Sampling timing for a reflected write signal

A method for adjusting the timing for sampling a reflected write signal is disclosed. The method comprises determining a reference timing event, and sampling the reflected write signal at first and second sampling times to produce first and second sampled signals, the first sampling time occurring after a first variable time delay following the reference timing event and the second sampling time occurring after a first fixed time delay following the first sampling time. The method further comprises comparing the magnitudes of the first and second sampled signals, and adjusting the first variable time delay in response to the comparison of the magnitudes of the first and second sampled signals so that the magnitudes of the first and second sampled signals are substantially equal.

FIELD OF THE INVENTION 
The present invention relates to optical data recording, and in particular, 
to providing accurate sampling of a reflected write signal relative to a 
reference timing event. 
BACKGROUND OF THE INVENTION 
A method for measuring mark formation effectiveness by sampling a reflected 
write signal at two instances in time T.sub.1 and T.sub.2 relative to an 
external reference timing event is described in U.S. Pat. No. 5,216,660 to 
Iimura, and is illustrated in FIG. 1. Such a reference timing event can 
be, for example, the rising edge of a write pulse sent to a recording 
laser. As shown in FIG. 1, the method disclosed in U.S. Pat. No. 5,216,660 
uses fixed, predetermined delays T.sub.1 and T.sub.2 between the reference 
timing event and the times that the reflected write signal is sampled. 
However, there are many sources of uncertainty and variation in the actual 
time delay between such a reference timing event and the reflected write 
signal. For example, the time delay can vary from one device to another 
and with time and temperature for a given device. Nonadjustable sampling 
delays between the reference timing event and the reflected write signal 
may be adequate for very low data recording rates such as "1X" speed in a 
CD recordable ("CD-R") system, where a given sample timing error may not 
cause significant error in the resulting measurement. However, as data 
recording rates increase, that same error in sample timing can lead to 
larger errors when attempting to measure a specific feature of the 
reflected write signal, such as its peak value. This is illustrated in 
FIG. 2, which is a graph of reflected write signals at 1X and 8X CD-R data 
rates, where X is the data rate associated with CD audio playback (i.e., 
4.321 Mbits per second data rate recorded to or read back from the disk). 
As shown in FIG. 2, a sampling timing error of 15 nanoseconds causes a 
sampled measurement error at 6X, which is significantly greater than at 
1X. 
A mark formation effectiveness measurement method described in 
commonly-assigned U.S. Pat. No. 5,495,466, to Dohmeier et al., the 
disclosure of which is herein incorporated by reference, effectively 
solves this timing error problem for a reflected write signal which has a 
well defined peak. Such a signal occurs, for example, with CD-R media. 
Rather than sampling the reflected write signal, this method works by 
dynamically adjusting one or more threshold levels relative to the 
reflected write signal. Each threshold level is adjusted until the 
reflected write signal exceeds that threshold for a predetermined time 
period. Referring to FIG. 3, a graph of a reflected write signal versus 
time is shown which illustrates the mark formation effectiveness 
measurement method disclosed in U.S. Pat. No. 5,495,466. In FIG. 3, the 
threshold level V.sub.1 is adjusted so that the reflected write signal 
exceeds it for a predetermined time period .DELTA.T.sub.1. Similarly, the 
threshold level V.sub.2 is adjusted so that the reflected write signal 
exceeds it for a predetermined time period .DELTA.T.sub.2. Mark formation 
effectiveness measurements can be made by processing the threshold levels 
V.sub.1 and V.sub.2, which is described in more detail in U.S. Pat. No. 
5,495,466. This method accomplishes accurate mark formation effectiveness 
measurements without needing to provide the precise sample timing of the 
reflected write signal, as required by U.S. Pat. No. 5,216,660. However, 
one of the limitations of the method described in U.S. Pat. No. 5,495,466 
is its speed of response. Since the threshold levels are dynamically 
adjusted, the threshold levels do not instantaneously follow changes in 
the reflected write signal. This time lag can be a problem, for example, 
when measuring rapid waveform fluctuations such as occur at the wobble 
frequency of CD-R. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved mark formation effectiveness measurement of a reflected write 
signal which responds quickly to changes in the reflected write signal. 
