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Viterbi algorithm

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be tracing back which states were used when calculating the maxima (which happens to be the best guess from each day but will not always be). In other words, given the observed activities, the patient was most likely to have been healthy on the first day and also on the second day (despite feeling cold that day), and only to have contracted a fever on the third day.
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From the table, it can be seen that the patient most likely had a fever on the third day. Furthermore, there exists a sequence of states ending on "fever", of which the probability of producing the given observations is 0.01512. This sequence is precisely (healthy, healthy, fever), which can be found
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have become standard terms for the application of dynamic programming algorithms to maximization problems involving probabilities. For example, in statistical parsing a dynamic programming algorithm can be used to discover the single most likely context-free derivation (parse) of a string, which is
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of possible outcomes, the Lazy Viterbi algorithm maintains a prioritized list of nodes to evaluate in order, and the number of calculations required is typically fewer (and never more) than the ordinary Viterbi algorithm for the same result. However, it is not so easy to parallelize in hardware.
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init = {"Healthy": 0.6, "Fever": 0.4} trans = { "Healthy": {"Healthy": 0.7, "Fever": 0.3}, "Fever": {"Healthy": 0.4, "Fever": 0.6}, } emit = { "Healthy": {"normal": 0.5, "cold": 0.4, "dizzy": 0.1}, "Fever": {"normal": 0.1, "cold": 0.3, "dizzy": 0.6}, }
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The probabilities for each of the following days can be calculated from the previous day directly. For example, the highest chance of being healthy on the second day and reporting to be cold, following reporting being normal on the first day, is the maximum of
167:(speech recognition), the acoustic signal is treated as the observed sequence of events, and a string of text is considered to be the "hidden cause" of the acoustic signal. The Viterbi algorithm finds the most likely string of text given the acoustic signal. 1404:
represent how likely each possible observation (normal, cold, or dizzy) is, given the underlying condition (healthy or fever). A patient who is healthy has a 50% chance of feeling normal; one who has a fever has a 60% chance of feeling dizzy.
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A doctor wishes to determine whether patients are healthy or have a fever. The only information the doctor can obtain is by asking patients how they feel. The patients may report that they either feel normal, dizzy, or cold.
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represent the change of health condition in the underlying Markov chain. In this example, a patient who is healthy today has only a 30% chance of having a fever tomorrow. The emission probabilities
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represents the doctor's belief about how likely the patient is to be healthy initially. Note that the particular probability distribution used here is not the equilibrium one, which would be
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Firstly, the probabilities of being healthy or having a fever on the first day are calculated. The probability that a patient will be healthy on the first day and report feeling normal is
917:{\displaystyle P_{t,s}={\begin{cases}\pi _{s}\cdot b_{s,o_{t}}&{\text{if }}t=0,\\\max _{r\in S}\left(P_{t-1,r}\cdot a_{r,s}\cdot b_{s,o_{t}}\right)&{\text{if }}t>0.\end{cases}}} 329: 1370:
from the doctor. On each day, the chance that a patient tells the doctor "I feel normal", "I feel cold", or "I feel dizzy", depends only on the patient's health condition on that day.
415: 1713:, has been proposed. For many applications of practical interest, under reasonable noise conditions, the lazy decoder (using Lazy Viterbi algorithm) is much faster than the original 1481: 1449: 54: 1702:, one can find the subsequence of an observation that matches best (on average) to a given hidden Markov model. This algorithm is proposed by Qi Wang et al. to deal with 1002: 2109: 577: 955: 643: 610: 526: 451: 975: 376: 1348: 1257: 1237: 1043: 1023: 703: 683: 663: 546: 491: 471: 349: 264: 244: 2513: 2132:
Qi Wang; Lei Wei; Rodney A. Kennedy (2002). "Iterative Viterbi Decoding, Trellis Shaping, and Multilevel Structure for High-Rate Parity-Concatenated TCM".
