  The probing of coherent lattice vibrations in solids has been conventionally
carried out using time-resolved transient spectroscopy where only the relative
oscillation amplitude can be obtained. Using time-resolved X-ray techniques,
absolute electron-phonon coupling strength could be extracted. However, the
complexity of such an experiment renders it impossible to be carried out in
conventional laboratories. Here we demonstrate that the electron-phonon,
anharmonic phonon-phonon coupling, and their relaxation dynamics can be probed
in real-time using high-harmonic spectroscopy. Our technique is background-free
and has extreme sensitivity directly in the energy domain. In combination with
the optical deformation potential calculated from density functional
perturbation theory and the absolute energy modulation depth, our measurement
reveals the maximum displacement of neighboring oxygen atoms in {\alpha}-quartz
crystal to tens of picometers in real space. By employing a straightforward and
robust time-windowed Gabor analysis for the phonon-modulated high-harmonic
spectrum, we successfully observe channel-resolved four-phonon scattering
processes in such highly nonlinear interactions. Our work opens a new realm for
accurate measurement of coherent phonons and their scattering dynamics, which
allows for potential benchmarking ab-initio calculations in solids