It is another object of the present invention to provide an improved mark 
formation effectiveness measurement of a reflected write signal which is 
insensitive to uncertainty and variation of a reference timing event. 
These objects are achieved by a method for adjusting the sampling time of a 
reflected write signal, comprising the steps of: 
(a) determining a reference timing event; 
(b) sampling the reflected write signal at first and second sampling times 
to produce first and second sampled signals, the first sampling time 
occurring after a first variable time delay following the reference timing 
event and the second sampling time occurring after a first fixed time 
delay following the first sampling time; 
(c) comparing the magnitudes of the first and second sampled signals; and 
(d) adjusting the first variable time delay in response to the comparison 
of the magnitudes of the first and second sampled signals so that the 
magnitudes of the first and second sampled signals are substantially 
equal. 
ADVANTAGES 
An advantage of the present invention is to provide accurate sampling of a 
reflected write signal despite uncertainty and variation in the reference 
timing event. 
Another advantage of the present invention is to provide an improved mark 
formation effectiveness measurement of a reflected write signal which 
responds quickly to changes in the reflected write signal.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 4, a representative recording apparatus which can be 
operated in accordance with the present invention is shown. As shown in 
FIG. 4, an incident radiation beam is produced by an optical source 10 for 
recording marks on an optical recording medium 12. The optical source 10 
can include, for example, a laser or a laser diode and a controller, and 
will be referred to hereinafter as a laser. It should be understood, 
however, that the techniques of the present invention can be used with 
other types of optical sources. The recording medium 12 typically includes 
data tracks (not shown) arranged in spiral or concentric circles on a data 
storage surface (not shown). 
The radiation beam from the laser 10 is collimated by a lens 14 and 
directed to surface 16 of a beam splitter 18. A portion of the collimated 
light beam is reflected by the surface 16 of the beam splitter 18 toward a 
detector focusing lens 20 and thereby focused onto a front facet detector 
22. The front facet detector 22 produces a front facet (FF) signal 24 from 
the incident beam. The FF signal 24 is used in a read laser power control 
circuit 26 to maintain the output power level of the laser 10 at a desired 
value during reading of the data or between recording pulses in a manner 
well known in the art. 
Referring again to FIG. 4, the portion of the light beam which is not 
reflected by surface 16 of the beam splitter 18 to the front facet 
detector 22 is transmitted through surface 16 to an objective lens 28 
which focuses the light beam onto the recording medium 12. By modifying 
the power level of the incident beam, data in the form of marks are 
recorded on the recording medium 12. 
Interaction of the incident beam with the recording medium 12 causes a 
portion of the incident beam to be reflected. The reflected beam, referred 
to hereinafter as a return beam, is recollimated by the objective lens 28. 
A portion of the return beam is reflected by surface 16 of the beam 
splitter 18 to a data focusing lens 30 which focuses the return beam onto 
a detector 32. 
Detector 32 produces a reflected write signals 34 which represents light 
received into the return beam aperture. Focus and tracking signals are 
also typically generated in the return path. A number of methods are well 
known to those skilled in the art and are not shown. 
The reflected write signal 34 is applied to a mark formation effectiveness 
(MFE) processor 36. The MFE processor 36 processes the reflected write 
signal 34 to produce a mark formation effectiveness (MFE) signal 38 
indicative of some aspect of mark formation quality, such as the resulting 
mark length, in a manner to be discussed in greater detail below. The 
timing aspects of the processing of the reflected write signal 34 are 
controlled by a pulsed signal 40 produced by a data source 42, in a manner 
to be discussed in more detail below. Using MFE signal comparison 
electronics 44, the MFE signal 38 is then compared to an optimum, or 
previously stored reference, value of an MFE signal which was previously 
determined during calibration. The previously determined MFE reference 
signal 46 can be stored, for example, in a portion of a recording system 
memory (not shown). The storage and retrieval of the MFE reference signal 
46 can be controlled, for example, by a recording system processor (not 
shown). 
