Originally Posted by CONF iture
the DFDR has a very accurate elapsed time-base
That may be so (and probably that makes the frame count an accurate measure of elapsed time), but apparently this DFDR records GMT (or UTC) obtained from an aircraft system clock. Large aircraft in the public transport category are equipped with flight data acquisition units (FDAUs). (*). The FDAU (or equivalent) orders data and sends them to the DFDR, whose function is limited to data recording.
Data acquisition systems output a binary file sequenced in four-second frames.
Each frame is divided into four one-second-subframes.
Each subframe is divided into 64, 128, 256 or 512 “words” of 12 bits each, depending on the FDR’s technology.
The bit is the basic binary unit whose value is either 0 or 1.
A 12 bit word can have values from 0 to 4095. The time parameter is recorded in the first 'word' of a data frame.
That 12 bit word can contain the 24*60=1440 minutes in a 24 hour day, but 24*60*60=86400 seconds requires at least 5 bits more.
According to the Habsheim accident report the DFDR recorded 200 parameters, including 141 'discretes' (yes/no parameters). The attachment shows that this number of parameters required reducing the recording frequency of many parameters to once every 4 seconds. Wouldn't it be a waste of valuable capacity to record GMT with a resolution of one second when 1 minute is sufficient because the seconds can be derived from the frame count?
(*) This and other quotes are from a BEA report "Flight Data Recorder Read-Out - Technical and Regulatory Aspects". The quotes are edited for continuity.
P.S.
BTW, subtracting 141 discretes from 200 recorded leaves 59 parameters. Counting the parameters in Tome 1 through 6 that readout must be pretty complete for the number of parameters.