A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a security mechanism to determine whether the user is human in the work flow of a user identification procedure.
Different types of CAPTCHA technologies exist, but the most common type requires that the user types random letters or digits that appear on a distorted image on the screen. The idea is that only a human user can read it and then type it into the adjacent text field.
This user identification procedure has received criticisms, especially with regard to web accessibility. The problem is that often even a human reader can not read the text. As logically no text equivalent can be provided, as that would be also accessible to the robots, to make it accessible, some alternative is needed for humans. One available solution is to use synthesised speech.
Examples and documentation
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Free to use. You do not need to fill in a request form. Please have a look at the documentation above.