Speech Recognition

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Speech to Text

This learning resource is about automatic conversion of spoken language into text, that can be stored as documents or processed as commands to control devices e.g. for handycapped people or elderly people or in a commercial setting allows to order goods and services by audio commands. The learning resource is based on the Open Community Approach so the used tools are Open Source to assure that learner have access to the tools.

Learning Tasks[edit | edit source]

  • (Applications of Speech Recognition) Analyse the possible applications of speech recognition and identify challenges of the application!
  • (Human Speech Recognition) Compare human comprehension of speech with the algorithmic speech recognition approach. What are the similarities and differences human and algorithmic speech recognition?
  • (Speech and Detection of Emotions) Speech contains more information than the encoded text. Is it possible to detect emotions in the speech with methods developed in computer science?
    • What are similarities and difference between text and emotion recognition in speech analysis?
    • What are possible application areas in digital assitants for both speech recognition and emotion recognition?
    • Analyze the different types of information systems and identify different areas of application of speech recognition and include mobile devices in your consideration!
  • (History) Analyse the history of speech recognition and compare the steps of development with current applications. Identify the major steps that are required for the current applications of speech recognition!
  • (Risk Literacy) Identify possible areas of risks and possible risk mitigation strategies if speech recognition is implemented in mobile devices, the Internet of things in general? What are required capacity building measures for business, research and development!
  • (Commercial Data Harvesting) Apply the concept of speech recognition to commercial data harvesting. What are potential benefits for generation of tailored advertisments for the users according to their generated profile? How is speech recognition contributing to user profile? What is the difference between offline and online speech recognition systems due to submission of recognized text or audio files submitted to remote servers for speech recognition?
  • (Context Awareness of Speech Recognition) The word "Fire" with a candle in your hand and with burning house in the background creates a different context and different expectations of people listening to what someone is going to tell you. Exlain why context awareness can be helpful to optimize the recognition correctness? How can a speech recognition system detect a context to the speech recognition. I.e. detecting the context without a user setting that switches to a dictation mode e.g. for medical report for X-Ray images.

Definition[edit | edit source]

Speech recognition is the interdisciplinary subfield of computational linguistics that develops methodologies and technologies that enables the recognition and translation of spoken language into text by computers. It is also known as automatic speech recognition (ASR), computer speech recognition or speech to text (STT). It incorporates knowledge and research in the linguistics, computer science, and electrical engineering fields.

Training of Speech Recognition Algorithms[edit | edit source]

Some speech recognition systems require "training" (also called "enrollment") where an individual speaker reads text or isolated vocabulary into the system. The system analyzes the person's specific voice and uses it to fine-tune the recognition of that person's speech, resulting in increased accuracy. Systems that do not use training are called "speaker independent"[1] systems. Systems that use training are called "speaker dependent".

Applications[edit | edit source]

Speech recognition applications include voice user interfaces such as voice dialing (e.g. "call home"), call routing (e.g. "I would like to make a collect call"), domotic appliance control, search (e.g. find a podcast where particular words were spoken), simple data entry (e.g., entering a credit card number), preparation of structured documents (e.g. a radiology report), determining speaker characteristics,[2] speech-to-text processing (e.g., word processors or emails), and aircraft (usually termed direct voice input).

The term voice recognition[3][4][5] or speaker identification[6][7] refers to identifying the speaker, rather than what they are saying. Recognizing the speaker can simplify the task of translating speech in systems that have been trained on a specific person's voice or it can be used to authenticate or verify the identity of a speaker as part of a security process.

From the technology perspective, speech recognition has a long history with several waves of major innovations. Most recently, the field has benefited from advances in deep learning and big data. The advances are evidenced not only by the surge of academic papers published in the field, but more importantly by the worldwide industry adoption of a variety of deep learning methods in designing and deploying speech recognition systems.

Models, methods, and algorithms[edit | edit source]

Both acoustic modeling and language modeling are important parts of modern statistically-based speech recognition algorithms. Hidden Markov models (HMMs) are widely used in many systems. Language modeling is also used in many other natural language processing applications such as document classification or statistical machine translation.

Learning Task: Applications[edit | edit source]

The following learning tasks focus on different applications of Speech Recognition. Explore the different applications.