2016:. Proc. 2003 Conf. of the North American Chapter of the Association for Computational Linguistics on Human Language Technology (NAACL). pp. 40–47. 1683:(HMM), with a limited number of connections between variables and some type of linear structure among the variables. The general algorithm involves 2518: 1416:
A particular patient visits three days in a row, and reports feeling normal on the first day, cold on the second day, and dizzy on the third day.
1868: 1706:. Iterative Viterbi decoding works by iteratively invoking a modified Viterbi algorithm, reestimating the score for a filler until convergence. 2296: 2243: 2385: 1931: 1381:
states (healthy, fever) form a hidden Markov model (HMM). From past experience, the probabilities of this model have been estimated as:
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prob ← new_prob prev ← r path ← empty array of length T path ← the state s with maximum prob
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The first step in the SOVA is the selection of the survivor path, passing through one unique node at each time instant,
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SOVA differs from the classical Viterbi algorithm in that it uses a modified path metric which takes into account the
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A tutorial for a Hidden Markov Model toolkit (implemented in C) that contains a description of the Viterbi algorithm
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Rabiner LR (February 1989). "A tutorial on hidden Markov models and selected applications in speech recognition".
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Viterbi AJ (April 1967). "Error bounds for convolutional codes and an asymptotically optimum decoding algorithm".
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Feldman J, Abou-Faycal I, Frigo M (2002). "A fast maximum-likelihood decoder for convolutional codes".
1422: 1808: 1366:. There are two states, "healthy" and "fever", but the doctor cannot observe them directly; they are 2331: 2226: 1079:
obs: sequence of T observations prob ← T × S matrix of zeroes prev ← empty T × S matrix
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is the number of edges in the graph, i.e. the number of non-zero entries in the transition matrix.
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Quach, T.; Farooq, M. (1994). "Maximum Likelihood Track Formation with the Viterbi Algorithm".
218:, where the track is computed that assigns a maximum likelihood to a sequence of observations. 2292: 2239: 2068: 1928: 1883: 555: 1717:(using Viterbi algorithm). While the original Viterbi algorithm calculates every node in the 927: 615: 582: 498: 423: 2448: 2336: 2270: 2231: 2203: 2172: 2141: 2091: 2058: 2050: 2017: 1987: 1828:. Since each node has 2 branches converging at it (with one branch being chosen to form the 1668: 180: 152: 144: 2159: 960: 354: 2482: 2455: 1935: 1893: 1718: 1714: 1664: 1660: 1640: 108:—that results in a sequence of observed events. This is done especially in the context of 2475: 2212:(note: the Viterbi decoding algorithm is described in section IV.) Subscription required. 1679:. The latent variables need, in general, to be connected in a way somewhat similar to a 2401: 2405: 2063: 2038: 1333: 1242: 1222: 1028: 1008: 688: 668: 648: 531: 476: 456: 334: 249: 229: 176: 164: 160: 2507: 1978: 2348: 2008: 2302: 2253: 1363: 2465: 2037:
Stanke, M.; Keller, O.; Gunduz, I.; Hayes, A.; Waack, S.; Morgenstern, B. (2006).
2470: 17: 2371:, "The sensitivity of the modified Viterbi algorithm to the source statistics," 2235: 2176: 1735: 1408: 1362:
It is believed that the health condition of the patients operates as a discrete
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Efficient parsing of highly ambiguous context-free grammars with bit vectors
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29 Apr 2005, G. David Forney Jr: The Viterbi Algorithm: A Personal History
1659:) can be used to find the most likely assignment of all or some subset of 1643:. The Viterbi path is essentially the shortest path through this trellis. 2360:, "Experiments in text recognition with the modified Viterbi algorithm," 2054: 2443: 1396:
according to the transition probabilities. The transition probabilities
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over noisy digital communication links. It has, however, a history of
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has an implementation as part of its support for stochastic processes
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The operation of Viterbi's algorithm can be visualized by means of a
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The rest of the probabilities are summarised in the following table:
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be the initial and transition probabilities respectively, and let
2353:(Describes the forward algorithm and Viterbi algorithm for HMMs). 2427:
signal processing framework provides the C++ implementation for
1986:. Proc. 20th Int'l Conf. on Computational Linguistics (COLING). 124: 2283:
Press, WH; Teukolsky, SA; Vetterling, WT; Flannery, BP (2007).
