Page 143 - Electrical Engineering Dictionary

Cockroft–Walton circuit a cascading bol to the code rate, where the amount of in-
voltage multiplier invented in 1932 by John formation is determined in accordance with
Cockroft and Ernest Walton. Shannon’s definition of entropy. It is a funda-
mental measure of performance of a coding
code (1) a technique for representing in- algorithm.
formation in a form suitable for storage or
transmission. code excited linear prediction (CELP) a
(2) a mapping from a set of messages into class of linear predictive speech coding meth-
binary strings. ods where the excitation is composed of sam-
ple vectors from VQ codebooks.
code acquisition the process of ini-
tial code synchronization (delay estimation) code hopping the use of a new spread-
between the transmitter and receiver in a ing code for each transmitted bit in a spread-
spread-spectrum system before the actual spectrum system. Compare with frequency
data transmission starts. It usually requires hopping.
the transmission of a known sequence. See
also code tracking. code letter See NEMA code letter.

code cache a cache that only holds in- code rate in forward error control and line
structions of a program (not data). Code codes, the ratio of the source word length to
caches generally do not need a write policy, the code word length, which is the average
but see self-modifying code. Also called an number of coded symbols used to represent
instruction cache. See also cache. each source symbol.

code combining an error control code code segment area in a process’ virtual
technique in which several independently re- address space used to contain the program’s
ceived estimates of the same codeword are instructions.
combined with the codeword to form a new
codeword of a lower rate code, thus providing code tracking the process of continu-
more powerful error correcting capabilities. ously keeping the code sequences in the re-
This is used in some retransmission protocols ceiver and transmitter in a spread-spectrum
to increase throughput efficiency. system synchronized during data transmis-
sion. See also code acquisition.
code converter a device for changing
codes from one form to another. code V a widely employed computer code
for design of optical systems by Optical Re-
code division multiple access (CDMA) search Associates.
a technique for providing multiple access to
common channel resources in a communica- codebook a set of codevectors (or code-
tion system. CDMA is based on spread spec- words) that represent the centroids of a given
trum techniques where all users share all the pattern probability distribution. See also
channel resources. Multiple users are distin- vector quantization.
guished by assigning unique spreading codes
to each user. Traditionally, individual detec- codebook design a fundamental prob-
tion is accomplished at the receiver through lem in vector quantization (VQ). The main
correlation or matched filtering. question addressed by codebook design is
how the codebook should be structured to
codeefficiency theunitlessratiooftheav- allow for efficient searching and good per-
erage amount of information per source sym- formance. Several methods (tree-structured,

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2000 by CRC Press LLC