improved error message when downsampling fails

src/ifc.py

+import sys
+import logging
 import numpy as num
 from pyrocko.guts import Object, String, Float
-from pyrocko import trace, autopick
+from pyrocko import trace, autopick, util
 from lassie import shifter, common
 
+logger = logging.getLogger('lassie.ifc')
+
 guts_prefix = 'lassie'
 
 
@@ -46,7 +50,7 @@ class WavePacketIFC(IFC):
         if not trs:
             return []
 
-        deltat_cf = trs[0].deltat
+        deltat_cf = max(tr.deltat for tr in trs)
         while deltat_cf * 2 < 0.25 / self.fmin:
             deltat_cf *= 2
 
@@ -63,7 +67,14 @@ class WavePacketIFC(IFC):
             tr.set_ydata(num.maximum(tr.ydata, 0.0))
             tr.shift(-(n/2.*tr.deltat))
 
-            tr.downsample_to(deltat_cf, snap=True, demean=False)
+            try:
+                tr.downsample_to(deltat_cf, snap=True, demean=False)
+            except util.UnavailableDecimation as e:
+                logger.fatal('%s, in_deltat = %g, out_deltat = %g' % (
+                    e, tr.deltat, deltat_cf))
+
+                sys.exit(1)
+
             trs_filt.append(tr)
 
         trs_by_nsl = common.grouped_by(trs_filt, lambda tr: tr.nslc_id[:3])