Patent Publication Number: US-8543395-B2

Title: Methods and systems for performing synchronization of audio with corresponding textual transcriptions and determining confidence values of the synchronization

Description:
BACKGROUND 
     Speech recognition (sometimes referred to as automatic speech recognition (ASR) or computer speech recognition) converts spoken words to text. The term “voice recognition” is sometimes used to refer to speech recognition where a recognition system is trained to a particular speaker to attempt to specifically identify a person speaking based on their unique vocal sound. 
     Speech recognition systems are generally based on Hidden Markov Models (HMM), which are statistical models that output a sequence of symbols or quantities. A speech signal can be viewed as a piecewise stationary signal or a short-time stationary signal, such that in a short-time, speech could be approximated as a stationary process. Speech could thus be thought of as a Markov model for many stochastic processes. 
     The HMMs output a sequence of n-dimensional real-valued vectors for each stationary signal. The vectors include cepstral coefficients, which are obtained by taking a Fourier transform of a short time window of speech, de-correlating the transform, and taking the first (most significant) coefficients. The HMM may have a statistical distribution that gives a likelihood for each observed vector. Each word or each phoneme may have a different output distribution. An HMM for a sequence of words or phonemes is made by concatenating individual trained HMMs for the separate words and phonemes. 
     Decoding of speech (e.g., when an ASR is presented with a new utterance and computes a most likely source sentence) may be performed using a Viterbi decoder that determines an optimal sequence of text given the audio signal, expected grammar, and a set of HMMs that are trained on a large set of data. 
     SUMMARY 
     In one example aspect, a method of processing audio signals is provided. The method includes receiving an audio signal comprising vocal elements, and performing an alignment of the vocal elements with corresponding textual transcriptions of the vocal elements. The method further includes based on the alignment, determining timing boundary information associated with an elapsed amount of time for a duration of a portion of the vocal elements, and outputting a confidence metric indicating a level of certainty for the timing boundary information for the duration of the portion of the vocal elements. 
     In one embodiment, a forward alignment of the vocal elements processed in a forward direction with corresponding textual transcriptions of the vocal elements is performed, and a reverse alignment of the vocal elements processed in a reverse direction with corresponding reverse textual transcriptions of the vocal elements is performed. In addition, the method includes determining forward timing boundary information associated with an elapsed amount of time for a duration of a portion of the vocal elements processed in the forward direction, and determining reverse timing boundary information associated with an elapsed amount of time for a duration of the portion of the vocal elements processed in the reverse direction. In this embodiment, the confidence metric is output based on a comparison between the forward timing information and the reverse timing information, for example. 
     In another embodiment, the audio signal is a song comprising lyrics, and the method further includes synchronizing the corresponding textual transcriptions of the vocal elements with the audio signal, and outputting time-annotated synchronized lyrics that indicate timing information of lines of the lyrics in relation to the audio signal. 
     In another example aspect, a computer readable storage medium having stored therein instructions executable by a computing device to cause the computing device to perform functions is provided. The functions include receiving an audio signal comprising vocal elements, and performing an alignment of the vocal elements with corresponding textual transcriptions of the vocal elements. The functions also include based on the alignment, determining timing boundary information associated with an elapsed amount of time for a duration of a portion of the vocal elements, and outputting a confidence metric indicating a level of certainty for the timing boundary information for the duration of the portion of the vocal elements. 
     In still another example aspect, a system is provided that comprises a Hidden Markov Model (HMM) database that may include statistical modeling of phonemes in a multidimensional feature space (e.g. using Mel Frequency Cepstral Coefficients), an optional expected grammar that defines words that a speech decoder can recognize, a pronunciation dictionary database that maps words to the phonemes, and a speech decoder. The speech decoder receives an audio signal and accesses the HMM, expected grammars, and a dictionary to map vocal elements in the audio signal to words. The speech decoder further performs an alignment of the audio signal with corresponding textual transcriptions of the vocal elements, and determines timing boundary information associated with an elapsed amount of time for a duration of a portion of the vocal elements. The speech decoder further determines a confidence metric indicating a level of certainty for the timing boundary information for the duration of the portion of the vocal elements. 
