Opinion for the court filed by Circuit Judge GAJARSA. Opinion concurring-in-part and dissenting-in-part filed by Circuit Judge LINN.
GAJARSA, Circuit Judge.
The issue before the court is whether or not a signal is patentable subject matter. Petrus A.C.M. Nuijten appeals the decision of the Board of Patent Appeals and Interferences (“Board”) of the United States Patent and Trademark Office (“PTO”), which rejected claims 14, 22, 23, and 24 in his patent application Serial No. 09/211,928 as unpatentable subject matter outside the scope of 35 U.S.C. § 101. The claims seek to patent any “signal” that has been encoded in a particular manner. Because we agree with the Board that the “signal” claims in Nuijten’s application are not directed to statutory subject matter, we affirm.
I. BACKGROUND
A. Nuijten’s Invention and Patent Application
Nuijten’s patent application discloses a technique for reducing distortion induced by the introduction of “watermarks” into signals. In the context of signal processing, watermarking is a technique by which an original signal (such as a digital audio file) is manipulated so as to embed within it additional data. The additional data is preferably imperceptible to someone who views or listens to the signal — for instance, a listener who plays back a watermarked digital audio file would, if the watermark is sufficiently unobtrusive, not be able to distinguish between the watermarked and un-watermarked versions. However, an analysis of the file by software capable of detecting the watermark will reveal the mark’s contents. This ability to encode additional data into a signal is useful to publishers of sound and video recordings, who can use watermarks to embed in the media they distribute information intended to protect that media against unauthorized copying. For these publishers and others, watermarking represents a trade-off: the desired additional data is encoded directly into the signal, but like any change to a signal, the watermark introduces some level of distortion. Thus, a key goal of water[1349] marking techniques is to minimize the distortion so that the resulting diminution in signal quality is as minimal as possible.
Nuijten’s technique improves existing watermark technology by further modifying the watermarked signal in a way that partially compensates for distortion introduced by the watermark. A helpful illustration is found in the diagrams of Nui-jten’s application:
[[Image here]]
This diagram, Figure 2 of the application, demonstrates a relatively simple form of digital audio encoding called “delta modulation.” 1 The smooth line in the upper graph (labeled V) represents a very small slice of the sound wave to be encoded. The lower graph represents a digital encoding of that signal. It takes on only two values. They are labeled here as T and - 1,’ rather than the usual labeling of these binary values as one and zero. The sound wave is reconstructed from the digital signal one step at a time, left to right. If the digital signal has value 1,’ the reconstructed sound wave’s value is increased slightly, and if the digital signal has value -1,’ the sound wave is decreased by the same amount. The recording is therefore represented by the change (or “delta”) over a very small increment of time, either T’ for an increase or -1’ for a decrease. Hence, the encoding scheme is known as “delta modulation.” The result is a close but imperfect approximation of the original sound wave, illustrated on the upper graph by ‘x’ with a caret above it. The fidelity of the reconstructed sound wave to the original will depend in large part on the “sample rate” — the length of the time interval represented by each discrete value in the digital signal. Representing all of the nuances of the original sound wave in order to produce a rich, clear recording may require tens or hundreds of thousands of samples per second.
The above-illustrated signal has no watermark. Nuijten’s application next illustrates in Figure 3 what occurs when the signal is modified to add a watermark:
[1350] [[Image here]]
The watermark Nuijten posits here is imposed on the signal by altering, if necessary, every hundredth value of the digital signal. A reader seeking to extract the watermark from the digital signal would therefore view only every hundredth value, disregarding the other 99 along the way; by stringing together all such values, the watermarked data may be discerned. Every point where a portion of the watermark is found represents a possibility that the signal may be distorted. If the watermark value designated for a certain position and the original value at that same position happen to coincide, there is no need to modify the original and hence no distortion. About half of the time, though, those values will not coincide and the digital signal will be altered. The result is shown in the diagram: the digital value at the point labeled ‘21’ and illustrated by a vertical dashed line has been changed from T in the original to -1.’ The reconstructed signal is thus decreased where it should be increased, and the encoded signal departs from the original in a pronounced manner.
Nuijten’s application teaches that this departure may be minimized by making an additional change to the watermarked digital signal, as shown in Figure 4:
[[Image here]]
Here, the value preceding the one that was modified by the watermark has also been modified: it was ‘-1’ in the original signal, but is now 1.’ The signal is therefore [1351] increased, then decreased (where in the original it was decreased, then increased). The resulting encoding has the same watermark as the above example, but as the diagram indicates, it tracks the original sound wave much more accurately. There is still some small loss in encoding quality relative to the unwatermarked original, but the magnitude of that loss has been greatly decreased.
