R/highPass.R

In FIACH: Retrospective Noise Control for fMRI

highPass <-function(x,freq,tr){
  x<-as.matrix(x)
  N<-nrow(x)
  K<-round(2*(N*tr/(freq + 1)))
  n <- t(0:(N-1))
  C <- matrix(0,N,K)
  if(ncol(C)<2){return(x)}
  
  C[,1]=1/sqrt(N)
  
  for (i in 2:K){C[,i] = sqrt(2/N)*cos(pi*(2*n+1)*(i-1)/(2*N))}
  basis<-C[,2:ncol(C)]
  
  beta<-crossprod(basis,x)
  resid<- x-basis%*%beta
  return(resid)
}