AC3
AC3 (or AC-3) is an abbreviation with multiple meanings in various fields. == Science and technology == == Transportation == == Video games == == Other uses ==Science and Technology
Dolby AC-3
Dolby AC-3, also known as Audio Coding 3 or Dolby Digital, is a perceptual audio compression format developed by Dolby Laboratories for efficient encoding of multichannel digital audio signals. It employs a lossy compression algorithm that discards inaudible audio data based on human psychoacoustics, enabling high-quality surround sound transmission at reduced bitrates compared to uncompressed PCM audio. The format was commercially released in 1991 as the first MDCT-based audio compression standard, initially targeting cinema applications to replace analog optical soundtracks on 35 mm film prints. Its development stemmed from Dolby's efforts in the late 1980s to create a digital alternative to existing surround systems like Dolby Stereo, culminating in the first theatrical release of an AC-3 soundtrack with Star Trek VI: The Undiscovered Country in December 1991. Although initially competing with Digital Theater Systems (DTS) for cinema adoption, AC-3 became the dominant standard following its selection by the Advanced Television Systems Committee (ATSC) in 1994. Technically, AC-3 utilizes the modified discrete cosine transform (MDCT) as its core compression method, dividing audio into 512-sample blocks for frequency-domain analysis and applying bit allocation to prioritize audible components. It supports up to 5.1 channels—comprising five full-bandwidth channels (left, center, right, left surround, right surround) plus a low-frequency effects (LFE) channel—while ensuring backward compatibility through matrixed stereo downmixing and rematrixing for legacy Dolby Surround decoders. Bitrates range from 32 kbit/s to 640 kbit/s, with common settings at 384 kbit/s for 5.1-channel content, and sample rates of 32 kHz, 44.1 kHz, or 48 kHz. Dynamic range is managed via dialogue normalization (dialnorm), a 5-bit metadata parameter that adjusts playback loudness relative to full-scale digital levels (providing 1–31 dB of headroom), alongside dynamic range control (dynrng) for scene-specific compression up to ±24 dB per audio block. These features maintain consistent volume across programs and prevent overload in consumer equipment.[1][2] AC-3 played a pivotal role in consumer media, serving as the primary audio codec for DVD-Video upon its 1995 launch, where every DVD worldwide included Dolby Digital 5.1 tracks encoded at 384 or 448 kbit/s. It was also mandated for ATSC HDTV broadcasts in the US and Canada starting in 1996, as well as digital cable and satellite TV standards, enabling efficient delivery of surround sound over limited bandwidth. In Blu-ray Disc, AC-3 supports up to 640 kbit/s for 5.1 channels, though higher-capacity formats like Dolby TrueHD have supplemented it. Dolby's licensing model required manufacturers of decoders, players, and broadcasters to pay royalties under patents covering AC-3 technology, generating revenue while ensuring widespread adoption; notable disputes included a 2005 U.S. court ruling that AC-3 did not infringe Lucent Technologies' patents, affirming Dolby's intellectual property position. The format evolved into Dolby Digital Plus (E-AC-3) in 2004, which extends AC-3's core while supporting higher channel counts and bitrates up to 6.144 Mbit/s for advanced applications.[2][1][3]AC-3 Algorithm
The AC-3 algorithm, short for Arc Consistency Algorithm #3, is a method for enforcing arc consistency in constraint satisfaction problems (CSPs), which are formal problems involving variables with finite domains and constraints defining compatible value assignments.[4] It improves upon earlier algorithms AC-1 and AC-2 by using a queue to process only potentially inconsistent arcs, avoiding redundant domain revisions and enhancing efficiency in practice for problems like scheduling or puzzle-solving.[4] AC-3 operates by iteratively revising variable domains to remove values lacking support from neighboring variables under the given binary constraints. For an arc (i, j) from variable X_i to X_j, a value x in domain D_i is supported if there exists a y in D_j such that the constraint P_{ij}(x, y) holds; unsupported values are deleted.[4] This process reduces the worst-case time complexity per arc revision to O(d^2), where d is the maximum domain size, compared to O(d^3) in naive implementations, though the overall complexity is O(e d^3) with e constraints due to potential queue expansions.[5] The algorithm begins by ensuring node consistency for all variables, then initializes a queue with all directed arcs in the constraint graph (excluding self-loops). While the queue is non-empty, it selects an arc (k, m), applies a revision step to D_k based on D_m, and if any deletions occur, adds all incoming arcs to k (except from m) back to the queue to propagate changes.[4] The core revision procedure, REVISE((i,j)), scans each value in D_i and deletes those without support in D_j, returning true if modifications were made:[4] Developed by Alan Mackworth in 1977, AC-3 has been foundational in AI for tackling CSPs in areas such as planning, configuration problems, and optimization tasks like resource allocation.[4] It is integrated into modern solvers, for example, as variants like GAC3rm in the Choco constraint programming library, where it supports efficient propagation for binary and generalized constraints.procedure REVISE((i,j)): DELETE := false; for each x ∈ D_i do if there is no y ∈ D_j such that P_{ij}(x, y) then delete x from D_i; DELETE := true; end; return DELETE; endprocedure REVISE((i,j)): DELETE := false; for each x ∈ D_i do if there is no y ∈ D_j such that P_{ij}(x, y) then delete x from D_i; DELETE := true; end; return DELETE; end