The above-described procedure is most naturally expressed as a series of steps for adding a low-distortion watermark to a signal, and indeed Nuijten has already obtained allowance of ten claims (Claims 1-10) directed to such a process. Claim 1 is the broadest process claim allowed. It reads:
A method of embedding supplemental data in a signal, comprising the steps of: encoding the signal in accordance with an encoding process which includes the step of feeding back the encoded signal to control the encoding; and modifying selected samples of the encoded signal to represent the supplemental data prior to the feedback of the encoded signal and including the modifying of at least one further sample of the encoded signal preceding the selected sample if the further sample modification is found to improve the quality of the encoding process.
Nuijten’s Claims 11-13, also allowed by the PTO, are directed to “[a]n arrangement for embedding supplemental data in a signal,” including “encoder means for encoding the signal” and other structural features that carry out the above process. Finally, Nuijten’s allowed Claim 15 is directed to “[a] storage medium having stored thereon a signal with embedded supplemental data,” where the stored signal has essentially the encoding properties described above. Thus, Nuijten has been allowed claims to the process he invented, a device that performs that process, and a storage medium holding the resulting signals. None of these claims is before us on appeal.
B. The Claims on Appeal
The claims whose disallowance Nuijten appeals are not traditional step-by-step process claims, nor are they directed to any apparatus for generating, receiving, processing, or storing the signals. As mentioned above, such claims have been allowed. The claims on appeal seek to cover the resulting encoded signals themselves. Claim 14 of Nuijten’s application is the only independent claim of the four rejected by the PTO. It reads:
A signal with embedded supplemental data, the signal being encoded in accordance with a given encoding process and selected samples of the signal representing the supplemental data, and at least one of the samples preceding the selected samples is different from the sample corresponding to the given encoding-process.
(emphasis added). Claims 22, 23, and 24 depend on Claim 14, respectively adding requirements that the embedded data be a watermark, that the signal be a video signal, and that the signal be an audio signal.
C. Procedural History
The Examiner rejected a number of claims in Nuijten’s application for obviousness-type double patenting, and rejected Claims 14, 15, and 22-24 as directed to nonstatutory subject matter under § 101. On appeal, the Board reversed the double-patenting rejections. As to Claim 15, it found that “[t]he storage medium in claim 15 nominally puts the claim into the statutory category of a ‘manufacture’ ” and thus reversed the Examiner’s § 101 rejection of that claim. However, it affirmed the Examiner’s § 101 rejections of Claims 14 and 22-24 on two grounds. First, it noted that “[t]he signal ... has no physical attributes [1352] and merely describes the abstract characteristics of the signal and, thus, it is considered an ‘abstract idea’ ” unpatentable under Diamond v. Diehr, 450 U.S. 175, 185, 101 S.Ct. 1048, 67 L.Ed.2d 155 (1981). Second, the Board determined that the claims at issue fell into none of the four statutory categories of patentable subject matter: “process, machine, manufacture, or composition of matter.” 35 U.S.C. § 101. In the Board’s view, the claims were not directed to a process because they did not “recite acts”; not a machine because “the signal ... has no concrete tangible physical structure”; and “not composed of matter and [therefore] clearly not a ‘composition of matter.’ ” Finally, the Board noted that “[t]he signal does not have any physical structure or substance and does not fit the definition of a ‘manufacture’ which requires a tangible object.” Accordingly, the Board rejected Claims 14 and 22-24 solely on the basis of unpatenta-bility under § 101. Nuijten timely appealed the Board’s decision to this court, which has jurisdiction under 28 U.S.C. § 1295(a)(4)(A).
II. DISCUSSION
Whether a claim is valid in light of § 101 is a question of law that we review de novo. AT & T Corp. v. Excel Commc’ns, Inc., 172 F.3d 1352, 1355 (Fed.Cir.1999). Section 101 states:
Whoever invents or discovers any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof, may obtain a patent therefor, subject to the conditions and requirements of this title.
Language setting forth a variety of categories of matter deemed patentable has existed throughout the history of American patent law. The country’s first patent statute permitted a patent on “any art, manufacture, engine, machine or device.” Patent Act of 1790 § 4,1 Stat. 109, 111 (1790). Soon thereafter, Congress amended the patent laws, changing the language to allow a patent for “any new and useful art, machine, manufacture or composition of matter.” Patent Act of 1793 § 1, 1 Stat. 318, 319 (1793).2 The next substantial amendment to the patent laws left this statutory language unchanged. Patent Act of 1836 § 6, 5 Stat. 117, 119 (1836). This four-category language has persisted to the present day, with the exception of the technical change of “art” to “process,” defined as “process, art or method,” in 1952. 35 U.S.C. § 100; see Part II.B.l, infra.
