Editing DARPA Speech Understanding Research (section)

===== HEARSAY-II =====
At the heart of this system, Hearsay-II put what are known as ''symbolic problem solvers'', also referred to as ''knowledge sources''. The need for multiple knowledge sources stemmed from the diverse transformations applied by speakers when creating acoustic signals and the corresponding inverse transformations required by listeners for interpretation<ref>Erman, L. D., Hayes-Roth, F., Lesser, V. R., & Reddy, D. R. (1980). The Hearsay-II speech-understanding system: Integrating knowledge to resolve uncertainty. ''ACM Computing Surveys (CSUR)'', ''12''(2), 213-253.</ref>. 

These knowledge sources communicated through a ''blackboard'' architecture, which served as a repository for data, partial results, and finished conclusions. This design allowed each knowledge source to know where to retrieve information from the blackboard and where to post partial conclusions. The range of knowledge sources included phonetic, phonemic, lexical, syntactic, semantic, prosodic, discursive, and even psychological aspects. Each of these sources independently proposed improved word string guesses for the given speech signal by 'writing' them on the virtual blackboard. Other knowledge sources could then build upon these suggestions.

Using this technique, Hearsay achieved recognition of 1,011 words in continuous speech from multiple speakers with limited syntax, achieving an accuracy rate of approximately 90%<ref name=":1">Erman, L. D., Hayes‐Roth, F., Lesser, V. R., & Reddy, R. (1976). The Hearsay‐II speech understanding system. ''The Journal of the Acoustical Society of America'', ''60''(S1), S11-S11.</ref>. However, a notable limitation of this system was the time spent deciding which knowledge source to utilize next, which detracted from real-time speech processing — contrary to one of the project's primary goals.