Page 225 - Electrical Engineering Dictionary

 
0.3536 0.3536 0.3536 0.3536 0.3536 0.3536 0.3536 0.3536
 0.4904 0.4157 0.2778 0.0975 −0.0975 −0.2778 −0.4157 −0.4904 
 
 0.4619 0.1913 −0.1913 −0.4619 −0.4619 −0.1913 0.1913 0.4619 
 
 0.4517 −0.0975 −0.4904 −0.2778 0.2778 0.4904 0.0975 −0.4517 
 
 0.3536 −0.3536 −0.3536 0.3536 0.3536 −0.3536 −0.3536 0.3536 
 
 0.2778 −0.4904 0.0975 0.4157 −0.4157 −0.0975 0.4904 −0.2778 
 
 0.1913 −0.4619 0.4619 −0.1913 −0.1913 0.4619 −0.4619 0.1913 
0.0975 −0.2778 0.4157 −0.4904 0.4904 −0.4157 0.2778 −0.0975
Discrete cosine transform (DCT).
discrete time signal a signal represented in order to position the read/write head(s)
by samples at discrete moments of time (usu- over the correct disk track.
ally regularly spaced). The samples may take
values from a continuous range, so the term disk arm a mechanical assembly that po-
is usually used to differentiate a sampled ana- sitions the head over the correct track for
log signal from a digital signal which is quan- reading or writing a disk device. The arm
tized. See signal. is not movable on a fixed-head disk, but is on
a moving-head disk.
discrete time system a process that trans-
forms discrete time input signals to discrete disk array a number of disks grouped
time output signals. together, acting together as a single logical
disk. By this, multiple I/O requests can be
discrete time white Gaussian noise noise serviced in parallel, or that the bandwidth of
samples modeled as independent and identi- severaldiskscanbeharnessedtogethertoser-
cally distributed Gaussian random variables. vice a single logical I/O request.

discrete Walsh-Hadamard transform disk cache a buffer memory area in main
See Hadamard transform. memory used to hold blocks of data from disk
storage. The cache can hide much of the ro-
discrete wavelet transform (DWT) a tational and seek latencies in disk accesses
computation procedure that calculates the co- because a complete data block (disk sector)
efficients of the wavelet series expansion for is read or written together. Disk caches are
a given finite discrete signal. normally managed by the machine’s operat-
ing system, unlike a processor cache, which
discriminator acircuitwhoseoutputvolt- is managed by hardware.
age varies in magnitude and polarity in direct
proportion to the difference between the in- disk capacitor a small single-layer ce-
put voltage and a standard signal. A discrim- ramic capacitor with a dielectric insulator
inator that converts phase deviations at the consisting of conductively silvered opposing
input to a linearly proportionate variation in surfaces.
output voltage is called a phase discriminator
and is used in FM detection. disk controller unit that carries out the
actions required for the proper operations of
disk See magnetic disk. Also spelled a disk unit.
“disc.”
disk drive assembly consisting of elec-
disk actuator a mechanical device that tronics and mechanical components, to con-
moves the disk arms over the disk surface(s) trol disk and disk-head movement and to ex-

c
2000 by CRC Press LLC