Friday, 13 March 2015

Deep Learning in Speech Recognition

What is the difference between natural learning (performed by humans) and machine learning?

Learning is to arrange/ classify objects in accordance to some feature set of interest. See how a child learns her/ his mother tongue. The mother utters the word once to the child. Then the child tries to repeat it (or replicate). Consider the word 'അമ്മ' (or mother in malayalam). Since this might be the first word introduced to the child, she/he just adds it to her/his dictionary set. A similar sounding word (say ആമ (tortoise in malayalam)) takes time for the child to distinguish. Only a feedback and learn or to learn from falling will help the child as the letter അ needs to be distinguished from ആ and the sound മ്മ from മ.Once the subtle differences in the sounds are known to the child she/ he can classify similar words. ശാസ്ത്രം | ശസ്ത്രം , മൂല്യം | മൂലം , ആദം | ആദ്യം , കാരണം | കരണം etc

Similar is what happens in spam classification also. From a data-set, we need to distinguish the the spam and the ham (or non-spam) emails. If we provide a learning set to the computer program, with markings of spam and ham, the algorithm can take care on its own. The future emails can be classified again based on the understanding/ pattern identified by the algorithm. How is this different from the learning of the language by the child? The learning performed by a machine is dependent on the code that is written into it. It just looks for patterns that are asked to look. (Ex. The words commonly occurring in a spam : SALE, PRIZE, !!!, WaTch, PassWord, etc. ) It does not look for new features or evolve on its own. On the contrary human systems can accommodate features on its own and is a non-linear evolution process.

The deep learning is associated with the human learning where the learner comes with new features relevant for the better classification spaces. It can be understood as a dimension spreading process where the learner adds one more feature (dimension) in the evolution process. The classification is said to be complete when the learner observes that all the points (or objects) in this space can be classified by different decision boundaries.

As is often quoted "Learning never ends", the machine capable of deep learning should be able to evolve to do the classification at any scale. From isolated words to recognizing sentences and to understand contexts and form interpretations.

 Fig : Example of decision regions in the 2D plane

Wednesday, 24 December 2014

On the recognition of Speech

Speech is a highly complicated random signal, yet as simple as a deterministic periodic signal. We cannot imagine a day without having a speech conversation with anyone else. This signal conveys a lot of meaning and understanding - it is considered as one of the most significant modes of information transfer.

Speech is very much omnipresent, that we wish it can be used to build speech recognition systems and make the systems (or machines)  around us also understand speech. Speech recognition consists of a receptor (or input device/ similar to the ear) and a processor (where the speech recognition system is built/ similar to the brain). 


The speech signal processing here is a two fold problem - one which is its generation and the second its processing. Thus, the speech recognition problem is the study of the two sides of the same coin - synthesis/analysis. The synthesis step enables us to determine the features that are useful in the analysis. Common feature of speech are the formant frequencies, pitch, vowel/consonant etc.  





An isolated word recognition system is a very realistic problem to start with. Suppose we are using a vehicle that uses an Automatic Speech Recognition (ASR) system, then by the utterance of single words the driver will be able to perform tasks like turning on the head lights, increasing the volume of the stereo, or turning the windshield wipers when it is raining. The most common words in this scenario will be "Lights on", "Lights off", "Mute Stereo", "plus Volume" etc.


It has been reported that an ASR with ten words in the dictionary can work with an accuracy of 0.5%. This suggests that it is possible to treat the systems around us also as human - yes they can understand speech too.