  Conditional averages are used to evaluate the effect of sweeps and ejections
on amplitude modulation. This is done numerically with a direct numerical
simulation (DNS) of a channel flow at friction Reynolds number $Re_{\tau} =
1000$ in a minimal stream-wise unit (MSU). The amplitude-modulation map of such
DNS is also compared to the one of a regular channel flow in a longer
streamwise domain (LSD), in order to assess its validity for this study. The
cheaper MSU is found to provide a good representation of the modulation
phenomena in the LSD. As for conditional averages, the amplitude-modulation
coefficient is conditioned on the sign of the large-scale fluctuations. Care
must be exerted in defining such a coefficient, as the conditioned large-scale
fluctuation has non-zero average, indeed as a consequence of conditioning. Both
sweeps and ejections (positive and negative large-scale fluctuation events) are
found to have a positive contribution to amplitude modulation in the buffer
layer, and a negative one in the outer layer. The negative-modulation region is
found to shrink in case of ejections, so that the positive-modulation region
extends farther away from the wall. Two more conditional statistics are used to
provide an alternative representation of amplitude modulation and insights into
the characteristics of the large-scale structures.
