We make a direct and model-independent measurement of the low pi+ pi- mass phase motion in the D+ <FONT FACE=Symbol>® p</FONT>-pi+pi+ decay. Our preliminary results show a strong phase variation, compatible with the isoscalar sigma(500) meson. This result confirms our previous result [1] where we found evidence for the existence of this scalar particle using full Dalitz-plot analysis. We apply the Amplitude Difference (AD) method [2] to the same Fermilab E791 data sample used in the preceding analysis. We also give an example of how we extract the phase motion of the scalar amplitude, looking at the f0(980) in Ds+ <FONT FACE=Symbol>® p</FONT>-pi+pi+ decay.
