3.337   8.50  ;
           430.66   4.124   7.10  ;   516.80   4.882   6.11  ;
           602.93   5.608   5.37  ;   689.06   6.301   4.79  ;
           775.20   6.959   4.32  ;   861.33   7.581   3.92  ;
           947.46   8.169   3.57  ;  1033.59   8.723   3.25  ;
          1119.73   9.244   2.95  ;  1205.86   9.734   2.67  ;
          1291.99  10.195   2.39  ;  1378.13  10.629   2.11  ;
          1464.26  11.037   1.83  ;  1550.39  11.421   1.53  ;
          1636.52  11.783   1.23  ;  1722.66  12.125   0.90  ;
          1808.79  12.448   0.56  ;  1894.92  12.753   0.21  ;
          1981.05  13.042  -0.17  ;  2067.19  13.317  -0.56  ;
          2153.32  13.577  -0.96  ;  2239.45  13.825  -1.38  ;
          2325.59  14.062  -1.79  ;  2411.72  14.288  -2.21  ;
          2497.85  14.504  -2.63  ;  2583.98  14.711  -3.03  ;
          2670.12  14.909  -3.41  ;  2756.25  15.100  -3.77  ;
          2842.38  15.283  -4.09  ;  2928.52  15.460  -4.37  ;
          3014.65  15.631  -4.60  ;  3100.78  15.795  -4.78  ;
          3186.91  15.955  -4.91  ;  3273.05  16.110  -4.97  ;
          3359.18  16.260  -4.98  ;  3445.31  16.405  -4.92  ;
          3531.45  16.547  -4.81  ;  3617.58  16.685  -4.65  ;
          3703.71  16.820  -4.43  ;  3789.84  16.951  -4.17  ;
          3875.98  17.079  -3.87  ;  3962.11  17.204  -3.54  ;
          4048.24  17.327  -3.19  ;  4134.38  17.447  -2.82  ;
          4306.64  17.680  -2.06  ;  4478.91  17.904  -1.32  ;
          4651.17  18.121  -0.64  ;  4823.44  18.331  -0.04  ; 
          4995.70  18.534   0.47  ;  5167.97  18.730   0.89  ;
          5340.23  18.922   1.23  ;  5512.50  19.108   1.51  ;
          5684.77  19.288   1.74  ;  5857.03  19.464   1.93  ; 
          6029.30  19.635   2.11  ;  6201.56  19.801   2.28  ;
          6373.83  19.963   2.46  ;  6546.09  20.120   2.63  ;
          6718.36  20.273   2.82  ;  6890.63  20.421   3.03  ; 
          7062.89  20.565   3.25  ;  7235.16  20.705   3.49  ;
          7407.42  20.840   3.74  ;  7579.69  20.971   4.02  ;
          7751.95  21.099   4.32  ;  7924.22  21.222   4.64  ;
          8096.48  21.341   4.98  ;  8268.75  21.457   5.35  ;
          8613.28  21.676   6.15  ;  8957.81  21.882   7.07  ;
          9302.34  22.074   8.10  ;  9646.88  22.253   9.25  ;
          9991.41  22.420  10.54  ; 10335.94  22.575  11.97  ;
         10680.47  22.721  13.56  ; 11025.00  22.857  15.31  ;
         11369.53  22.984  17.23  ; 11714.06  23.102  19.34  ;
         12058.59  23.213  21.64  ; 12403.13  23.317  24.15  ;
         12747.66  23.414  26.88  ; 13092.19  23.506  29.84  ;
         13436.72  23.592  33.05  ; 13781.25  23.673  36.52  ;
         14125.78  23.749  40.25  ; 14470.31  23.821  44.27  ;
         14814.84  23.888  48.59  ; 15159.38  23.952  53.22  ;
         15503.91  24.013  58.18  ; 15848.44  24.070  63.49  ;
         16192.97  24.124  68.00  ; 16537.50  24.176  68.00  ;
         16882.03  24.225  68.00  ; 17226.56  24.271  68.00  ;
         17571.09  24.316  68.00  ; 17915.63  24.358  68.00  ;
         18260.16  24.398  68.00  ; 18604.69  24.436  68.00  ;
         18949.22  24.473  68.00  ; 19293.75  24.508  68.00  ;
         19638.28  24.541  68.00  ; 19982.81  24.573  68.00
      ];
      
      N = length(TH(:, 1));
      
      % Convert frequencies to samples indecies.
      for i = 1:N,
         TH(i, INDEX) = round(TH(i, INDEX) / 44100 * 512);
      end
      
      % Generate a mapping between the FFT_SIZE / 2 samples of the input
      % signal and the N coefficients of the absolute threshold table.

      % Borders
      for j = 1:TH(1, INDEX),
         Map(j) = 1;
      end
      for j = TH(N, INDEX):FFT_SIZE/2,
         Map(j) = N;
      end
      % All the other (from table)
      for i = 2:N-1,
         for j = TH(i, INDEX):TH(i+1, INDEX) - 1,
            Map(j) = i;
         end
      end
      
      % An offset depending on the overall bit rate is used for the absolute
      % threshold. This offset is -12dB for bit rates >=- 96kbits/s and 0dB
      % for bit rates < 96 kbits/s per channel. [1, pp. 111]
      if (bitrate >= 96)
         for i = 1:N,
            TH(i, ATH) = TH(i, ATH) - 12;
         end
      end
      
      LTq = TH(:, ATH);
   else
      error('Frequency not supported.');
   end
else
   error('Layer not supported.');
end