multiple trend rate
“barrels”
Height is number of traces being saved
Trending Data Through Video 49
Archiving allows the history to be moved onto a removable media whether it is mag- netic tape, f loppy disk, optical disk, or solid state. Generally, when the historical infor- mation is archived, a record is maintained indicating which archived disk volume the information is kept. This may be an automatic and/or a manual process. Nevertheless, it provides a technique for retrieving data that could be months or even years old without having to memorize where everything is located.
The data can then be placed on a removable medium like a f loppy disk or an optical disk. Archive data is usually defined as data that is stored on a removable media.
In some systems, a library of the disk titles is kept in a station. At some later time the user may invoke a trend view that includes history that goes further back than what’s resident in the station. The station goes into its memory and notifies the user which disk volume number to locate and insert into the station. The station will then be able to blend this additional data into its own resident memory for display in that trend. Trended, historical, and archived data do not carry the same degree of urgency as alarmed and operating data. The movement of historical data from distant stations will carry a much lower priority in the data management of the “process operating net- work.” Historical and archiving activities for report generation and management reviews are likely to be operated over a separate subnetwork, connected to the opera- tor stations but operating independently of their primary role.
There is a whole field of data compression which is beyond the scope of this text. Nevertheless, when talking to vendors, or their engineers, you may encounter some terms and acronyms that are equally mysterious, but should not intimidate you. Some of them I list here, but I wish I knew even more about this myself (such as what the “SS” stands for in “LZSS” below):
Figure 13-8. Archiving Data Allows It to Be Saved Off Line
ARCHIVING can be:
!
“Poured” off “drum” before lost(painted over)
!
Series of “snapshots” sometime after saved ...but before lost...to be placed upon removable
media (floppy disk, optical disk, etc.)
asymmetrical compression—Data compression system that requires more process- ing capability to compress an image than to decompress it; typically used for mass dis- tribution of programs on media such as CD-ROMs.
CGM(IF) [Computer Graphics Metafile (Interchange Format)]—A standard for archiving and transferring graphics data.
compaction—See compression.
compression—Any of several techniques used to reduce the number of bits required to represent information in the storage or transmission of digital data (saving memory or bandwidth), in which the original form of the information can be reconstructed. Also called compaction; see asymmetrical, delta save, JPEG, MPEG, symmetrical, compression.
DCT (Discrete Cosine Transform)—A data compression technique.
EDM (Engineering Data Management)—Controls access to online and archived engi- neering data; prevents unauthorized users from getting information and erroneously making changes that others don’t know about.
JPEG (Joint Photographic Experts Group)—Working committee under the auspices of the ISO that is attempting to define a proposed universal standard for the digital compression and decompression of still images for use in computer systems and for image exchange between computers. The JPEG algorithm reduces image size by as much as 65:1 while maintaining image integrity by eliminating imperceptible color information.
lossless—Digital data technique that reduces the size of a file without sacrificing any of original data; this tool allows an expanded or restored file to be an exact replica of the original file before compression.
lossy compression—Digital data compression technique in which some data is delib- erately discarded so as to achieve massive reductions in the size of the compressed file.
L-Z algorithm—Lossless data compression technique developed by two researchers named Lempel and Ziv.
LZH Compression (Lempel-Ziv-Huffman)—A method of data compression that can reconstruct data exactly like the original with no loss.
LZSS—A refinement of the L-Z algorithm for data compression that can reconstruct data exactly like the original with no loss.
LZW (Lempel-Ziv & Welch)—Patented by Unisys, another refinement of the L-Z algo- rithm for data compression that can reconstruct data exactly like the original with no loss.
MPEG (Motion Picture Experts Group)—Standards committee under the auspices of the ISO that is working on algorithm standards that will allow the digital compression, storage, and transmission of moving image information such as motion video and CD quality audio and control data at CD-ROM bandwidth. The MPEG algorithm provides interframe compression of video images and can have an effective compression rate of 100:1 to 200:1.
MPEG 2—Proposed standard for ISO adaptation that allows for a higher than normal resolution motion video compression than the MPEG standard.
Trending Data Through Video 51
PLV (Production Level Video)—Video encoding using the oldest digital compression scheme, by Intel/IBM.
RLE (Run Length Encoding)—Data compression technique that saves data by a single count byte and a repeat byte rather than by using memory to save a repetitive group of bytes, for example: 777777 becomes “count of 6 with value of 7 (two bytes).
symmetrical compression—A system that requires equal processing capability for the compression and decompression of an image. This form of compression is used where both compression and decompression is used frequently.