Military[edit | edit source]

High-performance fighter aircraft[edit | edit source]

Substantial efforts have been devoted in the last decade to the test and evaluation of speech recognition in fighter aircraft. Of particular note have been the US program in speech recognition for the Advanced Fighter Technology Integration (AFTI)/F-16 aircraft (F-16 VISTA), the program in France for Mirage aircraft, and other programs in the UK dealing with a variety of aircraft platforms. In these programs, speech recognizers have been operated successfully in fighter aircraft, with applications including: setting radio frequencies, commanding an autopilot system, setting steer-point coordinates and weapons release parameters, and controlling flight display.

Working with Swedish pilots flying in the JAS-39 Gripen cockpit, Englund (2004) found recognition deteriorated with increasing g-loads. The report also concluded that adaptation greatly improved the results in all cases and that the introduction of models for breathing was shown to improve recognition scores significantly. Contrary to what might have been expected, no effects of the broken English of the speakers were found. It was evident that spontaneous speech caused problems for the recognizer, as might have been expected. A restricted vocabulary, and above all, a proper syntax, could thus be expected to improve recognition accuracy substantially.[8]

The Eurofighter Typhoon, currently in service with the UK RAF, employs a speaker-dependent system, requiring each pilot to create a template. The system is not used for any safety-critical or weapon-critical tasks, such as weapon release or lowering of the undercarriage, but is used for a wide range of other cockpit functions. Voice commands are confirmed by visual and/or aural feedback. The system is seen as a major design feature in the reduction of pilot workload,[9] and even allows the pilot to assign targets to his aircraft with two simple voice commands or to any of his wingmen with only five commands.[10]

Speaker-independent systems are also being developed and are under test for the F35 Lightning II (JSF) and the Alenia Aermacchi M-346 Master lead-in fighter trainer. These systems have produced word accuracy scores in excess of 98%.[11]

Helicopters[edit | edit source]

The problems of achieving high recognition accuracy under stress and noise pertain strongly to the helicopter environment as well as to the jet fighter environment. The acoustic noise problem is actually more severe in the helicopter environment, not only because of the high noise levels but also because the helicopter pilot, in general, does not wear a facemask, which would reduce acoustic noise in the microphone. Substantial test and evaluation programs have been carried out in the past decade in speech recognition systems applications in helicopters, notably by the U.S. Army Avionics Research and Development Activity (AVRADA) and by the Royal Aerospace Establishment (RAE) in the UK. Work in France has included speech recognition in the Puma helicopter. There has also been much useful work in Canada. Results have been encouraging, and voice applications have included: control of communication radios, setting of navigation systems, and control of an automated target handover system.

As in fighter applications, the overriding issue for voice in helicopters is the impact on pilot effectiveness. Encouraging results are reported for the AVRADA tests, although these represent only a feasibility demonstration in a test environment. Much remains to be done both in speech recognition and in overall speech technology in order to consistently achieve performance improvements in operational settings.

Training air traffic controllers[edit | edit source]

Training for air traffic controllers (ATC) represents an excellent application for speech recognition systems. Many ATC training systems currently require a person to act as a "pseudo-pilot", engaging in a voice dialog with the trainee controller, which simulates the dialog that the controller would have to conduct with pilots in a real ATC situation. Speech recognition and synthesis techniques offer the potential to eliminate the need for a person to act as pseudo-pilot, thus reducing training and support personnel. In theory, Air controller tasks are also characterized by highly structured speech as the primary output of the controller, hence reducing the difficulty of the speech recognition task should be possible. In practice, this is rarely the case. The FAA document 7110.65 details the phrases that should be used by air traffic controllers. While this document gives less than 150 examples of such phrases, the number of phrases supported by one of the simulation vendors speech recognition systems is in excess of 500,000.