Deviation of the processed MFE signal 38 from the stored MFE reference 
signal 46 is indicative of improper mark formation on the optical 
recording medium 12. MFE signal comparison electronics 44 then produces a 
write laser control signal 48 in response to the processed MFE signal 38 
and the stored MFE reference signal 46. The write laser control signal 48 
is applied to a write laser power control circuit 50 during actual data 
recording. The data source 42 produces a pulsed signal 40 which is also 
applied to the write laser power control circuit 50 to modulate the 
instantaneous output power of the laser 10 in response to the data to be 
recorded. The write laser power control circuit 50 produces a write laser 
power adjustment signal 52 which consists of a series of data pulses. The 
data pulses are produced in response to the pulsed signal 40 from the data 
source 42 and have an amplitude controlled by the write laser control 
signal 48. A summing amplifier 54 combines the write laser power 
adjustment signal 52 with a read laser power control signal 56 produced by 
the read laser power control circuit 26 to produce a combined laser power 
control signal 58. The combined laser power control signal 58 is applied 
to the laser 10 to adjust the power during both reading and recording of 
data. 
The generation of the MFE signal 38 will now be discussed. To accurately 
generate the MFE signal 38, it is desirable to sample the reflected write 
signal 34 at one or more sampling times which have a consistent timing 
relationship relative to the timing of the peak magnitude of the reflected 
write signal 34. FIGS. 5A-5C are graphs of the reflected write signal 34 
versus time illustrating adjustment of sampling timing relative to the 
timing of the peak magnitude of the reflected write signal 34 in 
accordance with the present invention. The pulsed signal 40 produced by 
the data source 42 is applied to the MFE processor 36 (shown in FIG. 4) 
and is used as a reference timing event to sample the reflected write 
signal 34. As shown in FIGS. 5A-5C, the reflected write signal 34 is 
sampled at a first sampling time T.sub.1 to produce a first sampled signal 
S.sub.1. T.sub.1 occurs after a first variable time delay .DELTA.T.sub.1 
following the reference timing event. The reflected write signal 34 is 
then sampled at a second sampling time T.sub.2 to produce a second sampled 
signal S.sub.2. T.sub.2 occurs after a fixed time delay .DELTA.T.sub.2 
following T.sub.1. The magnitudes of S.sub.1 and S.sub.2 are used to 
determine the timing of T.sub.1 and T.sub.2 relative to the timing of the 
peak magnitude of the reflected write signal 34. 
As shown in FIG. 5A, the magnitude of S.sub.1 is substantially equal to the 
magnitude of S.sub.2. The magnitude of S.sub.1 being substantially equal 
to the magnitude of S.sub.2 places T.sub.1 before and T.sub.2 after the 
timing of the peak magnitude of the reflected write signal 34. At this 
desired placement, T.sub.1 and T.sub.2 effectively straddle the timing of 
the peak magnitude of the reflected write signal 34. By maintaining 
S.sub.1 substantially equal to S.sub.2, T.sub.1 and T.sub.2 are maintained 
in a consistent relationship relative to the timing of the peak magnitude 
of the reflected write signal 34. This results in consistent measurements 
of the reflected write signal 34. 
In FIG. 5B, the magnitude of S.sub.1 is less than the magnitude of S.sub.2. 
