sonar_anal(fig)

function sonar_anal(fig)
% SONAR_ANAL is a callback for the sonar demo (tof.m).

% Copyright 2001-2010 The MathWorks, Inc.

% user data
ud = get(fig,'userdata');
ai = ud.ai;
ao = ud.ao;
h = ud.h;

% measurement
y = getdata(ai);             % get response when done

x = ud.x;                    % excitation chirp
N = ai.samplespertrigger;    % record length
Fs = ai.samplerate;          % samples/sec (Hz)
c = ud.c;                    % wave speed (m/sec)

% analysis
X = fft(x);         % forward transforms
Y = fft(y);                     
Z = conj(X).*Y;     % freq-domain equiv xcorr (circ)

% Hilbert transform => envelope
pos = 2:N/2;                    % double up positive frequencies
Z(pos) = 2*Z(pos);
neg = (N/2+1):N;                % zero out negative frequencies
Z(neg) = 0;                

z = fftshift(abs(ifft(Z)));     % inverse transform

i = (1:N)';                     % sample index
t = (i-1-N/2)/Fs;               % time index (sec)

[pk,loc] = max(z);              % height & location of peak
if length(loc)>1                % tie breaker
  if max(diff(loc))==1          % fence strattler
    lag = mean(t(loc));         % split the difference
  else                          % real competition
    lag = t(loc(1));            % early bird gets the worm 
  end
else                            % no contest
  lag = t(loc);                 % time delay (sec)
end
d = lag*c;                      % distance (m)

% display results
plot(t*c,z/pk), axis([0 25 0 1])
hold on, plot(d,mean(z(loc))/pk), hold off
set(h.text1,'string',sprintf('Distance = %.1f m',d))

% repeat
if strcmp(get(h.Stop,'Enable'),'on')    % repeating measurements
  putdata(ao,x)                         % preload output buffer
  start([ai ao])                        % do it again!
else                                    % single measurement
  set(h.Repeat,'enable','on')           % enable Repeat button
end