The USAF, USMC, US Army, US Navy, and FAA as well as a number of international ATC training organizations such as the Royal Australian Air Force and Civil Aviation Authorities in Italy, Brazil, and Canada are currently using ATC simulators with speech recognition from a number of different vendors.[citation needed]

Usage in education and daily life[edit | edit source]

For language learning, speech recognition can be useful for learning a second language. It can teach proper pronunciation, in addition to helping a person develop fluency with their speaking skills.[12]

Students who are blind (see Blindness and education) or have very low vision can benefit from using the technology to convey words and then hear the computer recite them, as well as use a computer by commanding with their voice, instead of having to look at the screen and keyboard.[13]

Students who are physically disabled or suffer from Repetitive strain injury/other injuries to the upper extremities can be relieved from having to worry about handwriting, typing, or working with scribe on school assignments by using speech-to-text programs. They can also utilize speech recognition technology to freely enjoy searching the Internet or using a computer at home without having to physically operate a mouse and keyboard.[13]

Speech recognition can allow students with learning disabilities to become better writers. By saying the words aloud, they can increase the fluidity of their writing, and be alleviated of concerns regarding spelling, punctuation, and other mechanics of writing.[14] Also, see Learning disability.

Use of voice recognition software, in conjunction with a digital audio recorder and a personal computer running word-processing software has proven to be positive for restoring damaged short-term-memory capacity, in stroke and craniotomy individuals.

Further applications[edit | edit source]

Performance[edit | edit source]

The performance of speech recognition systems is usually evaluated in terms of accuracy and speed.[17][18] Accuracy is usually rated with word error rate (WER), whereas speed is measured with the real time factor. Other measures of accuracy include Single Word Error Rate (SWER) and Command Success Rate (CSR).

Speech recognition by machine is a very complex problem, however. Vocalizations vary in terms of accent, pronunciation, articulation, roughness, nasality, pitch, volume, and speed. Speech is distorted by a background noise and echoes, electrical characteristics. Accuracy of speech recognition may vary with the following:[19][citation needed]

  • Vocabulary size and confusability
  • Speaker dependence versus independence
  • Isolated, discontinuous or continuous speech
  • Task and language constraints
  • Read versus spontaneous speech
  • Adverse conditions

Accuracy[edit | edit source]

As mentioned earlier in this article, accuracy of speech recognition may vary depending on the following factors:

  • Error rates increase as the vocabulary size grows:
e.g. the 10 digits "zero" to "nine" can be recognized essentially perfectly, but vocabulary sizes of 200, 5000 or 100000 may have error rates of 3%, 7% or 45% respectively.
  • Vocabulary is hard to recognize if it contains confusable words:
e.g. the 26 letters of the English alphabet are difficult to discriminate because they are confusable words (most notoriously, the E-set: "B, C, D, E, G, P, T, V, Z"); an 8% error rate is considered good for this vocabulary.[citation needed]
  • Speaker dependence vs. independence:
A speaker-dependent system is intended for use by a single speaker.
A speaker-independent system is intended for use by any speaker (more difficult).
  • Isolated, Discontinuous or continuous speech
With isolated speech, single words are used, therefore it becomes easier to recognize the speech.

With discontinuous speech full sentences separated by silence are used, therefore it becomes easier to recognize the speech as well as with isolated speech.
With continuous speech naturally spoken sentences are used, therefore it becomes harder to recognize the speech, different from both isolated and discontinuous speech.


  • Task and language constraints
    • e.g. Querying application may dismiss the hypothesis "The apple is red."
    • e.g. Constraints may be semantic; rejecting "The apple is angry."
    • e.g. Syntactic; rejecting "Red is apple the."

Constraints are often represented by a grammar.

  • Read vs. Spontaneous Speech – When a person reads it's usually in a context that has been previously prepared, but when a person uses spontaneous speech, it is difficult to recognize the speech because of the disfluencies (like "uh" and "um", false starts, incomplete sentences, stuttering, coughing, and laughter) and limited vocabulary.
  • Adverse conditions – Environmental noise (e.g. Noise in a car or a factory). Acoustical distortions (e.g. echoes, room acoustics)

Speech recognition is a multi-levelled pattern recognition task.

  • Acoustical signals are structured into a hierarchy of units, e.g. Phonemes, Words, Phrases, and Sentences;
  • Each level provides additional constraints;

e.g. Known word pronunciations or legal word sequences, which can compensate for errors or uncertainties at lower level;

