CROSS-REFERENCE TO RELATED APPLICATIONS This application is related to Application

Ser. No. 206,408 filed June 14, 1988, by Whitfield et al., Application Ser. No. 206,407 filed June 14, 1988, by Wash, and Application Ser. No. 206,553 filed June 14, 1988, by Whitfield, which Applications are being filed contemporaneously with this application. The entire disclosures of each of these applications are incorporated by reference herein. Each of these applications is copending and commonly assigned. FIELD OF THE INVENTION

This invention relates to a method for modulating a binary data stream into a code format suitable for encoding and decoding information. INTRODUCTION TO THE INVENTION Methods for encoding and decoding binary information or data extend to such disparate fields as e.g., magnetic systems or optical systems. The methods include the step of storing a binary 0 or 1 on a small area of material or medium. Storage is dynamic, for the medium on which the information is recorded typically moves past a reading or writing device, or vice versa. Typically, in writing information on a medium surface, the binary information is supplied to a recording circuitry, which then codes this information into a pattern which is recorded by a writing device. The techniques which are used to write information on the medium include the two well-known categories: the return-to-zero (RZ) techniques, and the non-return-to-zero (NRZ) techniques. Useful methods

for reading information using either technique also vary and are well known. Reference may be made, for example, to the Handbook authored by Jorgensen entitled THE COMPLETE HANDBOOK OF MAGNETIC RECORDING, (1980) published by Tab Books Inc., Blue Ridge Summit, PA.

The process for encoding and decoding binary information is not straightforward, and considerable attention has been given to both the recording patterns used to represent data 0 and 1 bits, and the means for determining the value recorded. Clearly, the reading and writing procedures should be as reliable as is possible. SUMMARY OF THE INVENTION As indicated above, the process for encoding and decoding binary information typically is dynamic, for the medium on which the information is recorded may move past a reading or writing device, or vice versa. The stated objective of reliability has been realized for this situation in systems where the information transfer rate is well-regulated e.g., disk drive, optical disk, or streaming tape. Here, the decoded information bits can be precisely positioned in a time opening known as a data window. If a bit is present in the data window, the decoded bit is a 1. If no bit is present in the data window, the bit is a zero.

In sharp contrast to this situation in which the information transfer rate is well regulated, is that situation where the information transfer rate is not, or cannot be, well-controlled i»e. , the transfer rate is dependent on unpredictable and variable transfer rate velocities and accelerations. For example, magnetic credit card readers fit into this last situation. Here,

the prior art encoding and decoding techniques may require the employment of expensive, multiple writing and/or reading devices, or fail or cease to provide a desired reliability. I have discovered a novel method for modulating a binary data stream into a code format suitable for encoding and decoding binary information. The novel method fits into the non-return-to-zero (NRZ) techniques which are used to write information on a medium. The novel method can be employed for the first situation where the information transfer rate is well-regulated. It is especially advantageously employed for the second situation, where the information transfer rate may be dependent on unpredictable and variable transfer rate velocities and accelerations. The novel method has the advantage of requiring the employment of only one writing or reading device, thus considerably reducing the expense of the presently required multiple reading and writing devices. At the same time, the novel method realizes the stated objective of reliability.

In a first aspect, the invention provides a method for modulating a binary data stream into a three-part code format, which method comprises:

(1) defining a bitcell as the time t between two adjacent clock transitions;

(2) writing a first clock transition at the beginning of the bitcell; and (3) encoding a binary data transition after the first clock transition in the ratio of t, , where t^ is the time duration

between the first clock transition and the data transition, with the proviso that t<j 1 .

Preferably, the ratio t^ < 1

—Γ- 2 encodes a data 0 bit, and the ratio t^ > 1

^~ T ^~ 2 encodes a data 1 bit. A particularly useful embodiment provides a ratio t<j «. 1 to encode a

-τ ^{~ "} 3 data 0 bit, and the ratio t^ _- 2 to encode a

^~ ^r ^* 3 data 1 bit. The usefulness of this last embodiment derives from the fact that, within the context of the method, it provides optimal data bit discrimination. The method can also be employed by providing a ratio t^ < 1 to τ ^~ 2 encode a data 1 bit, and the ratio t^ > 1

