Make sure that noise window does not end at the P arrival time

But only in case when the noise window length is not specified.
This ensures backward compatibility with the old behavior.

sourcespec/config_files/configspec.conf

@@ -127,7 +127,8 @@ p_arrival_tolerance = float(min=0, default=4.0)
 s_arrival_tolerance = float(min=0, default=4.0)
 
 # Start time (in seconds) of the noise window, respect to the P window
-# If None, it will be set to the length of the signal (P or S) window
+# If None, it will be set to the length of the signal (P or S) window plus
+# the value of "signal_pre_time" (see below)
 noise_pre_time = float(min=0.01, default=6.0)
 
 # Start time (in seconds) of the signal window, respect to the P or S arrival

sourcespec/ssp_process_traces.py

@@ -318,11 +318,13 @@ def _define_signal_and_noise_windows(config, trace):
         win_length = win_length_s
     if config.variable_win_length_factor:
         logger.info(f'{trace.id}: window length {win_length:.3f} seconds')
-    # use win_length if noise_pre_time is None
-    noise_pre_time = config.noise_pre_time or win_length
+    if config.noise_pre_time is None:
+        noise_pre_time = win_length + config.signal_pre_time
+    else:
+        noise_pre_time = config.noise_pre_time
     t1 = max(
         trace.stats.starttime,
-        p_arrival_time - noise_pre_time - config.signal_pre_time
+        p_arrival_time - noise_pre_time
     )
     t2 = t1 + win_length
     if t2 > (p_arrival_time - config.signal_pre_time):