The following experiments were designed to test the hypothesis that SETIEasy can detect at least some of the weak frequency-drifting signals in the 8-bit reverse linear-drift test set. The results are:
These results are essentially the same as recorded for the data with a +9.8 Hz/min frequency drift. Most notably, the same positive dechirp values give the biggest peaks in the number of signals, regardless of whether the frequency drift is positive or negative. It seems likely that artifact signals are generated during the dechirp routine. However, these artifacts do not seem to greatly obscure the injected small signal. The detection limit is a signal at 1.25% of the p-p noise level, as was observed using CoolEdit on signals with a small nonlinear drift. Does this mean that no set of frequency dechirp runs is necessary to optimize S/N? Probably not. It is disconcerting that SETIEasy v1.31a reports so many different signals between identical copies of the same file (e.g., 7rev vs. 7rev). The latest alpha version of SETIEasy, v1.33a, does not seem to have this problem, but the complete set of linear_drift and reverse files needs to be analyzed again with v1.33 to be certain.
Description of 8-Bit Reverse Linear Drift Test Files
The reverse linear drift files are a set of seven 15-minute noise files containing a weak, narrow signal with linear frequency drift. There is about 43 seconds of noise at the beginning, and then a signal in the last 14 minutes and 17 seconds of each file. The signal starts at 1540.8 Hz and ends at 1400 Hz, and a has a drift of a bit over -9.8 Hz/min. The first file has a signal that is small but clearly visible in CoolEdit. [A signal can be seen in linear_drift_2.wav in CoolEdit only if the whole signal is Scanned in the Frequency Analysis box after the proper dechirp (Stretch) is applied.] The intensity is then cut in half repeatedly until well after it completely vanishes. The signal level is given as a % of the peak noise.
5.00% http://monroe.pharm.uky.edu/seti/linear_drift_1rev.wav
2.50% http://monroe.pharm.uky.edu/seti/linear_drift_2rev.wav
1.25% http://monroe.pharm.uky.edu/seti/linear_drift_3rev.wav
0.63% http://monroe.pharm.uky.edu/seti/linear_drift_4rev.wav
0.32% http://monroe.pharm.uky.edu/seti/linear_drift_5rev.wav
0.16% http://monroe.pharm.uky.edu/seti/linear_drift_6rev.wav
0.08% http://monroe.pharm.uky.edu/seti/linear_drift_7rev.wav
The optimum dechirp factor in SETIEasy for these files should be about DC = -.042463
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