VoxForge was set up to collect transcribed speech for use in Open SourceSpeech Recognition Engines ("SRE"s) such as such as ISIP, HTK, Julius and Sphinx. We will
categorize and make available all submitted audio files (also called a 'Speech Corpus") and Acoustic Models in GPL format.
Why do we need GPL Transcribed Speech?
In order to recognize speech, Speech Recognition Engines require
two types of files: the first, called an Acoustic Model, is
created by taking a very large number of transcribed speech
recordings (called a Speech Corpus) and 'compiling' them into
statistical representations of the sounds that make up each word. The
second is a Grammar or Language Model. A Grammar is a
relatively small file containing sets of predefined combinations of
words. A Language Model is a much larger file containing the
probabilities of certain sequences of words.
Problems with Current Approaches:
Acoustic Models are Closed-Source
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. If they do give
you access, there are usually 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 Corpus in a form that can readily be used, or that is large
enough, to
create good quality Acoustic Models for Speech Recognition
Engines. Although there are a
few instances of small FOSS speech corpora that could be used to
create acoustic models, the vast majority of corpora (especially
large corpora best suited to building good acoustic models) must be
purchased under restrictive licenses.
As a result, Open
Source projects that want to distribute their code freely must purchase
restrictively licensed Speech Copora that limit distribution of the
'source' speech audio, but allow them to distribute any Acoustic
Models they create.
VoxForge will address this problem by providing all Acoustic
Models and their 'source' (i.e. transcribed speech audio) in GPL
licensing format - which requires that the distribution of
derivative works include access to the source used to create that
work.
Restrictive Licensing Creates an Access Barrier to Potential Contributors
Every project that wants to build
an acoustic model using a corpus with restrictive licensing must purchase their own copy.
This is difficult for FOSS projects, which usually have no
revenue. If a project does purchase such resources, the
license restrictions will require them to keep the resources
behind some kind of access barrier restricted to official project
members. This takes away freedom and flexibility from end users
and shrinks the pool of potential contributors to the project.
Acoustic Models are not Interchangeable
Most Open Source Speech Recognition Engines ("SRE"s)
come with an Acoustic Model. However, these Acoustic Models are
not interchangeable with other open source Speech Recognition
engines. The way to address this problem is to provide the
'source code' for the Acoustic Models (i.e. the Speech
Corpora used to create the Acoustic Models), and permit users to
'compile' it into Acoustic Models that can be used with the Open
Source SRE of their choice.
VoxForge hopes to address this problem by creating a repository of
'source' speech audio and transcriptions, and by creating Acoustic
Models for each of the main Open Source Speech Recognition Engines
(such as Sphinx,
Julius, HTK
and ISIP)
.
Open Source Acoustic Models Need to be Improved
Current Acoustic Models used by Open Source Speech Recognition
Engines are not at the level of quality of Commercial Speech
Recognition Engines.
VoxForge provides a central location that can collect GPL
speech audio and transcriptions. As more speech audio data is
collected, better Acoustic Models can be created, to the point that
someday they will be comparable to Commercial Speech Recognition.
No Open Source Dictation Software
Most Open Source SREs are designed for command and control and IVR
telephony type applications (e.g. Sphinx,
HTK and ISIP).
The Julius
Speech Recognition Engine was designed for dictation applications,
however the Julius
distribution only includes Japanese Acoustic Models. But
since it uses Acoustic Models trained using the HTK
toolkit, it can also use Acoustic Models trained in other languages -
like English. We just need hundreds of hours of transcribed
speech audio to create English dictation Acoustic Models. This
same audio data might also be used to permit the other open source
Speech Recognition Engines to work in dictation applications.
Although the current focus of VoxForge is on Speech Recognition
for IVR telephony applications or Command and Control applications on
the desktop, when the amount of audio data collected reaches a
certain threshold, this data can then be used in the creation of
Acoustic Models for Open Source Dictation Applications.
Why GPL?
Unrestricted Licenses for Speech Corpora will not be Effective
We believe that making Speech
Corpora available using an unrestrictive, BSD style license will not help the Open Source Community in this particular case. A BSD
style license permits users to distribute derivative works without
having to contribute the source of those modifications back to the
community. In our opinion, the Open Source Speech Recognition
community does not have the required threshold of users to create a
self-sustaining community using a BSD
style license. If there was a larger community, then there would
be a greater likelihood that a self-sustaining group would give back to
the community, even if not required to do so using a BSD style license.
GPL
licensing ensures that any contributions made by the Open Source
Community to VoxForge will benefit the community. This is because
the distribution of any derivative works based on the VoxForge Speech
Corpora must make the source (i.e. the transcribed speech audio) available to the community.
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