I noticed that IM4 transmission and MC2 transmission SUM spactra looked totally different. Specifically, it looked as if IM4 transmission had one extra dewhitening filter in digital land (see the plot).
This was tracked down to the modification of the dual QPD transimpedance amp box for HAM2 (D1002481, S1102815) where the unswitchable 0.4:40 whitening filter was disabled:
"02/13/2012 Charles Osthelder Modification Description R1, a 200 ohm resistor, was removed from each channel to negate the whitening filter. The resistors were placed in a poly bag and taped to the inside of the chassis to be reinstalled when necessary."
This modification was not done to other amplifiers (s1102819 for MC2 trans, s1102810 for POP sled). All QPD filters in the digital land had 40:0.4 dewhite enabled.
I removed 40:0.4 from the filter definition for IM4 trans and ISS QPDs.
I wanted to switch one stage of 1:10 switchable whitening for IM4 trans, which worked, but I couldn't enable any of three digital 10:1 dewhites manually (FM1, FM2, FM3 in H1:IMC-TM4_TRANS_SEG1 etc.) even though I can enable/disable Comb60 (FM6).
So for the moment IM4 transmission is left whitening-less.
