Numeral recognition is one among the most vital issues in pattern recognition. Its numerous applications like reading postal zip code, passport number, employee code, bank cheque processing and video gaming etc. To the best of our knowledge, little work has been done in Marathi language as compared with those for other Indian and non-Indian languages. This paper has discussed a novel technique for recognition of isolated Marathi numerals. It introduces a Marathi database and isolated numeral recognition system using Mel-Frequency Cepstral Coefficient (MFCC) and Distance Time Warping (DTW) as attributes. The precision of the pre-recorded samples is more than that of the real-time testing samples. We have also observed that the accuracy of the speaker dependent samples is more than that of the speaker independent samples. Another approach called HMM that statistically models the words is also presented. Experimentally, it is observed that recognition accuracy is better for HMM compared with DTW, but the training procedure in DTW is very simple and fast, as compared with the Hidden Markov Model (HMM). The time required for recognition of numerals using HMM is more as compared to DTW, as it has to go through the many states, iterations and many more mathematical modeling, so DTW is preferred for the real-time applications.
Keywords: Hidden Markov Model (HMM), Mel-Frequency Cepstral Coefficient (MFCC), Distance Time Warping (DTW).
Speech recognition systems are used in different applications in our daily life. Because of the rapid advancement in this field all over the world we can see many systems and devices with voice input 1. Speech Synthesis and Speech Recognition combinely form a speech interface. A speech synthesizer converts text into speech, so it can read out the textual contents from the screen. Speech recognizer had the capability to recognize the spoken words and convert it into text. We require such software’s to be present for Indian languages.
Speech recognition is the capability to listen spoken words and recognize different sounds present in it, and identify them as words of some familiar language. Speech recognition in computer domain involves many steps with issues attached with them. The steps required to make computers perform speech recognition are: Voice recording, word boundary detection, feature extraction, and recognition by using knowledge models.
The aim of the paper is to build a speech recognition tool for Marathi language, which is an isolated word speech recognition devices that uses Mel-Frequency Cepstral Coefficient (MFCC) for Feature Extraction and Distance Time Warping (DTW) for Feature Matching or to compare the test patterns.
3. Marathi Numeral Recognition using MFCC and DTW Features
The popularly used cepstrum based techniques to check the pattern to find their similarity are the MFCC and DTW. The MATLAB is used for the implementation of MFCC and DTW attributes.
FEATURE EXTRACTION (MFCC)
The MFCCs are used for feature extraction. The efficiency of this phase is important for the next phase since it affects its behavior 2. In MFCC feature extraction, the magnitude spectrum of windowed speech frame was filtered by employing a triangular Mel filter bank consisting of twenty Mel filters. From a group of twenty Mel-scaled log filter bank outputs, MFCC feature vector that consists of thirteen MFCC and the corresponding delta and acceleration coefficients (total thirty nine coefficients) is extracted from every frame.
The widespread use of the MFCCs is because of its low computational complexity and higher performance for ASR in the clean matched conditions. Performance of MFCC degrades drastically in presence of noise and degradation is directly proportional to signal-to noise ratio (SNR). The recognition accuracy for MFCC attribute is taken into account because it mimics the human ear perception 2.
The complete procedure of the MFCC is shown in Fig. 3.1. As shown in Fig.3.1, MFCC consists of seven computational steps. Every step has its function and approaches as mentioned in brief as follows.
Fig. 3.1.MFCC Block Diagram 2.
Step 1: Pre–emphasis
This method can increase the energy of signal at higher frequency. It permits the passing of every speech signal through a 1st order FIR filter which emphasizes higher frequencies. The 1st order FIR filter equation utilized is
Yn = xn-0.95 x n-1 …………………………………………………………..… (1)
Step 2: Framing.
Every speech signal is split into frames of thirty six ms (milliseconds) and most of spectral characteristics stay the constant in this period, with 50 % of overlapping.
Step 3: Windowing
To eliminate edge effects, every frame is formed with hamming window that
works better than other windows. The hamming window is represented by
W(n)=0.54-0.46 cos?(2?n/(N-1)) Where, 0?n?N-1……..….. (2)
Step 4: Fast Fourier Transformation (FFT)
The FFT is employed to get log magnitude spectrum to estimate MFCC. We have utilized 1024 point to obtain higher frequency resolution.
