Speech to Text

Apologies to the tl;dr brigade, this is going to be a long one... 

For a number of years I've been quietly working away with IBM research on our speech to text programme. That is, working with a set of algorithms that ultimately produce a system capable of listening to human speech and transcribing it into text. The concept is simple, train a system for speech to text - speech goes in, text comes out. However, the process and algorithms to do this are extremely complicated from just about every way you look at it – computationally, mathematically, operationally, evaluationally, time and cost. This is a completely separate topic and area of research from the similar sounding text to speech systems that take text (such as this blog) and read it aloud in a computerised voice.

Whenever I talk to people about it they always appear fascinated and want to know more. The same questions often come up. I'm going to address some of these here in a generic way and leaving out those that I'm unable to talk about here. I should also point out that I'm by no means a speech expert or linguist but have developed enough of an understanding to be dangerous in the subject matter and that (I hope) allows me to explain things in a way that others not familiar with the field are able to understand. I'm deliberately not linking out to the various research topics that come into play during this post as the list would become lengthy very quickly and this isn't a formal paper after all, Internet searches are your friend if you want to know more.

I didn't know IBM did that?

OK so not strictly a question but the answer is yes, we do. We happen to be pretty good at it as well. However, we typically use a company called Nuance as our preferred partner.

People have often heard of IBM's former product in this area called Via Voice for their desktop PCs which was available until the early 2000's. This sort of technology allowed a single user to speak to their computer for various different purposes and required the user to spend some time training the software before it would understand their particular voice. Today's speech software has progressed beyond this to systems that don't require any training by the user before they use it. Current systems are trained in advance in order to attempt to understand any voice.

What's required?

Assuming you have the appropriate software and the hardware required to run it on then you need three more things to build a speech to text system: audio, transcripts and a phonetic dictionary of pronunciations. This sounds quite simple but when you dig under the covers a little you realise it's much more complicated (not to mention expensive) and the devil is very much in the detail.

On the audial side you'll need a set of speech recordings. If you want to evaluate your system after it has been trained then a small sample of these should be kept to one side and not used during the training process. This set of audio used for evaluation is usually termed the held out set. It's considered cheating if you later evaluate the system using audio that was included in the training process – since the system has already “heard” this audio before it would have a higher chance of accurately reproducing it later. The creation of the held out set leads to two sets of audio files, the held out set and the majority of the audio that remains which is called the training set.

The audio can be in any format your training software is compatible with but wave files are commonly used. The quality of the audio both in terms of the digital quality (e.g. sample rate) as well as the quality of the speaker(s) and the equipment used for the recordings will have a direct bearing on the resulting accuracy of the system being trained. Simply put, the better quality you can make the input, the more accurate the output will be. This leads to another bunch of questions such as but not limited to “What quality is optimal?”, “What should I get the speakers to say?”, “How should I capture the recordings?” - all of which are research topics in their own right and for which there is no one-size-fits-all answer.

Capturing the audio is one half of the battle. The next piece in the puzzle is obtaining well transcribed textual copies of that audio. The transcripts should consist of a set of text representing what was said in the audio as well as some sort of indication of when during the audio a speaker starts speaking and when they stop. This is usually done on a sentence by sentence basis, or for each utterance as they are known. These transcripts may have a certain amount of subjectivity associated with them in terms of where the sentence boundaries are and potentially exactly what was said if the audio wasn't clear or slang terms were used. They can be formatted in a variety of different ways and there are various standard formats for this purpose from an XML DTD through to CSV.

If it has not already become clear, creating the transcription files can be quite a skilled and time consuming job. A typical industry expectation is that it takes approximately 10 man-hours for a skilled transcriber to produce 1 hour of well formatted audio transcription. This time plus the cost of collecting the audio in the first place is one of the factors making speech to text a long, hard and expensive process. This is particularly the case when put into context that most current commercial speech systems are trained on at least 2000+ hours of audio with the minimum recommended amount being somewhere in the region of 500+ hours.

Finally, a phonetic dictionary must either be obtained or produced that contains at least one pronunciation variant for each word said across the entire corpus of audio input. Even for a minimal system this will run into tens of thousands of words. There are of course, already phonetic dictionaries available such as the Oxford English Dictionary that contains a pronunciation for each word it contains. However, this would only be appropriate for one regional accent or dialect without variation. Hence, producing the dictionary can also be a long and skilled manual task.

What does the software do?

The simple answer is that it takes audio and transcript files and passes them through a set of really rather complicated mathematical algorithms to produce a model that is particular to the input received. This is the training process. Once system has been trained the model it generates can be used to take speech input and produce text output. This is the decoding process. The training process requires lots of data and is computationally expensive but the model it produces is very small and computationally much less expensive to run. Today's models are typically able to perform real-time (or faster) speech to text conversion on a single core of a modern CPU. It is the model and software surrounding the model that is the piece exposed to users of the system.

Various different steps are used during the training process to iterate through the different modelling techniques across the entire set of training audio provided to the trainer. When the process first starts the software knows nothing of the audio, there are no clever boot strapping techniques used to kick-start the system in a certain direction or pre-load it in any way. This allows the software to be entirely generic and work for all sorts of different languages and quality of material. Starting in this way is known as a flat start or context independent training. The software simply chops up the audio into regular segments to start with and then performs several iterations where these boundaries are shifted slightly to match the boundaries of the speech in the audio more closely.