  • This hierarchy of constraints are exploited. By combining decisions probabilistically at all lower levels, and making more deterministic decisions only at the highest level, speech recognition by a machine is a process broken into several phases. Computationally, it is a problem in which a sound pattern has to be recognized or classified into a category that represents a meaning to a human. Every acoustic signal can be broken in smaller more basic sub-signals. As the more complex sound signal is broken into the smaller sub-sounds, different levels are created, where at the top level we have complex sounds, which are made of simpler sounds on lower level, and going to lower levels even more, we create more basic and shorter and simpler sounds. The lowest level, where the sounds are the most fundamental, a machine would check for simple and more probabilistic rules of what sound should represent. Once these sounds are put together into more complex sound on upper level, a new set of more deterministic rules should predict what new complex sound should represent. The most upper level of a deterministic rule should figure out the meaning of complex expressions. In order to expand our knowledge about speech recognition we need to take into a consideration neural networks. There are four steps of neural network approaches:
  • Digitize the speech that we want to recognize

For telephone speech the sampling rate is 8000 samples per second;

  • Compute features of spectral-domain of the speech (with Fourier transform);

computed every 10 ms, with one 10 ms section called a frame;

Analysis of four-step neural network approaches can be explained by further information. Sound is produced by air (or some other medium) vibration, which we register by ears, but machines by receivers. Basic sound creates a wave which has two descriptions: amplitude (how strong is it), and frequency (how often it vibrates per second).

Security concerns[edit | edit source]

Speech recognition can become a means of attack, theft, or accidental operation. For example, activation words like "Alexa" spoken in an audio or video broadcast can cause devices in homes and offices to start listening for input inappropriately, or possibly take an unwanted action.[20] Voice-controlled devices are also accessible to visitors to the building, or even those outside the building if they can be heard inside. Attackers may be able to gain access to personal information, like calendar, address book contents, private messages, and documents. They may also be able to impersonate the user to send messages or make online purchases.

Two attacks have been demonstrated that use artificial sounds. One transmits ultrasound and attempt to send commands without nearby people noticing.[21] The other adds small, inaudible distortions to other speech or music that are specially crafted to confuse the specific speech recognition system into recognizing music as speech, or to make what sounds like one command to a human sound like a different command to the system.[22]

Further information[edit | edit source]

Conferences and journals[edit | edit source]

Popular speech recognition conferences held each year or two include SpeechTEK and SpeechTEK Europe, ICASSP, Interspeech/Eurospeech, and the IEEE ASRU. Conferences in the field of natural language processing, such as ACL, NAACL, EMNLP, and HLT, are beginning to include papers on speech processing. Important journals include the IEEE Transactions on Speech and Audio Processing (later renamed IEEE Transactions on Audio, Speech and Language Processing and since Sept 2014 renamed IEEE/ACM Transactions on Audio, Speech and Language Processing—after merging with an ACM publication), Computer Speech and Language, and Speech Communication.

Books[edit | edit source]

Books like "Fundamentals of Speech Recognition" by Lawrence Rabiner can be useful to acquire basic knowledge but may not be fully up to date (1993). Another good source can be "Statistical Methods for Speech Recognition" by Frederick Jelinek and "Spoken Language Processing (2001)" by Xuedong Huang etc. More up to date are "Computer Speech", by Manfred R. Schroeder, second edition published in 2004, and "Speech Processing: A Dynamic and Optimization-Oriented Approach" published in 2003 by Li Deng and Doug O'Shaughnessey. The recently updated textbook Speech and Language Processing (2008) by Jurafsky and Martin presents the basics and the state of the art for ASR. Speaker recognition also uses the same features, most of the same front-end processing, and classification techniques as is done in speech recognition. A most recent comprehensive textbook, "Fundamentals of Speaker Recognition" is an in depth source for up to date details on the theory and practice.[23] A good insight into the techniques used in the best modern systems can be gained by paying attention to government sponsored evaluations such as those organised by DARPA (the largest speech recognition-related project ongoing as of 2007 is the GALE project, which involves both speech recognition and translation components).

A good and accessible introduction to speech recognition technology and its history is provided by the general audience book "The Voice in the Machine. Building Computers That Understand Speech" by Roberto Pieraccini (2012).