This indicates that the variable time delay .DELTA.T.sub.1 is too short, 
placing T.sub.1 and T.sub.2 earlier than desired. In this case, 
.DELTA.T.sub.1 should be increased until the magnitude of S.sub.1 is 
substantially equal to the magnitude of S.sub.2 to achieve the consistent 
timing relationship of the sampled signals relative to the peak magnitude 
of the reflected write signal 34. Conversely, as shown in FIG. 5C, the 
magnitude of S.sub.1 is greater than the magnitude of S.sub.2, indicating 
that the variable time delay .DELTA.T.sub.1 is too long, placing T.sub.1 
and T.sub.2 later than desired. In this case, .DELTA.T.sub.1 should be 
decreased until the magnitude of S.sub.1 is substantially equal to the 
magnitude of S.sub.2 to achieve the consistent timing relationship of the 
sampled signals relative to the peak magnitude of the reflected write 
signal 34. In contrast to the present invention, the method disclosed in 
U.S. Pat. No. 5,216,660 uses a fixed delay between the reference timing 
event and one or more sampling times. With fixed delays, any changes in 
the timing between the reference timing event and the timing of the peak 
magnitude of the reflected write signal 34 will result in inconsistent 
sampling relative to the timing of the peak magnitude of the reflected 
write signal 34. These sampling timing errors will result in inaccurate 
mark formation effectiveness measurements. 
FIGS. 6-9 show various embodiments of using sampling timing adjustment 
strategy shown in FIGS. 5A-5C to measure mark formation effectiveness. 
FIG. 6 is a graph of the reflected write signal 34 versus time 
illustrating a first embodiment of mark formation effectiveness 
measurement in accordance with the present invention. As shown in FIG. 6, 
the reflected write signal 34 is sampled at a first sampled time T.sub.1, 
which occurs after a first variable time delay .DELTA.T.sub.1 following a 
reference timing event, to produce a first sampled signal S.sub.1. The 
reflected write signal 34 is then sampled at a second sampled time 
T.sub.2, which occurs after a first fixed time delay .DELTA.T.sub.2 
following the first sampling time T.sub.1, to produce a second sampled 
signal S.sub.2. The magnitudes of S.sub.1 and S.sub.2 are compared, and 
.DELTA.T.sub.1 is adjusted until the magnitudes of S.sub.1 and S.sub.2 are 
substantially equal in a manner previously described in connection with 
FIGS. 5A-5C. The reflected write signal 34 is also sampled at a third 
sampled time T.sub.3, which occurs after a second variable time delay 
.DELTA.T.sub.3 following the sampled time T.sub.1, to produce a third 
sampled signal S.sub.3. In accordance with the present invention, 
.DELTA.T.sub.3 is less than .DELTA.T.sub.2. As discussed above, T.sub.1 
and T.sub.2 effectively straddle the peak magnitude of the reflected write 
signal 34, and therefore, as shown in FIG. 6, S.sub.3 sampled at time 
T.sub.3 is substantially equal to the peak magnitude of the reflected 
write signal 34. The reflected write signal 34 is also sampled at a fourth 
sampled time T.sub.4, which occurs after the third fixed time delay 
.DELTA.T.sub.4 following sampled time T.sub.1, to produce a fourth sampled 
signal S.sub.4. In accordance with the present invention, .DELTA.T.sub.4 
is greater than .DELTA.T.sub.3 so that the reflected write signal 34 is 
sampled at a consistent time following the timing of the peak magnitude. 
The MFE signal 38 is produced by comparing the magnitudes of S.sub.3 and 
S.sub.4. For example, the MFE signal 38 can be produced by taking the 
ratio of S.sub.3 and S.sub.4. The magnitudes of S.sub.3 and S.sub.4 can be 
compared in various other ways to produce the MFE signal 38 in a manner 
well known to those skilled in the art. Examples of various comparison 
means are more fully described in U.S. Pat. No. 5,495,466. It should be 
noted that although this embodiment includes sampling the reflected write 
signal 34 at two sampling times T.sub.3 and T.sub.4 to measure mark 
formation effectiveness, mark formation effectiveness can be measured 
using additional sampling times. 
