Fix PCAL correction EPICS records

This MR is to account for the fact that the DARM signal has also received a delay going from the OMC model to the CALCS model. We had already been correctly accounting for the PCAL channel read from the corner station and propagating it back to the end station, we had not been doing the same for the DARM signal back to the OMC model. We are for now sticking this into the PCAL correction, though it should be fixed more appropriately in CALCS and GDS

pydarm/calcs.py

@@ -882,10 +882,25 @@ class CALCSModel(DARMModel):
 
         # Pcal corrections
         # See T1700106
+        # We are reading the PCAL and DARM signals from the CALCS model, both
+        # of which have an implicit 1 16k clock cycle delay. The
+        # PCAL_LINE*_CORRECTION is being divided into the DARM/PCAL transfer
+        # function (both in CALCS). The compute_pcal_correction uses the
+        # "endstation" boolean to indicate that the PCAL channel read is in the
+        # CALCS model so it has been divided by a delay.
+        # The darm_advance is therefore necessary so that the CALCS DARM signal
+        # is properly treated as though it is coming from the OMC model, which
+        # is where DARM originates.
+        # (DARM / PCAL)_calcs * (1/PCAL_LINE*_CORRECTION) =
+        #     (DARM_omc / PCAL_end)
+        # TODO: the front end and GDS needs to be fixed, and once it is fixed,
+        # then this math can be separate EPICS records and be more clear.
         pcal_correction = self.pcal.compute_pcal_correction(freq, endstation)
-        PCAL_LINE1_CORRECTION = pcal_correction[0]
-        PCAL_LINE2_CORRECTION = pcal_correction[4]
-        PCAL_LINE3_CORRECTION = pcal_correction[5]
+        darm_advance = signal.dfreqresp(
+            digital_delay_filter(-1, 16384), 2*np.pi*freq/16384)[1]
+        PCAL_LINE1_CORRECTION = pcal_correction[0] / darm_advance[0]
+        PCAL_LINE2_CORRECTION = pcal_correction[4] / darm_advance[4]
+        PCAL_LINE3_CORRECTION = pcal_correction[5] / darm_advance[5]
 
         # We divide out the pcal reference arm sign since we know which arm
         # the next two values will be assigned to. This makes the next two