The most recent book on speech recognition is Automatic Speech Recognition: A Deep Learning Approach (Publisher: Springer) written by D. Yu and L. Deng and published near the end of 2014, with highly mathematically oriented technical detail on how deep learning methods are derived and implemented in modern speech recognition systems based on DNNs and related deep learning methods.[24] A related book, published earlier in 2014, "Deep Learning: Methods and Applications" by L. Deng and D. Yu provides a less technical but more methodology-focused overview of DNN-based speech recognition during 2009–2014, placed within the more general context of deep learning applications including not only speech recognition but also image recognition, natural language processing, information retrieval, multimodal processing, and multitask learning.[25]

Software[edit | edit source]

In terms of freely available resources, Carnegie Mellon University's Sphinx toolkit is one place to start to both learn about speech recognition and to start experimenting. Another resource (free but copyrighted) is the HTK book (and the accompanying HTK toolkit). For more recent and state-of-the-art techniques, Kaldi toolkit can be used.[citation needed] In 2017 Mozilla launched the open source project called Common Voice[26] to gather big database of voices that would help build free speech recognition project DeepSpeech (available free at GitHub)[27] using Google open source platform TensorFlow[28].

A demonstration of an on-line speech recognizer is available on Cobalt's webpage.[29]

For more software resources, see List of speech recognition software.

See also[edit | edit source]

References[edit | edit source]

  1. "Speaker Independent Connected Speech Recognition- Fifth Generation Computer Corporation". Fifthgen.com. Archived from the original on 11 November 2013. Retrieved 15 June 2013. Unknown parameter |deadurl= ignored (help)
  2. P. Nguyen (2010). "Automatic classification of speaker characteristics".
  3. "British English definition of voice recognition". Macmillan Publishers Limited. Archived from the original on 16 September 2011. Retrieved 21 February 2012. Unknown parameter |deadurl= ignored (help)
  4. "voice recognition, definition of". WebFinance, Inc. Archived from the original on 3 December 2011. Retrieved 21 February 2012. Unknown parameter |deadurl= ignored (help)
  5. "The Mailbag LG #114". Linuxgazette.net. Archived from the original on 19 February 2013. Retrieved 15 June 2013. Unknown parameter |deadurl= ignored (help)
  6. Reynolds, Douglas; Rose, Richard (January 1995). "Robust text-independent speaker identification using Gaussian mixture speaker models". IEEE Transactions on Speech and Audio Processing 3 (1): 72–83. doi:10.1109/89.365379. ISSN 1063-6676. OCLC 26108901. Archived from the original on 8 March 2014. http://www.cs.toronto.edu/~frank/csc401/readings/ReynoldsRose.pdf. Retrieved 21 February 2014. 
  7. "Speaker Identification (WhisperID)". Microsoft Research. Microsoft. Archived from the original on 25 February 2014. Retrieved 21 February 2014. When you speak to someone, they don't just recognize what you say: they recognize who you are. WhisperID will let computers do that, too, figuring out who you are by the way you sound. Unknown parameter |deadurl= ignored (help)
  8. Englund, Christine (2004). Speech recognition in the JAS 39 Gripen aircraft: Adaptation to speech at different G-loads (PDF) (Masters thesis). Stockholm Royal Institute of Technology. Archived from the original (PDF) on 2 October 2008. Unknown parameter |deadurl= ignored (help)
  9. "The Cockpit". Eurofighter Typhoon. Archived from the original on 1 March 2017. Unknown parameter |deadurl= ignored (help)
  10. "Eurofighter Typhoon – The world's most advanced fighter aircraft". www.eurofighter.com. Archived from the original on 11 May 2013. Retrieved 1 May 2018. Unknown parameter |deadurl= ignored (help)
  11. Schutte, John (15 October 2007). "Researchers fine-tune F-35 pilot-aircraft speech system". United States Air Force. Archived from the original on 20 October 2007. Unknown parameter |deadurl= ignored (help)
  12. Cerf, Vinton; Wrubel, Rob; Sherwood, Susan. "Can speech-recognition software break down educational language barriers?". Curiosity.com. Discovery Communications. Archived from the original on 7 April 2014. Retrieved 26 March 2014. Unknown parameter |deadurl= ignored (help)
  13. 13.0 13.1 "Speech Recognition for Learning". National Center for Technology Innovation. 2010. Archived from the original on 13 April 2014. Retrieved 26 March 2014. Unknown parameter |deadurl= ignored (help)
  14. Follensbee, Bob; McCloskey-Dale, Susan (2000). "Speech recognition in schools: An update from the field". Technology And Persons With Disabilities Conference 2000. Archived from the original on 21 August 2006. Retrieved 26 March 2014. Unknown parameter |deadurl= ignored (help)
  15. "Projects: Planetary Microphones". The Planetary Society. Archived from the original on 27 January 2012. Unknown parameter |deadurl= ignored (help)
  16. Caridakis, George; Castellano, Ginevra; Kessous, Loic; Raouzaiou, Amaryllis; Malatesta, Lori; Asteriadis, Stelios; Karpouzis, Kostas (19 September 2007). Multimodal emotion recognition from expressive faces, body gestures and speech. IFIP the International Federation for Information Processing. 247. Springer US. pp. 375–388. doi:10.1007/978-0-387-74161-1_41. ISBN 978-0-387-74160-4.
  17. Ciaramella, Alberto. "A prototype performance evaluation report." Sundial workpackage 8000 (1993).
  18. Gerbino, E., Baggia, P., Ciaramella, A., & Rullent, C. (1993, April). Test and evaluation of a spoken dialogue system. In Acoustics, Speech, and Signal Processing, 1993. ICASSP-93., 1993 IEEE International Conference on (Vol. 2, pp. 135–138). IEEE.
  19. National Institute of Standards and Technology. "The History of Automatic Speech Recognition Evaluation at NIST Archived 8 October 2013 at the Wayback Machine.".
  20. "Listen Up: Your AI Assistant Goes Crazy For NPR Too". NPR. 6 March 2016. Archived from the original on 23 July 2017. Unknown parameter |deadurl= ignored (help)
  21. Claburn, Thomas (25 August 2017). "Is it possible to control Amazon Alexa, Google Now using inaudible commands? Absolutely". The Register. Archived from the original on 2 September 2017. Unknown parameter |deadurl= ignored (help)
  22. "Attack Targets Automatic Speech Recognition Systems". vice.com. 31 January 2018. Archived from the original on 3 March 2018. Retrieved 1 May 2018. Unknown parameter |deadurl= ignored (help)
  23. Beigi, Homayoon (2011). Fundamentals of Speaker Recognition. New York: Springer. ISBN 978-0-387-77591-3. Archived from the original on 31 January 2018. Unknown parameter |deadurl= ignored (help)
  24. Yu, D.; Deng, L. (2014). Automatic Speech Recognition: A Deep Learning Approach (Publisher: Springer). 
  25. Deng, Li; Yu, Dong (2014). "Deep Learning: Methods and Applications". Foundations and Trends in Signal Processing 7 (3–4): 197–387. doi:10.1561/2000000039. Archived from the original on 22 October 2014. http://research.microsoft.com/pubs/209355/DeepLearning-NowPublishing-Vol7-SIG-039.pdf. 
  26. https://voice.mozilla.org
  27. https://github.com/mozilla/DeepSpeech
  28. https://www.tensorflow.org/tutorials/sequences/audio_recognition
  29. https://demo-cubic.cobaltspeech.com/