Step 5: Mel Filter Bank Processing
The twenty Mel triangular filters are designed with 50% overlapping. From every filter the spectrum are included to obtain one coefficient each, hence we have considered the first thirteen coefficients as our attributes. These frequencies are converted to Mel scale utilizing conversion formula.
We have taken into account only 13 MFCC coefficients due to the fact it gives higher recognition accuracy than other coefficients.
Step 6: Discrete Cosine Transformation (DCT)
The DCT of every Mel frequency Ceptral are used for de-correlation and energy
compaction is called as MFCC. The group of coefficient are called MFCC Acoustic Vectors. So, every input speech signal is converted into a sequence of MFCC Acoustic Vector from which reference templates are obtained.
Step 7: Delta Energy and Delta Spectrum
The attributes associated to the variation in cepstral features over time are represented by thirteen delta features (12 cepstral features and one energy feature), and 13 double delta or acceleration attributes. Each of the 13 delta features gives the variation between frames, while each of the 13 double delta attributes gives the variation between frames in the corresponding delta features. In similar way, all the total 39 MFCC feature are estimated for each frame which has feature vector. The Mel filter bank created is shown in Fig.3.2.
Fig.3.2: Mel scale filter bank 2.
The operating procedure of the MFCC coefficient extraction is:
Pre-emphasis of the speech signal, frame, adding window, then use the
FFT to get the frequency information.
2. Pass the signal through the Mel frequency coordinate triangle filter sets to
match the human hearing techniques and the human hearing sensibility to
variant speech spectrum.
Estimate the logarithm value of the signal after the Mel filters to get the
4. Obtain the discrete cosine transform to the signal and get the MFCC
According to psychophysical studies, human perception of the frequency content of sounds follows a subjectively defined nonlinear scale called the Mel scale .The
speech signal consists of tones with different frequencies F or each tone with an actual frequency measured in Hz, a subjective pitch is measured on the ‘Mel’ scale. The mel-frequency scale is a linear spacing below 1000Hz and above 1000Hz is a logarithmic spacing 3.
4. Features Matching (DTW)
In this form of speech recognition technique the test data is transformed to templates. The recognition method then includes the matching the incoming speech with stored templates. The template with the lowest distance measure from the input pattern is the known word. The best choice (lowest distance measure) is based upon dynamic programming. This is called a Dynamic Time Warping (DTW) word recognizer 3.
To understand the concept of DTW, we require to know this parameters,
Features: the information in every signal has to be exhibited in some fashion.
Distances: some type of metric has be utilized so as to obtain a match path.
Since the feature, vectors may probably have multiple elements, a ways of calculating the local distance is needed. The distance measure between two feature vectors is estimated by the Euclidean distance metric. The Euclidean distance between two points P = (p1, p2…pn) and Q = (q1, q2…qn), is expressed as,
Speech is a time-dependent technique. So, the articulations of the same word will have variant durations, and articulations of the same word with the same duration will differ in the middle, as the different parts of the words are being spoken at variant rates. In order to obtain a global distance between two speech patterns (given as a sequence of vectors) a time alignment should be performed.
DTW algorithm is relies on Dynamic Programming methods. This method is for estimating similarity between two time series that may vary in time or speed. This method is also utilized to obtain the optimal alignment between two times series if one time series may be “warped” non-linearly by stretching or shrinking it along its time axis. The warping between two time series can then be utilized to obtain the corresponding regions between the two time series or to find the similarity between the two time series. Fig. 4.1 shows the example of how one times series is ‘warped’ to another 4.
In Fig. 4.1, every vertical line joins a point in one time series to its respectively similar point in the different time series. The lines have similar values on the y-axis, while they have been departed so the vertical lines between them can be seen more easily. When both of the time series in Fig. 4.1 were identical, all of the lines would be straight vertical lines as no warping would be required to ‘line up’ the two time series. The warp path distance is a measure of the variation between the two time series after they have been warped collectively, which is estimated by the sum of the distances between every pair of points joined by the vertical lines in Fig. 4.1. Hence, two time series that are same except for localized stretching of the time axis will have zero DTW distances. The aim of DTW is to compare two dynamic patterns and calculate its similarity by finding a minimum distance between them.
