Asynchronous timing for interpolated timing recovery

Details Asynchronous timing for interpolated timing recovery Asynchronous timing for interpolated timing recovery

2000-02-02

2002-12-10

Pham, Chi (Department: 2631)

Pulse or digital communications

Receivers

Angle modulation

Reexamination Certificate

active

06493403

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic recording and, particularly, to an improved system and method for interpolated timing recovery.
2. Description of the Related Art
In digital communications receivers, timing recovery circuits are used to acquire and then track the correct sampling and frequency of an analog signal. In an interpolated timing recovery circuit, this is accomplished by digitally resampling a stream of asynchronous samples of the analog signal. The digital resampling effectively reconstructs the values the signal takes on at points in time between the (asynchronous) times at which the signal was actually sampled. This is done by mathematically interpolating the asynchronous sample values of the signal. To allow for small errors in frequency, and to satisfy the Nyquist sampling criterion, the resampling period T
s
is slightly longer than the asynchronous sampling period T
a
.
In the case of magnetic recording, as shown in
FIG. 1
, data sectors
100
on magnetic disks are formatted to include an acquisition preamble
102
, a sync or synchronization mark
104
, and user data
106
. Timing recovery uses the acquisition preamble
102
to acquire the correct sampling frequency and phase before reading the user data
106
. The synchronization mark
104
demarcates the beginning of the user data. The preamble pattern is periodic, having period 4T
s
, where T
s
is the bit period.
The phase and frequency of the initial asynchronous samples of this waveform are unknown. The sampling phase (modulo the bit period T
s
) can be anything, and the sampling frequency can be in error by as much as half a percent. Interpolated timing recovery includes an asynchronous phase to estimate the initial sampling phase and to initialize the interpolator appropriately using the estimate. Then, a synchronous acquisition step is used to refine the initial estimate of the phase and correct the sampling frequency.
This is illustrated more clearly with reference to FIG.
2
. In particular,
FIG. 2
illustrates various initial sampling phases of the 2T acquisition preamble. Points y
k
on the curve
1000
are the asynchronous samples, sampled at a sampling period of T
a
. Points z
k
represent the interpolated (synchronous) points, at a resampling period of T
s
. The values &mgr;
k
represent the fractions of the asynchronous period T
a
at which to interpolate the next synchronous sample. The objective of zero phase restart (or asynchronous sampling) is to determine an initial interpolation interval &mgr;
0
, i.e., the fraction of an asynchronous period T
a
after the last asynchronous sample y
k
at which to interpolate the first synchronous sample.
SUMMARY OF THE INVENTION
One aspect of the present invention relates to an improved asynchronous sampling system and method, i.e., an improved zero phase restart system. Another aspect of the invention relates to synchronous acquisition, i.e., determining the phase error estimate. A timing error estimation unit is provided to calculate an acquisition timing error. The estimate is based on use of synchronous (interpolated) samples and an approximation to an arctangent function.
An interpolated timing recovery system according to the present invention is simpler to implement, requires fewer signal samples, and is more robust against signal distortions such as gain errors, DC offset errors, and magneto-resistive asymmetry.


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