Adenylate Cyclase 3
Adenylyl cyclase 3 (AC3), also known as ADCY3, is a membrane-bound isoform of adenylyl cyclase that catalyzes the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) and pyrophosphate (PPi), serving as a critical enzyme in G-protein-coupled receptor (GPCR) signaling pathways where cAMP functions as a second messenger to modulate intracellular processes such as ion channel activity and gene expression.[6] Structurally, AC3 features 12 transmembrane helices organized into two bundles that anchor it to the plasma membrane, along with two cytoplasmic catalytic domains (C1 and C2) that form a heterodimer essential for ATP binding and catalysis; the reaction is represented as: \text{ATP} \rightarrow \text{cAMP} + \text{PP}_\text{i} This enzyme is selectively activated by the stimulatory G protein alpha subunit (Gαs) upon GPCR stimulation, enhancing cAMP production in a calcium-independent manner, which distinguishes it from calcium-sensitive isoforms such as AC1 and AC8.[7] AC3 is predominantly expressed in olfactory sensory neurons, primary cilia of various cell types, and hypothalamic regions, playing pivotal roles in sensory transduction, energy homeostasis, and ciliary function. In olfaction, AC3 localizes to the cilia of olfactory receptor neurons, where it generates cAMP in response to odorant-induced GPCR activation, facilitating signal amplification for smell detection.[6] In the hypothalamus and brown adipose tissue, AC3 contributes to thermogenesis and body weight regulation by coupling β-adrenergic receptor signaling to cAMP-mediated lipolysis and heat production, thereby influencing metabolic rate and appetite control.[8] Additionally, its presence in primary cilia positions AC3 as a key regulator of ciliogenesis and ciliary signaling, impacting cellular polarity and sensory functions beyond olfaction.[9] Genetic studies, particularly in knockout mouse models, have revealed AC3's essentiality, with Adcy3-null mice displaying profound anosmia due to disrupted olfactory cilia morphology and impaired odorant signal transduction, alongside obesity characterized by increased fat mass, hyperphagia, reduced locomotor activity, and leptin resistance.[10] In humans, homozygous loss-of-function mutations in ADCY3 have been identified as a cause of monogenic severe early-onset obesity.[11] These phenotypes extend to ciliary dysfunction; AC3 deficiency produces phenotypes reminiscent of those in ciliopathies like Bardet-Biedl syndrome, involving disrupted ciliary signaling and defective leptin receptor trafficking in hypothalamic cilia.[12] A 2021 study highlighted AC3's involvement in neural crest-derived development, particularly in the enteric nervous system (ENS), where its absence leads to atrophy of the colonic mucosal neural network, underscoring its role in ENS maturation and gastrointestinal motility.[13] Furthermore, a truncated isoform, AC3-AT, emerges as a cold-induced regulator that limits excessive cAMP signaling in brown fat, acting as a rheostat to prevent metabolic imbalances; genetic depletion of AC3-AT in mice enhances energy expenditure, confers resistance to diet-induced obesity, and improves glucose homeostasis, positioning AC3-AT inhibition as a promising therapeutic strategy for obesity treatment.[14]Transportation
Comte AC-3
The Comte AC-3 was a high-wing semicantilever monoplane designed and built by the Swiss manufacturer Flugzeugbau A. Comte as a multi-role aircraft capable of serving in both bomber and transport capacities during the late 1920s.[15] It employed mixed construction, with a rectangular steel-tube fuselage that was fabric-covered and featured a plywood-lined cabin for structural integrity and ease of maintenance.[15] The aircraft was powered by two 600 hp Hispano-Suiza 12Lb inline engines arranged in tandem—one tractor and one pusher—mounted above the fuselage on eight struts, enabling a maximum bomb load of 2,000 kg or accommodation for up to 15 passengers via a dedicated cargo hatch and bomb rack positioned at the center of gravity.[15] Defensive armament options included machine guns operated from three open cockpits: one in the nose for a gunner or observer, one forward of the wing for the pilot, and one on the upper rear fuselage for a rear gunner.[15] Developed amid interwar military interests, the AC-3 prototype was constructed with an eye toward export, particularly to the Bolivian Air Force in anticipation of conflicts like the Chaco War (1932–1935). The prototype first flew on 22 February 1930 at Dübendorf airfield. Although an order for three was placed by Bolivia, it was cancelled due to financial issues, and only one was built.[16] Only one example was built due to funding limitations and performance shortfalls identified during testing, preventing any operational deployment or further production.[16] The aircraft's large size and twin-engine setup, while innovative for Swiss engineering at the time, contributed to its marginal speed and climb rates, ultimately leading to its dismantlement without entering service.[15]| Specification | Value |
|---|---|
| Wingspan | 26 m (85.3 ft) |
| Length | 18 m (59.1 ft) |
| Height | 5.8 m (19.0 ft) |
| Wing area | 94 m² (1,012 sq ft) |
| Empty weight | 3,400 kg (7,496 lb) |
| Maximum takeoff weight | 6,000 kg (13,228 lb) |
| Maximum speed | 195 km/h (121 mph) at sea level |
| Service ceiling | 7,300 m (23,950 ft) at 4,500 kg |