function get_sp( num_std )

if (~exist('z_data'))
    load('z_data');
end

if (~exist('num_std'))
    num_std = 5;
end

sp_T = 1/20e3;
sp_t = 0 : sp_T : 10-sp_T;

for channel = 1:64
    z = z_data(:,channel);
    threshold(channel) = num_std*std(z);
    erroneous_indices = find( abs( z(1:8) ) > 7*std(z) );
    z( erroneous_indices ) = z( erroneous_indices ) / 7;
    z_data(:,channel) = z;

    s4std_indices = find( abs(z) > threshold(channel) );
    s4std = sp_T * s4std_indices - sp_T;          % save these peak times in s4std
    
    k=0; m=0; mi=0;
    sp=[];
    for i=1:length(s4std)
       if (s4std(i)-k <= 0.4e-3)        % if we haven't jumped past a 0.4ms vicinity in this step
         if (z(s4std_indices) < m)
           m=z(s4std_indices);          % accumulates local minimum z(t)
           mi=s4std(i);                 % gets actual time of minimum
         end
       else
         sp(end+1)=mi;                  % saves the min peak time in sp
         m=z(s4std_indices);            % load in a new minimum amplitude
         mi=s4std(i);                   % and a new minimum time
       end
       k=s4std(i);                      % store the previous time in k
    end
    sp(end+1)=mi;
    sp=sp(2:end);                       % ignore the first point
    sp_data{channel} = sp;
    disp( sprintf( 'Channel %d done.', channel ) );
    
end

save( sprintf( 'sp%d_data_ext', num_std ), 'sp_data', 'z_data', 'threshold', 'sp_t', 'sp_T', 'num_std' );
save( sprintf( 'sp%d_data', num_std ), 'sp_data', 'num_std' );
return;