preprocess.md

@@ -5,6 +5,7 @@ Code preprocess_refact.py, git revision a2a50b7b
 - Cut the trace(s) between 00:00:00.000000 and 23:59:59.995000
 - Fill eventual gaps with zeroes
 - Butterworth Bandpass Filter between 0.01 and 30.0 Hz, order 4
+- 1-bit filtering
 - write h5 datasets, as :
 
 ```