WO1998038635A2

WO1998038635A2 - Time displacement of phase referenced measurement

Info

Publication number: WO1998038635A2
Application number: PCT/US1998/003854
Authority: WO
Inventors: Timothy Nix
Original assignee: Wea Manufacturing, Inc.
Priority date: 1997-02-28
Filing date: 1998-02-27
Publication date: 1998-09-03
Also published as: AU6671498A; WO1998038635A3

Abstract

A method and apparatus for recording and quantifying the amount of jitter in the circular drive means. It employs a clock (1) which is phase locked to the data as a time reference. The distance in time (2) from the reference clock transition to the land to pit (or pit to land) transition associated with the next clock cycle is used to construct a probability distribution function (pdf) (4) that can be approximated in the form of a histogram.

Description

TIME DISPLACEMENT OF PHASE REFERENCED MEASUREMENT FIELD OF THE INVENTION

This invention relates to a method for measuring jitter on an optical disc. In particular it relates to methods for compensating for the uncertainty in the constancy of the angular velocity of the rotating master during fabrication of an optical recording medium.

BACKGROUND OF THE INVENTION Compact discs are optical recording media in which the length of pits and lands encodes data. The pits and lands are created on a photoresist layer deposited on a disc shaped glass master surface. The glass master is rotated under a laser that pulses to expose the photoresist to a pattern of light that encodes the data. After development of the photoresist a pattern of pits and lands is created which is preserved by metallization of the surface. This surface or first or second generation imprints taken from it may be used in an injection molding process to create plastic surfaces having the pit and land pattern. These too may be metalized and the reflectivity from the metallized surface rotated under a scanning laser used to read the molded data pattern from the surface.

The prior art has relied upon the constancy of the -angular velocity of rotation of the disc surface in order to create and later to recover data from the pattern of pits and lands on a surface. Since the deviation from constancy of the angular velocity may be cumulative, the prior art has required the presence of synchronizing words placed periodically into the data stream. These synchronizing words permit the interpreting electronics to recognize the start of a new patter of position locations in the data.

Quality control also requires the feedback of information on the irregularities in rotation rate of the optical surface. In its crudest use this would permit the rejection of discs produced on a rotational drive with excessive jitter. By jitter is meant the stochastic variation in angular rota- tion rate and/or the stochastic variation in the start location for the various pits and lands constituting the recorded data.

BRIEF DESCRIPTION OF THE INVENTION The invention comprises a method and apparatus for recording and quantifying the amount of jitter in the circular drive means or in the data optically recorded on the surface of a compact disc. The invention provides a method to measure jitter.- It employs a clock which is phase locked to the data as a time reference. The distance in time from a reference clock transition to the land to pit (or pit to land) transition associated with the next clock cycle is used to construct a probability distribution function (pdf) that can be approximated in the form of a histogram. The number of observations in each range cell is derived from a counter, which is enabled at a specified time displacement from the clock transition. A delay line is used to set the time displacement. The histogram (and therefore the pdf) is approximated by changing the time displacement from the reference time to the counter enable time.

It is an object of the invention to proved a jitter control and measuring apparatus and method in which a phase reference generator shifts the time (phase with respect to the input signal) when a measuring device takes its reading.

It is a further object of the invention to provide an algorithm to subtract from (combine with) a current measurement, results contributed to a measurement or measurements taken with a dif- ferent phase/time reference than the current measurement to improve the time base resolution of the collection of measurements,

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of the present invention.

Figure 2 is an alternative embodiment in block form of the present invention. Figure 3 is a further embodiment in block form of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to Figure 1, the invention com- prises Phase Reference Generator 1 , Time Control Device 2 , and Measuring Device 3.. A Processing Device 4. may also be employed. Phase Reference Generator 1 produces a time reference related to the signal under test. Time Control Device 2 is used to control the time elapsed between the reference time and the time when the Measuring Device 3. takes its measurement. Using an algorithm, a Processing Device 4. may be used to make measurements of recurrent events with higher time posi- tion resolution than I possible when using the Measuring Device alone. Figure 2 depicts a further embodiment in which the time/phase displacement of the signal under test may be controlled.

The invention finds utility in the measure- ment of the jitter of a digital data signal with respect to a phase locked clock generated from the data signal. This particular application is depicted in Figure 3. Elements 1 , 2 , and 3. from the embodiment of Figure 1 become respectively Phase-locked loop 1 , Delay Line 2 and Counter 3. in Figure 3. The Additional Processing/Control Device 4_. is also used.

In operation, phase-locked-loop 1 generates a phase locked clock from the Input Data Signal. Delay Line 2 shifts the time when Counter 3. is allowed to increment its count. Processing/Control Device 4. controls and reads counts from Counter 3.» Also, Processing/Control Driver 4_. implements an algorithm to set the delay time of the Delay Line 2. and to compute jitter measurements from the counts read from Counter 3.. The algorithm subtracts from the current count a number derived from previous counts. In some applications the Input Data Signal may be applied to the Processing/Control Device 4. to allow it to determine the mark or space lengths surrounding the measurements.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the inven- tion.

Claims

What is claimed is:

1. An apparatus for determining jitter on an optical disc having data thereon encoded into pits and lands along a spiral track, said apparatus comprising data reading means for outputting a data transition signal when a transition is detected from land to pit or pit to land, a clock for outputting timing pulses, said clock outputting clock transition signals at each clock cycle, means for correlating said clock to the output signal from said data reading means, means for determining the time elapsed from a clock signal transition of a cycle to said data transition signal associated with the next clock cycle, time range storage means for counting numbers of impulses received in a time range.

2. The apparatus of claim 1, wherein signals from said means for determining are stored in said time range storage means to form a histogram representation of a probability distribution formula of said jitter.

3. The apparatus of claim 1, wherein said means for correlating comprises a phase locking circuit.

4. The apparatus of claim 2, wherein said means for correlating comprises a phase locking circuit.

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