     In one embodiment, the speech decoder synchronizes textual transcriptions of the vocal elements with the audio signal, and outputs time-annotated synchronized lyrics that indicate timing boundary information of lines of lyrics in relation to the audio signal. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an illustrative embodiment of a system for performing speech recognition and synchronizing text to the recognized speech. 
         FIG. 2  shows an illustrative embodiment of another system for performing speech recognition and synchronizing text to the recognized speech. 
         FIG. 3  illustrates a conceptual diagram showing the reversing of the input lyrics. 
         FIG. 4  is a conceptual illustration of an example of determining mismatches between forward and reverse alignments. 
         FIG. 5  is a conceptual illustration of an example of determining outliers of synchronized or mapped lines using either forward and reverse alignments. 
         FIG. 6  shows a flowchart of an illustrative embodiment of a method for processing audio signals. 
         FIG. 7  shows a flowchart of another illustrative embodiment of a method for processing audio signals. 
         FIG. 8  shows a flowchart of an illustrative embodiment of a method for processing audio signals in an iterative manner. 
         FIG. 9  is a block diagram illustrating a hierarchical HMM training and model selection. 
         FIG. 10  shows a flowchart of an illustrative embodiment of a method for adapting HMM using existing synchronized-lyrics data from a specific performer. 
         FIG. 11  is a block diagram illustrating an example parallel synchronization system. 
         FIG. 12  is a block diagram of an example system for selecting an appropriate HMM. 
         FIG. 13  is a block diagram of an example system for hybrid synchronization of audio and lyrics. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the Figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     In example embodiments, audio and a corresponding text (e.g., transcript) may be synchronized (using speech recognition techniques in some examples), and a resulting timing metadata may be used in many different applications, such as, for example, to enable a contextual search of audio, browsing of audio, as well as display of text as audio is being played (e.g., subtitles, karaoke-like display of lyrics, etc.). 
     Example embodiments describe methods for obtaining the timing metadata, computing confidence flags for the time-synchronization metadata, and enhancing an automated synchronization process using confidence information. For example, information obtained in an automated manner may not always be accurate due to a possible mismatch between input audio and acoustic models, as well as inaccuracies in a transcript, and thus, a confidence measure that describes a quality of timing information is generated to enhance a quality of inaccurate timing metadata using automated or manual methods. 
       FIG. 1  shows an illustrative embodiment of a system  100  for performing automated synchronization using speech recognition techniques. The system  100  receives an audio signal at an audio engine  102 . The audio signal may include a speech, a song or musical data, a TV signal, etc., and thus, may include spoken or sung words and accompanying instrumental music or background noise. The audio engine  102  suppresses any instrumental music or background noise and outputs the spoken or sung words (e.g., vocals) to an automated speech recognition (ASR) decoder  104 . When the input audio signal is a musical song, the spoken or sung words may correspond to lyrics of the song, for example. 
     The audio engine  102  may suppress any instrumental music in the audio signal using techniques that leverage the fact that vocals are usually centered in a stereo signal and instrumentals are not. Music (or other non-vocal data) can also be suppressed using frequency analysis methods to identify regions that are harmonically rich. As an example, the audio engine  102  may process the audio signal using the Vocal Remover product from iZotope, Inc. The audio engine  102  may suppress non-vocal data so as to extract the vocal data or data representing spoken utterances of words, for example. 
     The system  100  also receives a lyrics text file corresponding to the lyrics of the audio signal at a filter  106 . The filter  106  cleans and normalizes the lyrics text. For example, the filter  106  may correct misspelling errors using lookup tables, modify vocalizations (e.g., words like ‘heeeey’, ‘yeah’, etc.) can be reduced to a smaller set (e.g. ‘heeeey’ and ‘heeey’ will be changed to ‘heey’), perform grammatical changes capitalize first letter of each line), and remove extraneous non-lyrical text (e.g., name of the artist and the song, tags potentially identifying musical segments such as chorus or verse). 
