Function measure_alias 

pub fn measure_alias(
    base: &[f64],
    high: &[f64],
    oversample: usize,
    sample_rate: f64,
    start_secs: f64,
) -> Alias

The alias is what the base band holds and the oversamplex render does not. One bin grid for both, so no decimation filter blurs the top octave this is measuring.