  Transient responses are an inherent property of recursive filters due to
unknown or incorrectly selected initial conditions. Well-designed stable
filters are less affected by transient responses, as the impact of initial
conditions diminishes over time. However, applications that require very short
data acquisition periods (for example, as short as ten seconds), such as
biosignals recorded and processed by wearable technologies, can be
significantly impacted by transient effects. But how feasible is it to design
filters without transient responses?
  We propose a well-known filter design scheme based on constrained least
squares (CLS) optimization to create zero-transient effect notch filters for
powerline noise cancellation. We demonstrate that this filter is equivalent to
the optimal Wiener smoother in the stationary case. We also discuss its
limitations in removing powerline noise with nonstationary amplitude, where a
Kalman filter-based formulation can be used instead.