^~ t~ 2 to encode a data 0 bit. Preferably, the clock transitions are assigned a first identifiable characteristic, and the data transition a second identifiable characteristic. For example, an identifiable characteristic suitable for magnetic systems may be that of polarity, so that the clock transitions are the opposite polarity of the data transition. For example, the clock transition may have a positive polarity and the data transition may have a negative polarity, or vice versa. For optical systems, on the other hand, a suitable characteristic may be that of color, so that a clock transition could be assigned a light-to-dark transition, and a data transition have an assigned dark-to-light transition, or vice versa. In the same manner, a suitable characteristic in other appropriate systems can include readily differentiated changes, like transmissivity or absorption. In all cases, this feature of identifiable characteristics uniquely differentiates the clock transitions from the data transition- Consequently, this feature helps

provide "sel -clocking", which, in turn, permits velocity insensitive encoding.

This last concept, i.e., self-clocking and velocity insensitive encoding, highlights an important advantage of the novel method. The concept is also usefully understood by considering the case of a succession of bitcells. In the general case, the time t-, between two adjacent clock transitions of a first bitcell, may differ from the time t~ between two adjacent clock transitions of a second bitcell. This is a consequence of the information transfer rate being dependent on unpredictable and variable transfer rate velocities and accelerations. It should be clear, therefore, that if one employed an external, invariant clocking mechanism to indicate the advent of a bitcell, it would shortly be out of synchronization with the variable time dependent bitcell succession. The novel method, in sharp contrast, overcomes this problem by way of self-clocking i.e., no matter when a bitcell occurs, the clock transition defines the bitcell initiation, and is, at the same time, distinguished from the varying time location of the data transition for that bitcell.

This first aspect of the method may further comprise a step of decoding the encoded binary data stream, by determining the time between the first clock transition and the data transition, and comparing this time to the bitcell time.

In a second aspect, the invention provides a method for modulating a binary data stream into a three-part code format, which method comprises: (1) defining a bitcell as the space S between two adjacent clock transitions;

(2) writing a first clock transition at the beginning of the bitcell; and

(3) encoding a binary data transition after the first clock transition so that a data 0 bit is located in the first half of the bitcell space S, and a data 1 bit is located in the second half of the bitcell space S, or vice versa.

In a third aspect, the invention provides a method for modulating a binary data stream into a three-part code format, which method comprises:

(1) defining a bitcell as the time t between two adjacent clock transitions;

(2) writing a first clock transition at the beginning of the bitcell; and (3) encoding a binary data transition after the first clock transition in the ratio of t^, where t^ is the time duration between the τ ^~ first clock transition and the data transition, with the proviso that the ratio distinguishes a data 1 bit from a data 0 bit.

This third aspect of the method of the invention recites a different proviso, and demonstrates that the bitcell ratio t-j / 1 is a particular case of a generalized rati~o t^ ² fJ i- r- ⁿ

The point of the proviso, in all cases, is to avoid an ambiguity that may prevent the distinguishing of a data 1 bit from a data 0 bit. By generalizing the ratio from

1 to i, the argument is made that a "line of

2 n demarcation" can eliminate the possible ambiguity between the data 1 bit and the data 0 bit, and that the line of demarcation can be made at any point 1 within a bitcell. Now, n having established the "no ambiguity rule" that

t _d 1 the encoding locates a data 0 transition t ^~~ n according to t _j < t , and a data 1 transition n according to tj > t (or vice versa). In the same n way, the decoding proceeds by determining that if td ^< _L» then a data 0 bit has been encoded, and if t~ n t-d ^> λ» then a data 1 bit has been encoded (or vice versa). In a fourth aspect, the invention provides a method comprising:

(1) providing a composition comprising a photographic film,

(2) accelerating the composition to a transport velocity; and

(3) encoding a binary data stream into a three-part code format for writing said data stream onto the composition, the encoding being insensitive to the composition velocity, said encoding comprising the steps of:

(a) defining a bitcell as the time t between two adjacent clock transitions;

(b) writing a first clock transition at the beginning of the bitcell; and (c) encoding a binary data transition after the first clock transition in the

✓ ^• ratio of t, , where t _< j is the time duration between the^ irst clock transition and the data transition, with the proviso that t^ -TT ' — where 1. defines a line of demarcation between a data n 0 bit and a data 1 bit.