This first embodiment, as shown in FIG. 6, provides an advantage over the 
prior art method disclosed in U.S. Pat. No. 5,216,660 in that it provides 
sampling timing which adapts to changes in the timing of the reference 
timing event relative to the timing of the peak magnitude of the reflected 
write signal 34. Such timing changes can occur, for example, because of 
temperature changes or because of manufacturing tolerances. As a result, 
measurements in accordance with the present invention are made at 
consistent times relative to the timing of the peak magnitude of the 
reflected write signal 34. 
FIG. 7 is a graph of the reflected write signal 34 versus time illustrating 
a second embodiment of mark formation effectiveness measurement in 
accordance with the present invention. As shown in FIG. 7, the reflected 
write signal 34 is sampled at a first sampled time T.sub.1, which occurs 
after a first variable time delay .DELTA.T.sub.1 following a reference 
timing event, to produce a first sampled signal S.sub.1. The reflected 
write signal 34 is then sampled at a second sampled time T.sub.2, which 
occurs after a first fixed time delay .DELTA.T.sub.2 following the first 
sampling time T.sub.1, to produce a second sampled signal S.sub.2. The 
magnitudes of S.sub.1 and S.sub.2 are compared, and .DELTA.T.sub.1 is 
adjusted until the magnitudes of S.sub.1 and S.sub.2 are substantially 
equal in a manner previously described in connection with FIGS. 5A-5C. The 
reflected write signal 34 is also sampled at a third sampled time T.sub.3, 
which occurs after a second variable time delay .DELTA.T.sub.3 following 
the reference timing event, to produce a third sampled signal S.sub.3. The 
reflected write signal 34 is also sampled at a fourth sampled time 
T.sub.4, which occurs after the second fixed time delay .DELTA.T.sub.4 
following sampled time T.sub.3, to produce a fourth sampled signal 
S.sub.4. The magnitudes of S.sub.3 and S.sub.4 are compared, and 
.DELTA.T.sub.3 is adjusted until the magnitudes of S.sub.3 and S.sub.4 are 
substantially equal in a manner previously described in connection with 
FIGS. 5A-5C. The MFE signal 38 is produced in response to at least one of 
the first and second sampled signals S.sub.1 and S.sub.2, and at least one 
of the third and fourth sampled signals S.sub.3 and S.sub.4 after 
adjustment of the first variable time delay .DELTA.T.sub.1 and the second 
variable time delay .DELTA.T.sub.3. For example, the MFE signal 38 can be 
produced by taking the ratio of S.sub.2 and S.sub.4, or the ratio of 
(S.sub.1 +S.sub.2) and (S.sub.3 +S.sub.4). Various other comparison means 
can be used to produce the MFE signal 38 in a manner well known to those 
skilled in the art. It should be noted that although this embodiment 
includes sampling the reflected write signal 34 at two pairs of sampling 
times, T.sub.1 and T.sub.2, and T.sub.3 and T.sub.4, to measure mark 
formation effectiveness, mark formation effectiveness can be measured 
using additional sampling times or pairs of sampling times. 
It should be also noted that the methods illustrated in FIGS. 6 and 7 in 
accordance with the present invention also provide the advantage over the 
method disclosed in U.S. Pat. No. 5,495,466 in that the MFE signal 38 
responds more quickly to changes in the reflected write signal 34. One 
example of this decreased response time is shown in FIG. 8, which is a 
graph of the reflected write signal 34 during two reflected write pulses 
versus time. In this example, the reflected write signal 34 during a 
reflected write pulse B has a higher peak level than during a reflected 
write pulse A. As shown in FIG. 8, sampling times T.sub.1 and T.sub.2 have 
been established in a manner previously described in conjunction with 
FIGS. 5A-5C. During pulse A, the sampled signals taken at sampling times 
T.sub.1 and T.sub.2 are designated S.sub.1A and S.sub.2A, respectively. 