The claims on appeal cover transitory electrical and electromagnetic signals propagating through some medium, such as wires, air, or a vacuum. Those types of signals are not encompassed by any of the four enumerated statutory categories: “process, machine, manufacture, or composition of matter.” Before addressing in detail our rationale for this rejection, though, we begin by resolving a dispute between Nuijten and the PTO about the claims’ scope.
A. Claim Construction
As in any other context in which the scope and meaning of the claims bears on the ultimate determination at hand, we must start by considering the issue of claim construction. See State St. Bank & Trust Co. v. Signature Fin. Group, 149 F.3d 1368, 1370 (Fed.Cir.1998) (stating that “whether the ... patent is invalid for failure to claim statutory subject matter under § 101[ ] is a matter of both claim construction and statutory construction”). Claim construction is a question of law [1353] reviewed de novo on appeal. In re Baker Hughes, Inc., 215 F.3d 1297, 1301 (Fed.Cir.2000); Cybor Corp. v. FAS Techs., Inc., 138 F.3d 1448, 1451 (Fed.Cir.1998) {en banc).
The claim construction dispute between Nuijten and the PTO turns on a somewhat esoteric and metaphysical point, namely: are the claims at issue limited to covering only physical instances of signals, or do they also cover intangible, immaterial strings of abstract numbers? The PTO suggests that “claim 14 can be read to claim a signal that is merely data”—that is, merely numerical information without any physical embodiment. Nuijten disagrees, arguing that “a signal must have sufficient physical substance to be. discerned and recognized by a recipient.” That is, a signal can be sensed and received by some physical apparatus, if not directly by a person.
Nuijten’s position on this issue is correct in a limited way. A “signal” implies signaling—that is, the conveyance of information. To convey information to a recipient a physical carrier, such as an electromagnetic wave, is needed. Thus, in order to be a “signal,” as required by the claim, some carrier upon which the information is embedded is required. See Arrhythmia Research Tech., Inc. v. Corazonix Corp., 958 F.2d 1053, 1059 (Fed.Cir.1992) (“The view that there is nothing necessarily physical about ‘signals’ is incorrect.” (quotation marks omitted)).
However, while the claims are limited so as to require some physical carrier of information, they do not in any way specify what carrier element is to be used. The only limitations in Claim 14 address the signal’s informational content. Specifically, the signal must encode some supplemental data, it must have been encoded according to a “given encoding process,” and a sample, or single data point, located before the location of the supplemental data must be different from the original. The text of the claims is not limited by any specified physical medium, nor do the dependent claims add any physical limitations. They again require only that the signal carry certain information—a watermark, video, or audio. Therefore, any tangible means of information carriage will suffice for all of the claims at issue. Nui-jten’s claims can of course be embodied by conventional, known means, such as electrical signals, modulated electromagnetic waves, and pulses in fiber optic cable. So long as some object or transmission carries the information specified by Nuijten’s claim, it falls within that claim’s scope regardless of its physical form. In summary, some physical form for the signal is required, but any form will do, so long as a recipient can understand the message— the nature of the signal’s physical carrier is totally irrelevant to the claims at issue.
B. The Appealed Claims are Not in Any Statutory Category
Nuijten and the PTO agree that the claims include physical but transitory forms of signal transmission such as radio broadcasts, electrical signals through a wire, and light pulses through a fiber-optic cable, so long as those transmissions convey information encoded in the manner disclosed and claimed by Nuijten. We hold that such transitory embodiments are not directed to statutory subject matter.