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commonly called the "Viterbi parse". Another application is in
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on convolutional coding with viterbi decoding, by Chip Fleming
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IEEE Transactions on Pattern Analysis and Machine Intelligence
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IEEE Transactions on Pattern Analysis and Machine Intelligence
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is accumulated over the entire sliding window (usually equals
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The algorithm has found universal application in decoding the
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Proceedings of 33rd IEEE Conference on Decision and Control
2039:"AUGUSTUS: Ab initio prediction of alternative transcripts" 910: 493:, out of all possible sequences of states leading up to it. 2168: 2161:
A fast maximum-likelihood decoder for convolutional codes
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Given a hidden Markov model with a set of hidden states
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contains the maximum probability of ending up at state
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A generalization of the Viterbi algorithm, termed the
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Implementations in Java, F#, Clojure, C# on Wikibooks
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Proceedings IEEE 56th Vehicular Technology Conference
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may be too technical for most readers to understand
2289:Numerical Recipes: The Art of Scientific Computing 1929:"Speaker Diarization: A Review of Recent Research" 1555: 1517: 1475: 1443: 1342: 1322: 1251: 1231: 1211: 1037: 1017: 996: 969: 949: 916: 697: 677: 657: 637: 604: 571: 540: 520: 485: 465: 445: 409: 370: 343: 323: 258: 238: 135:modems, satellite, deep-space communications, and 2261:Forney GD (March 1973). "The Viterbi algorithm". 1137:new_prob ← prob * trans * emit] 991: 964: 799: 1966:. Pearson Education International. p. 246. 1556:{\displaystyle 0.04\times 0.4\times 0.4=0.0064} 1212:{\displaystyle O(T\times \left|{S}\right|^{2})} 528:tracks the previous state that was used before 139:wireless LANs. It is now also commonly used in 2010:A* parsing: fast exact Viterbi parse selection 1691:algorithm (which is the generalization of the 1067:init: initial probabilities of each state 1518:{\displaystyle 0.3\times 0.7\times 0.4=0.084} 8: 2375:, vol. PAMI-2, March 1980, pp. 181–185. 2364:, Vol. PAMI-l, April 1979, pp. 184–193. 2108:: CS1 maint: multiple names: authors list ( 2007:Klein, Dan; Manning, Christopher D. (2003). 1764:. Unsourced material may be challenged and 548:in this maximum probability state sequence. 