Electrical computers and digital processing systems: support – Synchronization of clock or timing signals – data – or pulses
Reexamination Certificate
1999-09-30
2003-08-05
Lee, Thomas (Department: 2185)
Electrical computers and digital processing systems: support
Synchronization of clock or timing signals, data, or pulses
C713S400000, C713S500000, C713S600000, C714S769000
Reexamination Certificate
active
06604204
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention is related generally to electronic circuits, and more particularly to a circuit and method for recovering synchronization information from a signal. In one embodiment, the circuit signals the beginning of a data stream to a Viterbi detector, and the circuit is separate from the Viterbi detector. In another embodiment, the circuit has a greater noise immunity than prior synchronization circuits, and thus can more accurately recover synchronization information from a read signal having a reduced signal-to-noise ratio (SNR). In yet another embodiment, the circuit can recover the synchronization information in fewer cycles, and thus with fewer pad bits, than prior synchronization circuits.
BACKGROUND OF THE INVENTION
FIG. 1
is a partial block diagram of a conventional disk drive
10
, which includes a magnetic storage disk
12
and a read channel
14
for reading data-synchronization information and encoded data from the disk
12
. The read channel
14
includes a read head
16
for sensing the data-synchronization information and the encoded data stored on the disk
12
and for generating a corresponding read signal. A clock circuit
18
recovers a clock from the read signal, and a read circuit
20
amplifies the read signal, samples the read signal on the edges of the clock, and digitizes the samples. Using the data-synchronization information to locate the first data bit, the Viterbi detector
22
recovers the encoded data from the digitized samples. A decoder
24
, which uses the data-synchronization information to locate the first recovered data bit from the Viterbi detector
22
, decodes the recovered data.
FIG. 2
is a timing diagram of the data-synchronization information and the data stored on the disk
12
(
FIG. 1
) in the order sensed by the read head
16
(FIG.
1
). The disk
12
includes a number of concentric tracks (not shown) that each include one or more respective data sectors, each sector including respective data-storage locations. Each data sector to which data has been written stores a data forerunner and the data in its storage locations. The data forerunner includes a synchronization wedge, a preamble, a synchronization mark (hereinafter sync mark), and a pad. Typically, the disk drive
10
(
FIG. 1
) writes a respective wedge at the beginning of each data sector during the formatting of the disk
12
, and writes the preamble, sync mark, and pad to a data sector each time one writes data to the data sector. As the disk
12
rotates, the read head
16
first senses the wedge at time t
0
, and then senses the preamble, sync mark, pad, and data at relative times t
1
, t
2
, t
3
, and t
4
, respectively.
Referring to
FIGS. 1 and 2
, the read channel
14
operates as follows. A front-end circuit (not shown) receives the read signal and activates the clock circuit
18
in response to the synchronization wedge. Once activated, the clock circuit
18
, which typically includes a phase-locked loop (PLL, not shown), aligns the phase and frequency of the clock signal with the phase and frequency of the preamble. Next, the Viterbi detector
22
recovers from the sync mark the time—typically the clock edge—at which the detector
22
will receive the first data sample. The pad includes a number of don't-care bits, and thus provides a delay between the end of the sync mark and the beginning of the data. This delay allows the detector
22
to reliably recover this first-data-sample time before it actually occurs. The detector
22
then begins recovering the data from the read signal at the first-data-sample time. After a delay equal to its latency, the detector
22
provides the first recovered data bit to the decoder
24
at a first-recovered-bit time, and synchronizes the decoder
24
such that it begins decoding the recovered data at the first-recovered-bit time. But as discussed below, if the detector
22
fails to accurately recover the first-data-sample time, then it begins recovering the data at the wrong sample time, and thus typically generates fatal read errors.
One problem with the Viterbi detector
22
is that it often requires the read signal to have a relatively high signal-to-noise ratio (SNR), and thus often limits the data-storage density, and thus the data-storage capacity, of the disk
12
.
The storage density of the disk
12
is a function of the distances between the storage locations within the data sectors and the distances between the disk tracks. The smaller these distances, the greater the storage density, and vice-versa. The storage capacity of the disk
12
is proportional to its surface area and its storage density. But because the diameter of the disk
12
, and thus its surface area, is typically constrained to industry-standard sizes, the option of increasing the surface area of the disk
12
to increase its storage capacity is usually unavailable to disk-drive manufacturers. Therefore, increasing the storage density is typically the only available technique for increasing the storage capacity of the disk
12
.
Typically, the greater the storage density of the disk
12
, the closer the surrounding storage locations are to the read head
16
while it is reading the surrounded storage location, and thus the lower the signal-to-noise ratio (SNR) of the read signal. Specifically, the closer the surrounding locations are to the read head
16
, the greater the magnitudes of the magnetic fields that these locations respectively generate at the head
16
, and thus the greater the Inter Symbol Interference (ISI). The greater the ISI, the smaller the root-mean-square (RMS) amplitude of the read signal. In addition, as the storage density of the disk
12
increases, the media noise also increases. Generally, the media noise results from the uncertainty in the shapes of the read pulses that compose the read signal. This uncertainty is caused by unpredictable variations in the relative positions of the storage locations from one data-write cycle to the next. Moreover, for a given spin rate of the disk
12
, as one increases the linear storage density within the data sectors, he/she must also increase the bandwidth of the read head
16
to accommodate the increased number of storage locations that the read head
16
must sense in a given time period. This increase in bandwidth causes a proportional increase in the white noise generated by the read head
16
. The SNR of the read signal for a particular storage location is the ratio of the RMS amplitude of the corresponding portion of the read signal to the sum of the amplitudes of the corresponding media and white noise. Thus, the lower the RMS amplitude of the read signal and the greater the amplitudes of the media and/or white noise, the lower the SNR of the read signal.
Unfortunately, as the SNR of the read signal decreases, the data-recovery speed of the Viterbi detector
20
often decreases as well. Specifically, the lower the SNR of the read signal, the lower the accuracy of the detector
20
. As discussed above, the failure of the detector
20
to accurately recover the first-data-sample time from the sync mark often causes serious read errors. If the error processing circuit (not shown) initially detects a read error, then it tries to correct the error using conventional error-correction techniques. If the processing circuit cannot correct the error using these techniques—typically the case when the detector
20
recovers an inaccurate first-data-sample time—then it identifies the error as “fatal” and instructs the read channel
14
to re-read the data from the disk
12
. The time needed by the processing circuit for error detection and error correction and the time needed by the read channel
14
for data re-read increase as the number and severity of the read errors increase. As the error-processing and data re-read times increase, the effective data-read speed of the channel
14
, and thus of the disk drive
10
, decreases.
Therefore, to maintain an acceptable effective data-read speed, the manufacture rates the Viterbi detector
22
for a m
Ozdemir Hakan
Rezzi Francesco
Jorgenson Lisa K.
Lee Thomas
Patel N
Santarelli Bryan A.
STMicroelectronics Inc.
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