Further reading[edit | edit source]

  • Pieraccini, Roberto (2012). The Voice in the Machine. Building Computers That Understand Speech. The MIT Press. ISBN 978-0262016858.
  • Woelfel, Matthias; McDonough, John (2009-05-26). Distant Speech Recognition. Wiley. ISBN 978-0470517048.
  • Karat, Clare-Marie; Vergo, John; Nahamoo, David (2007). "Conversational Interface Technologies". In Sears, Andrew; Jacko, Julie A. (eds.). The Human-Computer Interaction Handbook: Fundamentals, Evolving Technologies, and Emerging Applications (Human Factors and Ergonomics). Lawrence Erlbaum Associates Inc. ISBN 978-0-8058-5870-9.
  • Cole, Ronald; Mariani, Joseph; Uszkoreit, Hans; Varile, Giovanni Battista; Zaenen, Annie; Zampolli; Zue, Victor, eds. (1997). Survey of the state of the art in human language technology. Cambridge Studies in Natural Language Processing. XII–XIII. Cambridge University Press. ISBN 978-0-521-59277-2.
  • Junqua, J.-C.; Haton, J.-P. (1995). Robustness in Automatic Speech Recognition: Fundamentals and Applications. Kluwer Academic Publishers. ISBN 978-0-7923-9646-8.
  • Pirani, Giancarlo, ed. (2013). Advanced algorithms and architectures for speech understanding. Springer Science & Business Media. ISBN 978-3-642-84341-9.

External links[edit | edit source]

Page Information[edit | edit source]

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