1. A device for determining spectral samples of a finite length sequence at non-uniformly spaced frequency values, the device comprising: a phase rotation set generator configured to generate sample-specific phase-value sets, each phase-value set associated with a respective time index of a sample from the finite length sequence and a set of predetermined frequencies; an iso-phasor mapper configured to map each of the sample-specific phase-value set to a corresponding sample-specific iso-phasor set; a multiplier configured to calculate an altered iso-phasor set from each sample-specific iso-phasor set and its corresponding sample; an unmapper configured to generate a rotated sample value set by unmapping the altered iso-phasor set to obtain real and imaginary components of the sample rotated by the corresponding sample-specific phase value set; and a summer configured to sum selected rotated sample values from each of the rotated sample value sets to generate a spectral sample vector.