Preferably, the composition comprises a magnetic recording material. Preferred compositions are in the form of a tape. A suitable composition for employment in the

present method is disclosed in U.S. Pat. Nos. 3,782,947, 4,613,911, U.K. Application No. 2,083,652A, and the publication entitled DataKode Magnetic Control Surface, Eastman Kodak Company, 1983. The disclosure of each of these publications and applications is incorporated herein by reference. BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated in the accompanying drawing, in which FIG. 1 shows, in a generalized fashion, the three-part code concept, FIGS. 2 and 3 illustrate the method encoding and decoding schemes, respectively, for a magnetic system, and FIG. 4 illustrates the method as used in an optical system. DETAILED DESCRIPTION OF,THE. INVENTION

Attention is now directed to FIG. 1, which shows, in a generalized fashion, the three-part code concept of the invention. FIG. 1 shows two adjacent clock transitions (CLK and CLK ), with a data 0

1 2 bit and a data 1 bit located between the two clock transitions. The time t defines the time between the two adjacent clock transitions. The time t thus defines the bitcell. The time t also corresponds to the space S between the two adjacent clock transitions. FIG. 1 further shows a pulse representation of a data 0 bit and a data 1 bit. Both the data 0 bit and the data 1 bit are encoded, according to the method of the invention, after the first clock transition, and in the ratio of t,, where t^ is the time duration between the

first clock transition and the data transition.

FIG. 1, in particular, shows that the ratio td < 1 encodes the data 0 bit, and the ratio ~t 2

t _< j > 1 encodes the data 1 bit. Looked at from T— 2 the second aspect of the invention, Fig. 1 also shows the data 0 bit located in the first half of the bitcell space S, and the data 1 bit located in the second half of the bitcell space S.

FIG. 1 helps illustrate a particularly important advantage of the method of the invention discussed above. Namely, the clock transitions are "built-in" to the encoding scheme. This feature means that the encoding can be self-clocking i.e., velocity insensitive. In a general situation, this means that the information transfer rate is not bounded. In a practical situation, the information rate can change more than 10-25%, in particular greater than 100%, especially greater than 1000%, from a first bitcell to a second adjacent bitcell.

Attention is now directed to FIG. 2, which shows an encoding scheme suitable for a magnetic system and in accordance with the method of the invention. Consider first the case for a data 0 bit. Here, the data 0 bit is encoded by first writing a clock transition at the beginning of the bitcell. The information or data i.e., the 0 bit, then follows. The data 0 bit is shown here as a transition written 33% of the time of the bitcell, shown by way of six equally spaced hatch marks, with the data 0 bit written through two of the hatch marks. In other words, td τ~ 3

Now consider an encoding scheme for the case of a data 1 bit. Here, the data 1 bit is encoded by first writing a clock transition at the beginning of the bitcell. The information or data i.e., the 1 bit, then follows. The data 1 bit is shown here as a transition written 67% of the time t

of the bitcell, shown by way of six equally spaced hatch marks, with the 1 bit written through four of the hatch marks.

In other words, td •= 2 .

Note that for both the data 0 bit and the data 1 bit, the clock transitions are the opposite polarity of the data transition. FIG. 2 shows that the clock transitions are negative, and the data transitions are positive. In other embodiments, not shown, this representation may be reversed, so that the clock transitions are positive and the data transitions are negative. The advantage of this representation is that it provides an easily decoded clock which cannot be confused with data, for each bitcell, and regardless of a variable or unpredictable information transfer rate.

Attention is now directed to FIG. 3, which shows a decoding scheme suitable for a magnetic system and in accordance with the method of the invention. The data or information, 0 bit or 1 bit, may be decoded by determining the time between the data transition and the preceding clock transition, and comparing this time to the bitcell time. If the data transition occurs in the first half of the bitcell, the data bit' is a 0 bit. On the other hand, if the data transition occurs in the second half of the bitcell, the data bit is a 1 bit.

Preferred electrical circuits that may be employed for realizing the encoding and decoding schemes of FIGS. 2 and 3 are disclosed in the Application Ser. Nos. 206,407, 206,408, and 206,553, cross-referenced above.

Attention is now directed to FIG. 4 which illustrates the method as used in an optical

system. FIG. 4 shows a clock transition is assigned a light-to-dark transition, and a data transition is assigned a dark-to-light transition. In other embodiments, not shown, this representation may be reversed, so that the clock transition is assigned a light-to-dark transition, and the data transition is assigned a dark-to-light transition.