Fig. 4.1 Warping between two time series4.
Suppose we have 2 time series Q and C, of length n and m respectively, where:
Q = q1, q2,………..qi…, qn …………………………………… (5)
C = c1, c2…………cj….cn ……………………………………. (6)
To align 2 sequences using DTW, an n-by-m matrix where the (ith, jth) element of the matrix has the distance d (qi, cj) between the two points qi and cj is designed. Then, the absolute distance between the values of two sequences is measured by using the Euclidean distance as:
d(qi, cj) = (qi-cj)2 ……………………………….………. (7)
Every matrix element (i, j) matches to the alignment between the points qi and cj. Then, accumulated distance is given as:-
Di, j = minD(i-1, j-1), D(i-1, j), D(I, j-1)+ d(i, j) …..………..… (8)
This is shown in Fig.4.4, where the horizontal axis gives the time of test input signal, and the vertical axis gives the time sequence of the reference template. The path shown gives the minimum distance between the input and template signal. The shaded region shows the search region for the input time to template time mapping function.
Fig.4.2 Example Dynamic time warping (DTW)4.
Using dynamic programming methods, the search for the minimum distance
path can be obtained in polynomial time P(t), using the equation as:-
p(t)=oN^2 V ……………………………………………………. (9)
Where, N is that the length of the sequence, and V is that the number of
templates to be considered.
Theoretically, the major optimizations to the DTW algorithm come from observations on the nature of good paths through the grid. These are given in Sakoe and Chiba and can be summarized as.
Monotonic condition: -The path will not return back on itself, both i and j indexes either remain the constant or increase, they never decrease.
Fig.4.3 Template Matching Issues: Dynamic Time Warping 5
Continuity condition: – The path forwards one step at a time. Both i and j can only increase by 1 on every step along the path.
Boundary condition: – The path begins at the bottom left and stops at the top right.
Adjustment window condition: – A good path is the one that wander away from the diagonal. The distance that the path is allowed to wander is the window length r.
Slope constraint condition: -The path should not be very steep or shallow. This restraints very short sequences matching very long ones. The condition is given as a ratio n/m where m is the number of steps in the x direction and n is the number in the y direction.
Finding for the best path that matches two time-series signals is the main task for many researchers, due to its importance in these applications. Dynamic Time-Warping (DTW) is one of the main techniques to accomplish this task, especially in speech recognition systems to deal with different speaking speeds. DTW is a cost minimization matching method, in which a test signal is stretched or compressed as per a reference template. Dynamic time warping (DTW) is such a typical method for a template based approach matching for speech recognition and also DTW stretches and compresses different parts of utterance in order to obtain alignment that gives the best possible match between template and utterance on frame by frame manner. The template with nearest match defined in a manner chosen as recognized word 6.
The DTW is mainly used in the small-scale embedded-speech recognition systems like those embedded in cell phones. The reason for this is due to the simplicity of the hardware implementation of the DTW, hat makes it suitable for different mobile devices. Also, the training procedure in DTW is very simple and fast, as compared with the HMM and ANN. DTW has been used in various areas, like speech recognition, data mining, and movement recognition 3.
5. Hidden Markov Model Training and Recognition
The Hidden Markov Models (HMMs) are widely used statistical tools in recognition system. It covers from isolated speech recognition to very large vocabulary unconstrained continuous speech recognition and speaker identification fields. Therefore most of the current speech recognitions are conducted based on Hidden Markov Model (HMMs)7.
The Hidden Markov Model is a statistical model where the system is modelled as Markov process which can be represented as a state machine with unknown parameter through it (Ting, 2007). The main important process is to determine the unknown parameter from the observable parameter. After determining the parameter, it then used to perform further analysis. One of the examples of the Hidden Markov Model is normally use in pattern application. For example, Hidden Markov Model in pattern application is speech, signature, handwriting, gesture recognition and bioinformatics and genomics in medical.
The hidden Markov Model is given by the formula, ?=(?, A, B).
? = initial state distribution vector.
A = State transition probability matrix.
B = continuous observation probability density function matrix.
Fig. 5.1: Overview of HMM speech recognition process 7.