1851:five constraint lengths), to indicate the 324:{\displaystyle o_{0},o_{1},\dots ,o_{T-1}} 2330: 2225: 2062: 2021: 1991: 1784:Learn how and when to remove this message 1530: 1492: 1456: 1424: 1412:Graphical representation of the given HMM 1335: 1305: 1289: 1268: 1244: 1224: 1200: 1191: 1172: 1030: 1010: 983: 962: 935: 929: 893: 878: 867: 848: 823: 802: 777: 767: 756: 743: 731: 716: 710: 690: 670: 650: 623: 617: 590: 584: 563: 557: 533: 506: 500: 478: 458: 431: 425: 398: 386: 362: 356: 336: 309: 290: 277: 271: 251: 231: 98:maximum a posteriori probability estimate 73:Learn how and when to remove this message 57:, without removing the technical details. 1568: 1167:The time complexity of the algorithm is 1056:Viterbi(states, init, trans, emit, obs) 410:{\displaystyle T\times \left|{S}\right|} 2196:IEEE Transactions on Information Theory 1938:, retrieved 19. August 2010, IEEE TASLP 1920: 2101: 27:Finds likely sequence of hidden states 1957: 1955: 1813:of the input symbols, and produces a 1377:(normal, cold, dizzy) along with the 1025:at the final timestep, and following 705:are given by the recurrence relation 175:The Viterbi algorithm is named after 104:sequence of hidden states—called the 55:make it understandable to non-experts 7: 1762:adding citations to reliable sources 1687:and is substantially similar to the 1263:one can show that the complexity is 1107:// t = 0 has been dealt with already 2514:Eponymous algorithms of mathematics 2500:includes code for Viterbi decoding. 2171:. December 2002. pp. 371–375. 2134:IEEE Transactions on Communications 1071:trans: S × S transition matrix 1962:Daniel Jurafsky; James H. Martin. 1869:Expectation–maximization algorithm 1476:{\displaystyle 0.4\times 0.1=0.04} 25: 2220:. Vol. 1. pp. 371–375. 2090:. Vol. 1. pp. 271–276. 1444:{\displaystyle 0.6\times 0.5=0.3} 2285:"Section 16.2. Viterbi Decoding" 1734: 1394:{'Healthy': 0.57, 'Fever': 0.43} 1075:emit: S × O emission matrix 645:be the probability of observing 34: 2431:codes and channel equalization 2169:Vehicular Technology Conference 2049:(Web Server issue): W435–W439. 2519:Error detection and correction 1964:Speech and Language Processing 1855:measure of reliability of the 1709:An alternative algorithm, the 1317: 1314: 1282: 1273: 1259:in the inner loop. Then using 1206: 1177: 1: 1798:soft output Viterbi algorithm 1726:Soft output Viterbi algorithm 1063:states: S hidden states 2236:10.1109/VETECF.2002.1040367 2177:10.1109/VETECF.2002.1040367 197:natural language processing 2545: 1879:Forward-backward algorithm 1859:of the Viterbi algorithm. 1700:iterative Viterbi decoding 1693:forward-backward algorithm 997:{\displaystyle \arg \max } 957:is identical, except that 110:Markov information sources 1698:With an algorithm called 1677:conditional random fields 157:computational linguistics 2429:Forward error correction 2208:10.1109/TIT.1967.1054010 1094:prob = init * emit] 572:{\displaystyle \pi _{s}} 2319:Proceedings of the IEEE 2263:Proceedings of the IEEE 2096:10.1109/CDC.1994.410918 2023:10.3115/1073445.1073461 1993:10.3115/1220355.1220379 1977:Schmid, Helmut (2004). 1927:Xavier Anguera et al., 950:{\displaystyle Q_{t,s}} 638:{\displaystyle b_{s,o}} 605:{\displaystyle a_{r,s}} 521:{\displaystyle Q_{t,s}} 446:{\displaystyle P_{t,s}} 195:. It was introduced to 2279:Subscription required. 