For pulse B, the sampled signals taken at sampling times T.sub.1 and 
T.sub.2 are designated S.sub.1B and S.sub.2B, respectively. If pulse A is 
immediately followed by pulse B, the sampled signals S.sub.1 and S.sub.2 
will change directly from S.sub.1A to S.sub.1B and from S.sub.2A to 
S.sub.2B. In contrast to the present invention in which the reflected 
write signal is sampled at two sampling times separated by a fixed time 
interval, the prior art method disclosed in U.S. Pat. No. 5,495,466 (shown 
in FIG. 3) works by dynamically adjusting a threshold level relative to 
the reflected write signal until the peak portion of the reflected write 
signal exceeds that threshold for a fixed time interval. With the Dohmeier 
method, if pulse A is immediately followed by pulse B, the threshold level 
would gradually change from the level of S.sub.1A and S.sub.1B to the 
level of S.sub.2A and S.sub.2B, rather than directly change as in the 
present invention. This gradual change results in a longer response time, 
which is a problem, for example, when measuring rapid fluctuations in the 
reflected write signal such as occur at the wobble frequency of CD-R. 
It would be understood by one skilled in the art that the sampling timing 
strategy described above can be applied to measurements of a reflected 
write signal made immediately following a write pulse. This is shown in 
FIG. 9, where a first sampling time T.sub.1 occurs after a variable time 
delay .DELTA.T.sub.1 following a reference timing event associated with 
the end of a write pulse. A second sampling time T.sub.2 occurs after a 
fixed time delay AT.sub.2 after the first sampling time T.sub.1. 
.DELTA.T.sub.1 is adjusted so that the magnitude of a first sampled signal 
S.sub.1 taken at T.sub.1 is substantially equal to the magnitude of a 
second sampled signal S.sub.2 taken at T.sub.2. The magnitude of S.sub.1 
being substantially equal to the magnitude of S.sub.2 places T.sub.1 
before and T.sub.2 after the timing of the minimum magnitude of the 
reflected write signal 34 following a write pulse. At this desired 
placement, T.sub.1 and T.sub.2 effectively straddle the timing of the 
minimum magnitude of the reflected write signal 34. By maintaining S.sub.1 
substantially equal to S.sub.2, T.sub.1 and T.sub.2 are maintained in a 
consistent relationship relative to the timing of the minimum magnitude of 
the reflected write signal 34. This results in consistent measurements of 
the reflected write signal 34. In adjusting the variable time delay 
.DELTA.T.sub.1, if the magnitude of S.sub.1 is less than the magnitude of 
S.sub.2, .DELTA.T.sub.1 should be decreased until the magnitude of S.sub.1 
is substantially equal to the magnitude of S.sub.2 to achieve the 
consistent timing relationship of the sampled signals relative to the 
minimum magnitude of the reflected write signal 34. Conversely, if the 
magnitude of S.sub.1 is greater than the magnitude of S.sub.2, 
.DELTA.T.sub.1 should be increased until the magnitude of S.sub.1 is 
substantially equal to the magnitude of S.sub.2. The techniques used to 
measure mark formation effectiveness which were previously described for 
sampling during a write pulse also apply to mark formation effectiveness 
measurements when the sampling occurs after a write pulse, as shown in 
FIG. 9. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention. 
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TS LIST 
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10 optical source 
12 recording medium 
14 lens 
16 surface of beam splitter 
18 beam splitter 
20 detector focusing lens 
22 front facet detector 
24 front facet signal 
26 read laser power control circuit 
28 objective lens 
30 data focusing lens 
32 detector 
34 reflected write signal 
36 mark formation effectiveness processor 
38 mark formation effectiveness signal 
40 pulsed signal 
42 data source 
44 mark formation effectiveness signal comparison electronics 
46 stored mark formation effectiveness reference signal 
48 write laser control signal 
50 write laser power control circuit 
52 write laser power adjustment signal 
54 summing amplifier 
56 read laser power control signal 
58 combined laser power control signal 
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