kmaclean writes, "VoxForge collects free GPL Transcribed Speech Audio that can be used in the creation of Acoustic Models for use with Open Source Speech Recognition Engines. We are essentially creating a user-submitted repository of the 'source' speech audio for the creation of Acoustic Models to be used by Speech Recognition Engines. The Speech Audio files will then be 'compiled' into Acoustic Models for use with Open Source Speech Recognition engines such as Sphinx, HTK, CAVS and Julius." Read on for why we need free GPL speech audio.
Why free GPL Speech Audio?
Speech Recognition Engines require two types of files to recognize speech. The first is an Acoustic Model, which is created by taking a very large number of audio recordings of speech and their transcriptions (called Speech Corpus or Corpora) and 'compiling' them into statistical representations of the sounds that make up each word. The second is a Language Model or Grammar file. A Language Model is a very large file containing the probabilities of certain sequences of words. A Grammar is a much smaller file containing sets of predefined combinations of words.
Most Acoustic Models used by 'Open Source' Speech Recognition engines are 'closed source'. They do not give you access to the speech audio (the 'source') used to create the acoustic model, or if they do, there are licensing restrictions on the distribution of the 'source' (i.e. you can only use it for personal or research purposes). The reason for this is because there is no free Speech Corpora in a form that can readily be used to create Acoustic Models for Speech Recognition Engines. Open Source projects are required to purchase Speech Copora which has restrictive licensing — i.e. they are not permitted to distribute the 'source' speech audio, but they are permitted them to distributed the 'compiled' Acoustic Model.
A GPL-style license will ensure that user contributions will always benefit the open source community, since it requires any distribution of derivative Acoustic Models to include access to the 'source' speech audio.