  We implement and investigate a method for measuring departures from
scale-invariance, both scale-dependent as well as scale-free, in the primordial
power spectrum of density perturbations using cosmic microwave background (CMB)
C_l data and a principal component analysis technique. The primordial power
spectrum is decomposed into a dominant scale-invariant Gaussian adiabatic
component plus a series of orthonormal modes whose detailed form only depends
the noise model for a particular CMB experiment. However, in general these
modes are localised across wavenumbers with 0.01 < k < 0.2 Mpc^-1, displaying
rapid oscillations on scales corresponding the acoustic peaks where the
sensitivity to primordial power spectrum is greatest. The performance of this
method is assessed using simulated data for the Planck satellite, and the full
cosmological plus power spectrum parameter space is integrated out using Markov
Chain Monte Carlo. As a proof of concept we apply this data compression
technique to the current CMB data from WMAP, ACBAR, CBI and VSA. We find no
evidence for the breaking of scale-invariance from measurements of four PCA
mode amplitudes, which is translated to a constraint on the scalar spectral
index n_S(k_0=0.04 Mpc^-1)=0.94+-0.04 in accordance with WMAP studies.
