% ALTCODE - returns a strong alternating code set of specified length
%   one scan per row.  For input not a power of 2, a truncated set of the
%   next power of 2 is returned.
% The code sets are taken from [Nygrén, 1996, table 6.2 and 6.3]
%   and from inspection of the tau0 code
%
%   See also WTT, TAU0CODE, AC_LP_WTT, AC_GATE_WTT, AC_LPH_WTT,
%            BCAC_LP_WTT, BCAC_GATE_WTT.

% The 16-bit code marked tau0 below is what is used in the tau0 experiment,
%  randomised with the vector 45F1, or
%   [ 1 -1  1  1  1 -1  1 -1 -1 -1 -1 -1  1  1  1 -1 ]

%   $Id: altcode.m,v 1.3 2008-11-18 04:57:14 tom Exp $

%   Copyright © 2003-2008 Tom Grydeland <Tom.Grydeland@phys.uit.no>
%   This is free software, licensed under GNU GPL version 2 or later

function codeset = altcode(bauds)

w32  = walsh(32);
if bauds <= 4,		% 4-bit strong alternating code
    codeset  = w32(1:8, [1 2 3 5]);
elseif bauds <= 8,
    codeset  = w32(1:16, [1 2 3 5 9  4 8 15]);
    codeset  = w32(1:16, [1 2 3 5 9 13 6 12]);
elseif bauds <= 16,
    codeset = w32(:, [1 2 3 5 9 17 16 32 18 13 26 30 21  7 14 27]);
    codeset = w32(:, [1 2 3 5 9 17 31  4  8 15 29  7 14 27 11 22]);	% tau0
elseif bauds <= 32,
    w = walsh(64);
    codeset = w(:, [ 1  2  3  5  9 17 33  7 14 27 53 47 28 56 41 24 ...
		    48 25 50 37 16 32 63 59 52 34  6 12 23 45 31 62]);
elseif bauds <= 64,
    w = walsh(128);
    codeset = ...
	w(:,[ 1   2   3   5   9  17  33  65  96  97  31  62 123  44  88 114 ...
	     61 122  46  92 106  13  26  51 101  24  48  95 100  25  50  99 ...
	     28  56 111   4   8  15  29  57 113  64 128  34  68  90 109   7 ...
	     14  27  53 105  16  32  63 125  40  80  66  93 103  19  38  75]);
else
    error(sprintf('Codeset %d too large', baud));
end

codeset = codeset(:, 1:bauds);