*** note1 ***
The cadence of course depends on the data recording rate. These values (128 s for FFIs and 32 s for 2x2 PFIs) are those for the high recording rate (32Kbit/s). The cadence is 8 times as long for the medium recording rate (4Kbit/s), which happens when the interval of two downlinks is too long for the data recorder to keep all the data in the high rate. Usually the telemetry allocation for FFIs is four times as much as that for PFIs.

*** note2 ***
Of cource, the cadence will be longer if we take too long exposure images, say 30s.