Table 1
Properties of travel time maps (center-to-annulus geometry, outward minus inward travel times) computed from compressed data.
| Compression method | Parameter | Correlation | S/N at kR⊙ = 122 | S/N at kR⊙ = 221 | File size [bits/pixel] |
|
|
|||||
| no compression | 1 | 40 ± 2 | 8.0 ± 0.3 | 32 | |
| quantization | n = 2 | 0.965 | 35.1 ± 0.8 | 6.5 ± 0.1 | 0.71 |
| n = 4 | 0.975 | 36.8 ± 0.7 | 6.81 ± 0.09 | 0.72 | |
| n = 8 | 0.998 | 39.8 ± 0.2 | 7.84 ± 0.03 | 0.92 | |
| JPEG compression | q = 5 | 0.957 | 31 ± 1 | 5.8 ± 0.2 | 0.23 |
| q = 10 | 0.991 | 38.2 ± 0.4 | 7.50 ± 0.08 | 0.43 | |
| q = 20 | 0.998 | 39.2 ± 0.3 | 7.91 ± 0.03 | 0.71 | |
| 2 × 2 subsampling and | n = 2 | 0.948 | 32 ± 1 | 5.9 ± 0.2 | 0.23 |
| quantization | n = 4 | 0.960 | 34 ± 1 | 6.2 ± 0.2 | 0.24 |
| n = 8 | 0.989 | 37.9 ± 0.6 | 7.36 ± 0.09 | 0.30 | |
| 2 × 2 subsampling and | q = 5 | 0.612 | 7 ± 2 | 0.8 ± 0.3 | 0.06 |
| JPEG compression | q = 10 | 0.927 | 27 ± 2 | 4.6 ± 0.2 | 0.12 |
| q = 15 | 0.972 | 34.2 ± 0.9 | 6.0 ± 0.2 | 0.17 | |
Notes. We evaluate the quality of the compression for selected compression efficiencies (number of velocity bins n or quality factor q) by computing the correlation of the travel times with the travel times computed from uncompressed data, the signal-to-noise ratio (S/N) of the travel times for two wavenumbers (kR⊙ = 122 and kR⊙ = 221) and the file size of the Dopplergrams in bits/pixel (relative to the data with full spatial resolution) after applying the compression. The 2 × 2 subsampled data were interpolated back to the original resolution using Fourier interpolation for computing the correlations. Before determining the S/N, we averaged the power of the travel times from kR⊙ = 98−147 and kR⊙ = 196−245, respectively. Our measurement of the S/N for the uncompressed case has an uncertainty of about 2. The other error estimates are for differential S/N measurements relative to the uncompressed case.
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