2275:10.1109/PROC.1973.9030 2043:Nucleic Acids Research 1904:Part-of-speech tagging 1817:output indicating the 1810:a priori probabilities 1711:Lazy Viterbi algorithm 1557: 1519: 1477: 1445: 1413: 1344: 1324: 1253: 1233: 1213: 1039: 1019: 998: 971: 951: 918: 699: 679: 659: 639: 606: 573: 542: 522: 487: 467: 447: 411: 372: 345: 325: 260: 240: 201:part-of-speech tagging 2369:Godfried T. Toussaint 2358:Godfried T. Toussaint 1889:Error-correcting code 1663:in a large number of 1657:max-product algorithm 1558: 1520: 1478: 1446: 1411: 1345: 1325: 1254: 1234: 1214: 1040: 1020: 999: 972: 970:{\displaystyle \max } 952: 919: 700: 685:. Then the values of 680: 660: 640: 607: 574: 543: 523: 488: 468: 448: 412: 381:Two matrices of size 373: 371:{\displaystyle o_{t}} 346: 326: 261: 241: 1874:Baum–Welch algorithm 1758:improve this section 1673:Markov random fields 1529: 1491: 1455: 1423: 1334: 1267: 1243: 1223: 1171: 1029: 1009: 982: 961: 928: 709: 689: 669: 649: 616: 583: 556: 532: 499: 477: 457: 424: 385: 355: 335: 270: 250: 230: 189:Needleman and Wunsch 114:hidden Markov models 2524:Dynamic programming 2408:(scholarpedia.org). 1909:A* search algorithm 1899:Hidden Markov model 1681:hidden Markov model 1141:new_prob > prob 181:convolutional codes 121:convolutional codes 91:dynamic programming 2481:2012-05-02 at the 2466:Julia (HMMBase.jl) 2454:2014-05-04 at the 2188:General references 2055:10.1093/nar/gkl200 1934:2016-05-12 at the 1689:belief propagation 1553: 1515: 1473: 1441: 1414: 1340: 1320: 1261:amortized analysis 1249: 1229: 1209: 1035: 1015: 994: 967: 947: 914: 909: 813: 695: 675: 655: 635: 602: 569: 538: 518: 483: 463: 443: 407: 368: 341: 321: 256: 246:and a sequence of 236: 203:as early as 1987. 193:Wagner and Fischer 185:multiple invention 163:. For example, in 141:speech recognition 131:digital cellular, 96:for obtaining the 2406:Andrew J. Viterbi 2402:Viterbi algorithm 2367:Shinghal, R. and 2356:Shinghal, R. and 2298:978-0-521-88068-8 2245:978-0-7803-7467-6 2146:10.1109/26.975743 2122:Xing E, slide 11. 1884:Forward algorithm 1857:hard bit decision 1821:of the decision. 1794: 1793: 1786: 1669:Bayesian networks 1653:max-sum algorithm 1633: 1632: 1343:{\displaystyle E} 1252:{\displaystyle s} 1232:{\displaystyle r} 1157:path ← prev] 1038:{\displaystyle Q} 1018:{\displaystyle P} 977:is replaced with 896: 798: 780: 698:{\displaystyle P} 678:{\displaystyle s} 658:{\displaystyle o} 541:{\displaystyle s} 486:{\displaystyle t} 466:{\displaystyle s} 417:are constructed: 344:{\displaystyle t} 259:{\displaystyle T} 239:{\displaystyle S} 211:Viterbi algorithm 87:Viterbi algorithm 83: 82: 75: 18:Viterbi Algorithm 16:(Redirected from 2536: 2352: 2334: 2313: 2311: 2310: 2301:. Archived from 2278: 2257: 2229: 2211: 2181: 2180: 2166: 2156: 2150: 2149: 2129: 2123: 2120: 2114: 2113: 2107: 2099: 2083: 2077: 2076: 2066: 2034: 2028: 2027: 2025: 2015: 2004: 1998: 1997: 1995: 1985: 1974: 1968: 1967: 1959: 1950: 1945: 1939: 1925: 1789: 1782: 1778: 1775: 1769: 1738: 1730: 1665:graphical models 1661:latent variables 1569: 1562: 1560: 1559: 1554: 1524: 1522: 1521: 1516: 1482: 1480: 1479: 1474: 1450: 1448: 1447: 1442: 1403: 1399: 1395: 1391: 1349: 1347: 1346: 1341: 1329: 1327: 1326: 1321: 1313: 1309: 1297: 1293: 1258: 1256: 1255: 1250: 1238: 1236: 1235: 1230: 