     A grammar processor  108  receives the lyrics text from the filter  106 , and creates “grammars” that indicate text that is expected to be in the vocals in the audio signal. The lyrics text can be transformed into a sequence of words accompanied by “words” modeling instrumental (music-only) portions of the signal inserted at the beginning and end. Optional instrumental and/or filler models can be inserted between words in the lyrics to account for voice rest and possible background accompaniment. 
     The ASR decoder  104  receives the vocals from the audio engine  102  and grammars from the grammar processor  108  and performs lyric synchronization. In an example where accurate lyrics are known ahead of time, the ASR decoder  104  will perform a forced-alignment of audio and lyrics, i.e., the expected response in the grammars will be mapped to corresponding words that are sung. Accurate lyrics may be determined based on a source of the lyrics text. If the lyrics text is received from a trusted source, then accurate lyrics can be assumed, and forced-alignment can be used to map the lyrics to the audio signal. Thus, using force alignment, grammars are defined so that there is no branching, i.e., only certain possible sequences of words can be recognized. Timing information can be stored for a beginning and ending time for each line of lyrics in relation to elapsed amount of time of the song, for example, by including a timestamp or counter (not shown) in the system  100  or as a function of the ASR decoder  104 . 
     The ASR decoder  104  may have access to a pronunciation dictionary database  110  that defines phonetic representations of a word (e.g., phonemes). Although the dictionary database  110  is illustrated separate from the system  100 , in other examples, the dictionary database  110  may be a component of the system  100  or may be contained within components of the system  100 . 
     The filter  106  may clean the lyrics text and prepare the lyrics for the grammar processor  108 . The grammar processor  108  will create expected response grammars from the cleaned lyrics. If the lyric source is not trusted, or if the lyrics text is not likely to fully match the words in the audio signal, the grammar processor  108  may create a stochastic grammar. To create stochastic grammar, the grammar processor  108  may place all the lines of lyrics in parallel and allow any arbitrary sequence of lyric lines to be recognized. The grammar processor  108  may insert optional and multiple words modeling instrumentals between words and at a beginning and an end of the grammar. In addition, filler word models may be used to model occurrences of non-words (vocalizations, etc.). Thus, in examples of untrusted lyric sources, grammars can be defined in a manner that allows for branching (e.g., any line of lyrics can follow any other line). 
     The audio engine  102  may analyze the suppressed audio signal by extracting feature vectors about every 10 ms (e.g., using Mel Frequency Cepstral Coefficients or (MFCC)). The ASR decoder  104  may then map the sequence of feature vectors to the expected response defined in the grammar. The ASR decoder  104  will expand the word grammar created by the grammar processor  108  into a phonetic grammar by using the dictionary database  110  to expand words into phonemes. The ASR decoder  104  may use a Hidden Markov Model (HMM) database  112  that statistically describes each phoneme in the features space (e.g., using MFCC) to obtain an optimal sequence of words from the phonemes that matches the grammar of the audio signal and corresponding feature vector. Although the HMM database  112  is illustrated separate from the system  100 , in other examples, the HMM database  112  may be a component of the system  100  or may be contained within components of the system  100 . 
     HMMs are typically trained on a large set of relevant data; in the context of lyric synchronization that could be a large set of songs. Estimation of model parameters can be performed using the Baum-Welch algorithm, for example. Parameters of the model can be determined by re-estimation given a set of training examples corresponding to a particular model, for example. 
     The ASR decoder  104  may use an HMM from the database  112  to decode the audio signal using a Viterbi decoding algorithm that determines an optimal sequence of text given the audio signal, expected grammar, and a set of HMMs that are trained on a large set of data, for example. Thus, the ASR decoder  104  uses the HMM database  112  of phonemes to map spoken words to a phonetic description, and uses the dictionary database  110  to map words to the phonetic description, for example. 
     The ASR decoder  104  will perform speech recognition or force alignment on the audio signal to create a sequence of word and phonetic transcriptions corresponding to speech in the audio signal. 
     When performing lyric synchronization, the ASR decoder  104  will also perform a timing analysis of the phonetic description. In one example, a set of input lyrics text and corresponding phonetic transcriptions are as shown below in Table 1. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Lyric 
                   
