Reconstruction filter is used when you need to remove higher frequencies in the digital signal in order to retrieve your baseband signal and it is usually a LPF. Sometimes reconstruction filter is also called a smoothing circuit or smoothing filter.
It is unlike an anti-aliasing which is used to remove unwanted signals or suppress energies in high frequencies NOT of interest before further processing.
1)Do we need to calculate the THD before smoothing filter or after it?
2)If the answer is after the filtering, I'm confused the distorted signal
can be recovered by filter?