1218: 1216: 1215: 1210: 1205: 1204: 1199: 1195: 1044: 1042: 1041: 1036: 1024: 1022: 1021: 1016: 1003: 1001: 1000: 995: 976: 974: 973: 968: 956: 954: 953: 948: 946: 945: 924:The formula for 923: 921: 920: 915: 913: 912: 897: 894: 890: 886: 885: 884: 883: 882: 859: 858: 840: 839: 812: 781: 778: 774: 773: 772: 771: 748: 747: 727: 726: 704: 702: 701: 696: 684: 682: 681: 676: 664: 662: 661: 656: 644: 642: 641: 636: 634: 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choice. 1838:amount of error 1790: 1779: 1773: 1770: 1755: 1739: 1728: 1715:Viterbi decoder 1685:message passing 1649: 1641:trellis diagram 1527: 1526: 1489: 1488: 1453: 1452: 1421: 1420: 1401: 1397: 1393: 1389: 1386: 1356: 1332: 1331: 1301: 1285: 1265: 1264: 1241: 1240: 1221: 1220: 1187: 1186: 1169: 1168: 1165: 1051: 1027: 1026: 1007: 1006: 980: 979: 959: 958: 931: 926: 925: 908: 907: 891: 874: 863: 844: 819: 818: 814: 795: 794: 775: 763: 752: 739: 732: 712: 707: 706: 687: 686: 667: 666: 647: 646: 619: 614: 613: 586: 581: 580: 559: 554: 553: 530: 529: 502: 497: 496: 475: 474: 473:at observation 455: 454: 427: 422: 421: 394: 383: 382: 358: 353: 352: 333: 332: 305: 286: 273: 268: 267: 248: 247: 228: 227: 224: 216:target tracking 199:as a method of 173: 79: 68: 62: 59: 51:help improve it 48: 39: 35: 28: 23: 22: 15: 12: 11: 5: 2542: 2540: 2532: 2531: 2526: 2521: 2516: 2506: 2505: 2502: 2501: 2495: 2490: 2485: 2473: 2468: 2463: 2458: 2446: 2441: 2436: 2422: 2414: 2411: 2410: 2409: 2399: 2394: 2388: 2381: 2380:External links 2378: 2377: 2376: 2365: 2354: 2325:(2): 257–286. 2314: 2297: 2280: 2269:(3): 268–278. 2258: 2244: 2213: 2202:(2): 260–269. 2189: 2186: 2183: 2182: 2151: 2124: 2115: 2078: 2029: 1999: 1969: 1951: 1940: 1919: 1918: 1916: 1913: 1912: 1911: 1906: 1901: 1896: 1891: 1886: 1881: 1876: 1871: 1864: 1861: 1792: 1791: 1774:September 2023 1742: 1740: 1733: 1727: 1724: 1648: 1645: 1631: 1630: 1625: 1622: 1619: 1615: 1614: 1611: 1606: 1601: 1597: 1596: 1593: 1590: 1587: 1583: 1582: 1579: 1576: 1573: 1552: 1549: 1546: 1543: 1540: 1537: 1534: 1514: 1511: 1508: 1505: 1502: 1499: 1496: 1472: 1469: 1466: 1463: 1460: 1440: 1437: 1434: 1431: 1428: 1388:In this code, 1383: 1355: 1352: 1339: 1319: 1316: 1312: 1308: 1304: 1300: 1296: 1292: 1288: 1284: 1281: 1278: 1275: 1272: 1248: 1239:which link to 1228: 1208: 1203: 1198: 1194: 1190: 1185: 1182: 1179: 1176: 1052: 1050: 1047: 1034: 1014: 993: 990: 987: 966: 944: 941: 938: 934: 911: 906: 903: 900: 892: 889: 881: 877: 873: 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Index

Viterbi Algorithm
help improve it
make it understandable to non-experts
Learn how and when to remove this message
dynamic programming
algorithm
maximum a posteriori probability estimate
likely
Markov information sources
hidden Markov models
convolutional codes
CDMA
GSM
dial-up
802.11
speech recognition
speech synthesis
diarization
keyword spotting
computational linguistics
bioinformatics
speech-to-text
Andrew Viterbi
convolutional codes
multiple invention
Needleman and Wunsch
Wagner and Fischer
natural language processing
part-of-speech tagging
target tracking

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