Just as an appendix to A-S' great explanation of data aquisition. This is cribbed from the Loral Fairchild 2100 Manual:
Quote:
(2) Record State
Once the FA2100 Solid State Flight Data Recorder has been initialized, recording will commence with the first data forwarded to the SMP from the FDP. The FDP is responsible for all of the processing needed to format the received FDAU bit stream into a form ready for storage into the CSMU. The process starts with bit synchronization and ends with TDM bus transmission of formatted data to the SMP. The entire FDP portion of this sequential process is best described by the steps below:
(a) The incoming FDAU bit stream is sampled by the hardware and sent to
the bit sync software module. Once the bit sync has locked to the incoming
bit rate (equivalent to 64, 128, 256, 512, or 1024 WPS), the individual
bits are passed to the bit packer module. The synchronization process
continues for every bit received.
(b) The bit packer accumulates the data stream into 12 bit words (the basic
unit of the flight data stream, and passes these words along to the frame
sync module. Once frame sync is achieved, the bit packer will align the 12
bit flight data words to the actual word boundary of the frame structured
flight data. Refer to Figure 4 for an illustration of a 64wps frame structure.
(c) The frame sync module searches the 12 bit flight data word stream for the
frame and subframe markers which define the frame structure of the data.
After one full frame of markers has been verified, frame sync is declared.
If frame sync is not detected within ten seconds, a fault is reported. Any
unsynchronized data continues to be passed through, uncompressed, to
the SMP for storage in the CSMU.
(d) Frame synchronized data is passed to the delta compressor module.
Initially, a baseline frame is sent to the SMP for storage. The data words
from subsequent frames of data are compared to the words in the previous
frame to determine the delta, or difference, between sequential
frames of data. Compression is accomplished by storing only the encoded
delta values from frame to frame.
(e) The delta values are encoded using the Huffman encoder module. This is
an algorithmic method of converting the delta values to a minimum number
of bits that can be packed together for efficient memory utilization.
(f) The compressed data is then processed by the page builder module. This
module counts packed data words in order to track 64-word “page” units.
(g) Finally, the Hamming encoder module adds an error correction code for
each page of data. The error correction code enables errors in the stored
data stream to be detected following data retrieval. Each word is queued
for transfer to the SMP using the TDM bus as it is completed.
(h) Data words are received off the TDM bus by the FDR Executive task
running on the SMP. As the data is received, it is organized for storage in
flash memory, then is sent to the Flash Manager task for storage in the
CSMU. A read-after-write operation is performed on each word written to
CSMU memory to validate storage.
(i) The FDR Executive task also manages the erase look-ahead function to
insure that there is always a new erased block of flash memory available
for impending memory write operations.