Cockroft–Walton circuit
a cascading
voltage multiplier invented in 1932 by John
Cockroft and Ernest Walton.
code
(1) a technique for representing in-
formation in a form suitable for storage or
transmission.
(2) a mapping from a set of messages into
binary strings.
code acquisition
the process of ini-
tial code synchronization (delay estimation)
between the transmitter and receiver in a
spread-spectrum system before the actual
data transmission starts. It usually requires
the transmission of a known sequence. See
also
code tracking
.
code cache
a cache that only holds in-
structions of a program (not data).
Code
caches generally do not need a write policy,
but see self-modifying code. Also called an
instruction cache. See also
cache
.
code combining
an error control code
technique in which several independently re-
ceived estimates of the same codeword are
combined with the codeword to form a new
codeword of a lower rate code, thus providing
more powerful error correcting capabilities.
This is used in some retransmission protocols
to increase throughput efficiency.
code converter
a device for changing
codes from one form to another.
code division multiple access (CDMA)
a technique for providing multiple access to
common channel resources in a communica-
tion system. CDMA is based on spread spec-
trum techniques where all users share all the
channel resources. Multiple users are distin-
guished by assigning unique spreading codes
to each user. Traditionally, individual detec-
tion is accomplished at the receiver through
correlation or matched filtering.
code efficiency
the unitless ratio of the av-
erage amount of information per source sym-
bol to the code rate, where the amount of in-
formation is determined in accordance with
Shannon’s definition of entropy. It is a funda-
mental measure of performance of a coding
algorithm.
code excited linear prediction (CELP)
a
class of linear predictive speech coding meth-
ods where the excitation is composed of sam-
ple vectors from VQ codebooks.
code hopping
the use of a new spread-
ing code for each transmitted bit in a spread-
spectrum system. Compare with
frequency
hopping
.
code letter
See
NEMA code letter
.
code rate
in forward error control and line
codes, the ratio of the source word length to
the code word length, which is the average
number of coded symbols used to represent
each source symbol.
code segment
area in a process’ virtual
address space used to contain the program’s
instructions.
code tracking
the process of continu-
ously keeping the code sequences in the re-
ceiver and transmitter in a spread-spectrum
system synchronized during data transmis-
sion. See also
code acquisition
.
code V
a widely employed computer code
for design of optical systems by Optical Re-
search Associates.
codebook
a set of codevectors (or code-
words) that represent the centroids of a given
pattern probability distribution.
See also
vector quantization
.
codebook design
a fundamental prob-
lem in vector quantization (VQ). The main
question addressed by codebook design is
how the codebook should be structured to
allow for efficient searching and good per-
formance. Several methods (tree-structured,
c
2000 by CRC Press LLC