RESULTS and DISCUSSIONS
We recorded Marathi database of numerals zero to nine. In this we have intended to implement a password system with numerals and many other such applications in everyday life. The 20 samples for each word were recorded from different people and these samples were then normalized by dividing their maximum values. Then they were decomposed using Dynamic Time Warping. Out of 20 samples recorded, 16 samples are used to train the DTW and the unused 4 samples are used for test purpose.
In this project, speech recognition software had been developed using MFCC ; DTW algorithms. The reference file was created for different-different pre recorded speech signals. When the microphone input signal was applied, its MFCC coefficients were compared to the pre-recorded speech’s MFCC coefficients using DTW algorithm. The Output scores of DTW calculate the nearest sound of the recorded speech signals. End of the software output was displayed on MATLAB output screen. Software would display correct numeral if applied microphone signal would be compared with pre-recorded & online signals.
The Resultsof some of the extracted features of recorded database of numerals zero to nine in Marathi are shown in the figures below.
Fig.6.1: Mel Frequency Cepstrum Coefficients Fig.6.2: Mel Frequency Cepstrum
of Shunya. Coefficients of pach.
Graphical User Interface (GUI) of the system
We have created the GUI of the system for the recognition of the numerals. The DTW 0-9 Digit Recognizer has the various command buttons like record, open, play, recognize etc. It shows the opened wave file.
In this project, we have designed a DTW digit recognizer, in which the command button open reads the pre-recorded numerals and the command button record the online numeral spoken by the speaker. We can play the pre-recorded & online numeral spoken by the speaker, and then we can recognize the numeral using the DTW for feature matching. It matches the template by taking into account the minimum warping distance between the various numerals. The Template with closest match defined in manner chosen as recognized numeral & it is displayed on GUI display.
Fig.6.3: GUI of DTW Digit Recognizer. Fig.6.4: GUI of opened wave file.
Fig.6.5: GUI of pattern matching of shunya. Fig.6.6: GUI of recognized numeral shunya.
Fig.6.7: GUI of pattern matching of ek. Fig.6.8: GUI of recognized numeral ek.
Fig.6.9: GUI of pattern matching of saha. Fig.6.10: GUI of recognized numeral saha.
Fig.6.11: GUI of pattern matching of nau. Fig.6.12: GUI of recognized numeral nau.
6.2 Testing And Results
6.2.1 Testing with pre-recorded samples
Out of the 20 samples recorded for each word, 16 were used for training purpose. We tested our program’s accuracy with these 4 unused samples. A total of 20 samples were tested (4 samples each for the 5 words) and the program yielded the right result for all 20samples. Thus, we obtained 100% accuracy with pre- recorded samples.
6.2.2 Real-time testing
For real-time testing, we took a sample using microphone and directly executed the program using this sample. A total of 30 samples were tested, out of which 24 samples gave the right result. This gives an accuracy of about 80% with real-time samples.
Case1: Speaker independent (20 templates per digit 10 male, 10 female)
The above implemented work is tested for 100 samples of each word spoken by 50 Different speakers with 2 samples of each digit per head.
The testing work leads to the results given in Table 6.1.
Table 6.1: Accuracy of the Speaker Independent Test Results.
DIGIT 0 1 2 3 4 5 6 7 8 9
% ACCURACY 87 88 82 78 79 84 85 81 78 87
Case2: Speaker Dependent (one template per digit).
The above implemented work is tested for 10 samples of each word spoken by single speaker. The results are:
Table 6.2: Accuracy of the Speaker Dependent Test Results.
DIGIT 0 1 2 3 4 5 6 7 8 9
% ACCURACY 90 91 84 90 87 88 92 84 86 92
It is observed that the accuracy of the pre-recorded samples is more than that of the real-time testing samples. We have also observed that the accuracy of the speaker dependent samples is more than that of the speaker independent samples.
