  The amplitude of displacive excitation of coherent phonons ({\it DECP}) in
$YBa_2Cu_3O_7$ observed in time-resolved spectroscopy increases by a very large
factor of 15 between 20 K (below $T_c$) and 100 K (above $T_c$). Microscopic
modelling of phonon excitation reveals that these amplitude changes of the {\it
$A_{1g}$} mode phonons are related to the existence of the {\it Van-Hove}
singularity (VHS) below but very close to Fermi energy. PACS numbers: 78.47.+p,
74.25.Gz, 74.25.Jb, 71.15.Fv