Our inquiry here, like that of the Board, will consider whether a transitory, propagating signal is within any of the four statutory categories: process, machine, manufacture, or composition of matter. Before embarking on an analysis considering each of the four categories, we must address a prior statement of this court which Nuijten argues forecloses such an analysis. In State Street Bank & Trust Co. v. Signature Financial Group, Inc., [1354] 149 F.3d 1368, 1375 (Fed.Cir.1998), we noted that “[t]he question of whether a claim encompasses statutory subject matter should not focus on which of the four categories of subject matter a claim is directed to — process, machine, manufacture, or composition of matter — but rather on the essential characteristics of the subject matter, in particular, its practical utility.” However, we do not consider this statement as a holding that the four statutory categories are rendered irrelevant, non-limiting, or subsumed into an overarching question about patentable utility. Indeed, State Street recognized that “the [claimed] subject matter must fall into at least one category of statutory subject matter,” id. at 1375 n. 9, and specifically found that the claim at issue was directed to a machine, id. at 1375. In telling courts where they “should not focus” their analysis, State Street was advising not to be concerned about debates over “which of the four categories,” id. (emphasis added), subject matter falls into — that is, not to be overly concerned with pigeonholing subject matter once the court assures itself that some category has been satisfied. If, for instance, a court determines that a claim encompasses either a process or machine but is unsure which category is appropriate, it need not resolve the ambiguity. The claim must be within at least one category, so the court can proceed to other aspects of the § 101 analysis.3 See State Street, 149 F.3d at 1371 (“[I]t is of little relevance whether claim 1 is directed to a ‘machine’ or a ‘process.’”). State Street sets forth a sound premise, but this case presents a different situation. The essence of the dispute between the parties is whether a transitory signal is covered by any statutory category. The four categories together describe the exclusive reach of patentable subject matter. If a claim covers material not found in any of the four statutory categories, that claim falls outside the plainly expressed scope of § 101 even if the subject matter is otherwise new and useful. We must therefore determine whether any of the four categories encompass the claims on appeal, and it is appropriate to consider each of the categories in turn.
1. Process
Nuijten suggests that a signal of the type covered by the claims is a “process” under that term’s statutory meaning, arguing both that a process need not be defined by reference to an act or series of steps, and that his signal claims do refer to the performance of acts. Nuijten first notes that “process” is defined as “process, art or method,” 35 U.S.C. § 100, and suggests that an “art” is somehow broader than the usual understanding of “process,” transcending any requirement of steps or action. The presence of “art” in the statutory definition and its meaning can best be understood by reference to the legislative history of the Patent Act of 1952. The report of the Senate Committee on the Judiciary notes:
The present law [i.e., the pre-1952 patent statute] states that any person who has invented or discovered any “new and useful art, machine, manufacture, or any new or useful improvement thereof’ may obtain a patent. That language has been preserved except that the word “art” which appears in the present statute has been changed to the word “process.” “Art” in this place in the present statute has a different meaning than the words “useful art” in the Constitution, [1355] and a different meaning than the use of the word “art” in other places in the statutes, and it is interpreted by the courts to be practically synonymous with process or method. The word “process” has been used to avoid the necessity of explanation that the word “art” as used in this place means “process or method,” and that it does not mean the same thing as the word “art” in other places. The definition of “process” has been added in section 100 to make it clear that “process or method” is meant, and also to clarify the present law as to the patentability of certain types of processes or methods as to which some insubstantial doubts have been expressed.
S.Rep. No. 82-1979, at 5 (1952), U.S.Code Cong. & AdmimNews 1952, pp. 2394, 2398-99. The Supreme Court and this court have consistently interpreted the statutory term “process” to require action. See Gottschalk v. Benson, 409 U.S. 63, 70, 93 S.Ct. 253, 34 L.Ed.2d 273 (1972) (“A process is a mode of treatment of certain materials to produce a given result. It is an act, or a series of acts, performed upon the subject-matter to be transformed and reduced to a different state or thing.” (emphasis added) (quoting Cochrane v. Deener, 94 U.S. 780, 788, 24 L.Ed. 139 (1876))); NTP, Inc. v. Research in Motion, Ltd., 418 F.3d 1282, 1316 (Fed.Cir.2005) (“A process is a series of acts.” (quoting Minton v. Nat’l Ass’n of Sec. Dealers, Inc., 336 F.3d 1373, 1378 (Fed.Cir.2003))); In re Kollar, 286 F.3d 1326, 1332 (Fed.Cir.2002) (“[A] process ... consists of a series of acts or steps.... It consists of doing something, and therefore has to be carried out or performed.”). Nuijten’s argument that his claims might be covered by the “process” category even if they do not recite acts therefore lacks merit.
Nuijten also notes that his signal claims recite acts, noting that the claimed signal must be “encoded in accordance with a given encoding process.” But all that recitation implies is that these are potentially product-by-process claims “in which the product is defined at least in part in terms of the method or process by which it is made.” SmithKline Beecham Corp. v. Apotex Corp., 439 F.3d 1312, 1315 (Fed.Cir.2006) (quoting Bonito Boats, Inc. v. Thunder Craft Boats, Inc., 489 U.S. 141, 158, 109 S.Ct. 971, 103 L.Ed.2d 118 (1989)). Such claims are still directed to the ultimate product, not the underlying process. See Id. at 1317 (“Regardless of how broadly or narrowly one construes a product-by-process claim, it is clear that such claims are always to a product, not a process.”). The presence of acts recited in the claim does not transform a claim covering a thing — the signal itself — into one covering the process by which that thing was made.
