Azimuthally averaged power of travel times (center-to-annulus geometry, outward minus inward travel times) derived from observations (blue curve) and a noise model (red curve). Top left: uncompressed data, top right: quantization (two velocity bins), bottom left: JPEG compression (quality = 5), bottom right: 2 × 2 subsampling combined with JPEG compression with a quality of 10. The power shown is an average from twenty time-series, each of them having a length of eight hours. The shaded areas show the 1σ scatter of the individual realizations. The compression slightly decreases the signal and increases the noise. The vertical arrows denote the wavenumbers used for determining the S/N in Fig. 6 and Table 1.