Table 6.3: Confusion Matrix of the MFCC ; DTW Recognition.
ek don teen char pach saha sat aath nau shunya Avg. %
ek 1 1 1 4 1 1 1 1 1 0 80
don 2 2 2 2 3 2 2 2 2 2 90
teen 3 3 3 3 9 3 3 2 2 2 80
char 4 4 5 4 4 4 4 6 4 4 80
pach 5 5 5 5 5 5 5 5 5 3 90
Saha 6 6 6 6 1 6 6 4 6 6 80
Sat 7 7 8 7 7 7 7 7 7 7 90
Aath 2 8 8 8 8 7 8 8 8 8 80
nau 9 9 4 9 9 5 9 9 9 9 80
shunya 0 0 0 0 5 0 0 2 0 0 80
Table 6.4: Confusion Matrix of the MFCC ; HMM Recognition.
ek don teen char pach saha sat aath nau shunya Avg. %
ek 1 1 1 1 1 1 1 3 1 1 90
don 2 2 2 2 2 2 2 2 2 5 90
teen 3 3 3 3 3 3 1 3 3 3 90
char 4 3 4 4 4 4 4 4 8 4 80
pach 5 5 5 5 5 5 5 5 5 5 100
Saha 6 6 6 8 6 6 6 6 6 6 90
Sat 7 7 7 7 7 7 7 5 7 7 90
Aath 8 8 8 8 8 8 8 8 5 8 90
nau 9 9 9 9 9 7 9 9 9 9 90
shunya 0 0 0 7 0 0 0 5 0 0 80
Table 6.5: Comparison Digit Recognition Accuracy Test Results.
Numeral DTW Accuracy HMM Accuracy
ek 80 90
don 90 90
teen 80 90
char 80 80
pach 90 100
Saha 80 90
Sat 90 90
Aath 80 90
nau 80 90
shunya 80 80
Average % 83% 89%
Experimentally, it is observed that recognition accuracy is better for HMM compared with DTW, but the training procedure in DTW is very simple and fast, as compared with the HMM.
Fig.6.13: Recognition accuracy of the DTW ; HMM.
The time required for recognition of numerals using HMM is more as compared to DTW, as it has to go through the many states, iteratations; many more mathematical modeling, so DTW is preferred for the real-time applications as compared with the HMM.
CONCLUSIONS and FUTURE SCOPES
Though the advances accomplished throughout the last decades, automatic speech recognition (ASR) is still a challenging and difficult task.
The non-parametric method for modeling the human auditory perception system, Mel Frequency Cepstral Coefficients (MFCCs) isused as extraction techniques. The nonlinear sequence alignment known as Dynamic Time Warping (DTW) has been used as features matching techniques. The nonlinear sequence alignment known as Dynamic Time Warping (DTW) has been used as features matching techniques. Since it’s obvious that the voice signal tends to have different temporal rate, the alignment is important to produce the better performance.
This paper proposed that higher recognition rates can be achieved using MFCC features with DTW which is useful for different time varying numeral speech utterances.
MFCC analysis provides better recognition rate than LPC as it operates on a logarithmic scale which resembles human auditory system whereas LPC has uniform resolution over the frequency plane. This is followed by pattern recognition. Since the voice signal tends to have different temporal rate, DTW is one of the methods that provide non-linear alignment between two voice signals.
Another method called HMM that statistically models the words is also presented. Experimentally it is observed that recognition accuracy is better for HMM compared with DTW, but the training procedure in DTW is very simple and fast, as compared with the HMM.
The time required for recognition of numerals using HMM is more as compared to DTW, as it has to go through the many states, iteratations& many more mathematical modeling, so DTW is preferred for the real-time applications as compared with the HMM .
DTW is a cost minimization matching technique, in which a test signal is stretched or compressed according to a reference template.
The accuracy of the pre-recorded samples is more than that of the real-time testing samples. We have also observed that the accuracy of the speaker dependent samples is more than that of the speaker independent samples.
7.2 Future Scope
One of the key areas where future work can be concentrated is the large vocabulary generation & to improve robustness of speech recognition performance.
Another key area of research is focused on an opportunity rather than a problem. This research attempts to take advantage of the fact that in many applications there is a large quantity of speech data available, up to millions of hours. It is too expensive to have humans transcribe such large quantities of speech, so the research focus is on developing new methods ofmachine learning that can effectively utilize large quantities of unlabeled data.
The better understanding of human capabilities and to use this understanding to improve machine recognition performance.
The future work could be towards Online Speech Summarization. The
majority of speech summarization research has focused on extracting the most informative dialogue acts from recorded, archived data.
The future work could be towards minimizing the time required for recognition of numerals using HMM.