#windowed sinc low pass filter
#y - vector to filter
#t - time interval between measurements (s)
#f - low pass frequency (Hz)
wlpf <- function( y, t, f ) {
    #sinc function of frequency f
    sinc <- function( x, f )
        ifelse(x==0, 2*pi*f, sin(2*pi*f*x)/x)
    #Blackman window from 0..m
    Blackman <- function( m )
        0.42-0.5*cos(2*pi*(0:m)/m)+0.08*cos(4*pi*(0:m)/m)
    m  <- min(floor(length(y)/2), 500)
    rk <- sinc(-m:m, f*t)  
    bk <- Blackman(2*m) * rk
    k  <- c(bk, rep(0,length(y)-length(bk)))
    fy  <- fft(fft(k)*fft(y), inverse=TRUE)
    return(Re(fy))
}

ecg  <- scan("http://biostatmatt.com/csv/ecg.csv")
ecg  <- ( ecg - mean(ecg) ) / sd(ecg)