The next phase is context dependent training. This phase starts to make the model a little more specific and tailored to the input being given to the trainer. The pronunciation dictionary is used to refine the model to produce an initial system that could be used to decode speech into text in its own right at this early stage. Typically, context dependent training, while an iterative process in itself, can also be run multiple times in order to hone the model still further.

Another optimisation that can be made to the model after context dependent training is to apply vocal tract length normalisation. This works on the theory that the audibility of human speech correlates to the pitch of the voice, and the pitch of the voice correlates to the vocal tract length of the speaker. Put simply, it's a theory that says men have low voices and women have high voices and if we normalise the wave form for all voices in the training material to have the same pitch (i.e. same vocal tract length) then audibility improves. To do this an estimation of the vocal tract length must first be made for each speaker in the training data such that a normalisation factor can be applied to that material and the model updated to reflect the change.

The model can be thought of as a tree although it's actually a large multi-dimensional matrix. By reducing the number of dimensions in the matrix and applying various other mathematical operations to reduce the search space the model can be further improved upon both in terms of accuracy, speed and size. This is generally done after vocal tract length normalisation has taken place.

Another tweak that can be made to improve the model is to apply what we call discriminative training. For this step the theory goes along the lines that all of the training material is decoded using the current best model produced from the previous step. This produces a set of text files. These text files can be compared with those produced by the human transcribers and given to the system as training material. The comparison can be used to inform where the model can be improved and these improvements applied to the model. It's a step that can probably be best summarised by learning from its mistakes, clever!

Finally, once the model has been completed it can be used with a decoder that knows how to understand that model to produce text given an audio input. In reality, the decoders tend to operate on two different models. The audio model for which the process of creation has just been roughly explained; and a language model. The language model is simply a description of how language is used in the specific context of the training material. It would, for example, attempt to provide insight into which words typically follow which other words via the use of what natural language processing experts call n-grams. Obtaining information to produce the language model is much easier and does not necessarily have to come entirely from the transcripts used during the training process. Any text data that is considered representative of the speech being decoded could be useful. For example, in an application targeted at decoding BBC News readers then articles from the BBC news web site would likely prove a useful addition to the language model.

How accurate is it?

This is probably the most common question about these systems and one of the most complex to answer. As with most things in the world of high technology it's not simple, so the answer is the infamous “it depends”. The short answer is that in ideal circumstances the software can perform at near human levels of accuracy which equates to in excess of 90% accuracy levels. Pretty good you'd think. It has been shown that human performance is somewhere in excess of 90% and is almost never 100% accuracy. The test for this is quite simple, you get two (or more) people to independently transcribe some speech and compare the results from each speaker, almost always there will be a disagreement about some part of the speech (if there's enough speech that is).

It's not often that ideal circumstances are present or can even realistically be achieved. Ideal would be transcribing a speaker with a similar voice and accent to those which have been trained into the model and they would speak at the right speed (not too fast and not too slowly) and they would use a directional microphone that didn't do any fancy noise cancellation, etc. What people are generally interested in is the real-world situation, something along the lines of “if I speak to my phone, will it understand me?”. This sort of real-world environment often includes background noise and a very wide variety of speakers potentially speaking into a non-optimal recording device. Even this can be a complicated answer for the purposes of accuracy. We're talking about free, conversational style, speech in this blog post and there's a huge different in recognising any and all words versus recognising a small set of command and control words for if you wanted your phone to perform a specific action. In conclusion then, we can only really speak about the art of the possible and what has been achieved before. If you want to know about accuracy for your particular situation and your particular voice on your particular device then you'd have to test it!

What words can it understand? What about slang?

The range of understanding of a speech to text system is dependent on the training material. At present, the state of the art systems are based on dictionaries of words and don't generally attempt to recognise new words for which an entry in the dictionary has not been found (although these types of systems are available separately and could be combined into a speech to text solution if necessary). So the number and range of words understood by a speech to text system is currently (and I'm generalising here) a function of the number and range of words used in the training material. It doesn't really matter what these words are, whether they're conversational and slang terms or proper dictionary terms, so long as the system was trained on those then it should be able to recognise them again during a decode.

Updates and Maintenance

For the more discerning reader, you'll have realised by now a fundamental flaw in the plan laid out thus far. Language changes over time, people use new words and the meaning of words changes within the language we use. Text-speak is one of the new kids on the block in this area. It would be extremely cumbersome to need to train an entire new model each time you wished to update your previous one in order to include some set of new language capability. The models produced are able to be modified and updated with these changes without the need to go back to a full standing start and training from scratch all over again. It's possible to take your existing model built from the set of data you had available at a particular point in time and use this to bootstrap the creation of a new model which will be enhanced with the new materials that you've gathered since training the first model. Of course, you'll want to test and compare both models to check that you have in fact enhanced performance as you were expecting. This type of maintenance and update to the model will be required to any and all of these types of systems as they're currently designed as the structure and usage of our languages evolve.

Conclusion

OK, so not necessarily a blog post that was ever designed to draw a conclusion but I wanted to wrap up by saying that this is an area of technology that is still very much in active research and development, and has been so for at least 40-50 years or more! There's a really interesting statistic I've seen in the field that says if you ask a range of people involved in this topic the answer to the question “when will speech to text become a reality” then the answer generally comes out at “in ten years time”. This question has been asked consistently over time and the answer has remained the same. It seems then, that either this is a really hard nut to crack or that our expectations of such a system move on over time. Either way, it seems there will always be something new just around the corner to advance us to the next stage of speech technologies.