Since a process claim must cover an act or series of acts and Nuijten’s signal claims do not, the claims are not directed to a process.
2. Machine
The Supreme Court has defined the term “machine” as “a concrete thing, consisting of parts, or of certain devices and combination of devices.” Burr v. Duryee, 68 U.S. (1 Wall.) 531, 570, 17 L.Ed. 650 (1863). This “includes every mechanical device or combination of mechanical powers and devices to perform some function and produce a certain effect or result.” Corning v. Burden, 56 U.S. 252, 267, 15 How. 252, 14 L.Ed. 683 (1853). A transitory signal made of electrical or electromagnetic variances is not made of “parts” or “devices” in any mechanical sense. While such a signal is physical and real, it does not possess concrete structure in the sense implied by these definitions. No part of the signal — the crests or troughs of the electromagnetic wave, or perhaps the particles that make it up (modern physics teaches that both features are present simultaneously) is a mechani[1356] cal “device” or “part.” A propagating electromagnetic signal is not a “machine” as that term is used in § 101.4
3. Manufacture
The question of whether the claimed signals are “manufactures” is more difficult. They are man-made, in the sense of having been encoded, generated, and transmitted by artificial means. However, artificiality is insufficient by itself to render something a “manufacture.” The Supreme Court has defined “manufacture” (in its verb form) as “the production of articles for use from raw or prepared materials by giving to these materials new forms, qualities, properties, or combinations, whether by hand-labor or by machinery.” Diamond v. Chakrabarty, 447 U.S. 303, 308, 100 S.Ct. 2204, 65 L.Ed.2d 144 (1980) (emphasis added) (quoting Am. Fruit Growers, Inc. v. Brogdex Co., 283 U.S. 1, 11, 51 S.Ct. 328, 75 L.Ed. 801 (1931)).5 The term is used in the statute in its noun form, Bayer AG v. Housey Pharms., Inc., 340 F.3d 1367, 1373 (Fed.Cir.2003), and therefore refers to “articles” resulting from the process of manufacture. The same dictionary the Supreme Court relied on for its definition of “manufacture” in turn defines “article” as “a particular substance or commodity: as, an article of merchandise; an article of clothing; salt is a necessary article.” 1 Century Dictionary 326 (William Dwight Whitney ed., 1895) (emphasis in original).
These definitions address “articles” of “manufacture” as being tangible articles or commodities. A transient electric or electromagnetic transmission does not fit within that definition. While such a transmission is man-made and physical — it exists in the real world and has tangible causes and effects — it is a change in electric potential that, to be perceived, must be measured at a certain point in space and time by equipment capable of detecting and interpreting the signal. In essence, energy embodying the claimed signal is fleeting and is devoid of any semblance of permanence during transmission.6 Moreover, any tangibility arguably attributed to a signal is embodied in the principle that it is perceptible — e.g., changes in electrical potential can be measured. All signals within the scope of the claim do not themselves comprise some tangible article or commodity.7 This is particularly true [1357] when the signal is encoded on an electromagnetic carrier and transmitted through a vacuum — a medium that, by definition, is devoid of matter.8 Thus, we hold that Nuijten’s signals, standing alone, are not “manufacture^]” under the meaning of that term in § 101.9
4. Composition of matter
As to the final statutory category, Nuijten does not- challenge in his opening brief the Board’s conclusion that “[t]he signal is not composed of matter and is clearly not a ‘composition of matter.’ ” We note, however, that the Supreme Court has defined “composition of matter” to mean “all compositions of two or more substances and all composite articles, whether they be the results of chemical union, or of mechanical mixture, or whether they be gases, fluids, powders or solids.” Chakrabarty, 447 U.S. at 308, 100 S.Ct. 2204. A signal comprising a fluctuation in electric potential or in electromagnetic fields is not a “chemical union,” nor a gas, fluid, powder, or solid. Nuijten’s signals are not “composition^ of matter.”
III. CONCLUSION
A transitory, propagating signal like Nuijten’s is not a “process, machine, manufacture, or composition of matter.” Those four categories define the explicit scope and reach of subject matter patentable under 35 U.S.C. § 101; thus, such a signal cannot be patentable subject matter. The Board’s rejection of the application’s Claims 14, 22, 23, and 24 is therefore
AFFIRMED.
Each party shall bear its own costs.