               
               
                 Line 
                 Input Lyrics Text (words and corresponding phonetic transcription) 
               
               
                   
               
             
            
               
                 1 
                 Would You Believe Your Eyes 
               
               
                   
                 W UH D . Y UW . B IH L IY V . Y AO R . AY Z 
               
               
                 2 
                 As I Fell Asleep If Fireflies 
               
               
                   
                 AE Z . AY . F EH L . AH S L IY P . IH F . F AY ER F L AY Z 
               
               
                 3 
                 Produce Light For The World 
               
               
                   
                 PROH DOOCE . L AY T . F OUR . DH AH . W ER L D 
               
               
                   
               
            
           
         
       
     
     The phonetic transcription may be a standard dictionary transcription, such that, for example, the word “asleep” may be phonetically transcribed as “AH SH L IY P”, and periods and spaces are used for clarity to indicate beginning/end of word transcriptions, to indicate pauses in the speech, or to indicate background instrumentals that may be heard between words. Note that for simplicity purposes, only a first three (out of N total) lines of the lyrics text are displayed in Table 1. 
     After performing speech recognition, the audio signal may be matched to the input lyrics, so as to generate output lyrics as shown below in Table 2. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Start 
                   
                 End 
               
               
                 Lyric 
                 Time 
                 Output lyrics text  
                 Time 
               
               
                 Line 
                 [sec] 
                 (words and corresponding phonetic transcription) 
                 [sec] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 22 
                 Would You Believe Your Eyes 
                 24.4 
               
               
                   
                   
                 W UH D . Y UW . B IH L IY V . Y AO R . AY Z 
                   
               
               
                 2 
                 24.7 
                 As I Fell Asleep If Fireflies 
                 27 
               
               
                   
                   
                 AE Z . AY . F EH L . AH S L IY P . IH F .  
                   
               
               
                   
                   
                 F AY ER F L AY Z 
                   
               
               
                 3 
                 27.4 
                 Produce Light For The World 
                 30.2 
               
               
                   
                   
                 PROH DOOCE . L AY T . F OUR . DH AH .  
                   
               
               
                   
                   
                 W ER L D 
               
               
                   
               
            
           
         
       
     
     In addition, timing information may be output with the output lyrics, as shown Table 2. The timing information may indicate an elapsed amount of time from a beginning of a song from which the audio signal was obtained, or an elapsed amount of time from a beginning of the received audio signal to a beginning of the line of text (e.g., lyrics), and an elapsed amount of time from a beginning of the audio signal to an end of the line of lyrics. The timing information may alternatively (or additionally) include an amount of time elapsed during a line, a word, or a phoneme of the lyrics. 
     As shown in Table 2, a first line of the output lyrics may have a start time of 22 seconds and an end time of 24.4 seconds. The start and end times are an elapsed amount of time from a beginning of the audio signal, for example. A second line of output lyrics is shown in Table 2 to have a start and end time of 24.7 and 27 seconds, and a third line of output lyrics is shown in Table 2 to have a start and end time of 27.4 and 30.2 seconds. 
     To determine the timing information, the ASR decoder  104  identifies an elapsed amount of time from a beginning of the audio signal to a time when vocals of the audio signal begin when the audio signal is played in a forward direction. Note that in the above example, timing information is specified at the line level, so the first line starts at 22 seconds and ends at 24.4 seconds. However, timing information may also be provided at a word level as well. 
     The ASR decoder  104  may determine timing information as a by-product of performing speech recognition. For example, a Viterbi decoder determines an optimal path through a matrix in which a vertical dimension represents HMI states and a horizontal dimension represents frames of speech (e.g., 10 ms). When an optimal sequence of HMM states is determined, an optimal sequence of corresponding phonemes and words is available. Because each pass through the HMM state consumes a frame of speech, the timing information at the state/phoneme/word level is available as the output of the automated speech recognition. 
     Alternatively, the ASR decoder  104  may include, have access to, or be operated according to a timer to determining the timing information, for example. 
     The system  100  in  FIG. 1  may perform time-synchronization of lyrics and audio in a batch mode (i.e., not in real-time but instead by using a recording of the audio signal stored in file) so as to create the timing information as shown in Table 2 above for a number of audio signals or songs. 
     Components of the system  100  in  FIG. 1  include engines, filters, processors, and decoders, any of which may include a computing device or a processor to execute functions of the components. Alternatively, any of the components of the system  100  in  FIG. 1  may have functions embodied by computer software, which when executed by a computing device or processor cause the computing device or processor perform the functions of the components, for example. Thus, although not shown, the system  100  may include memory to store the computer software as well. 
       FIG. 2  shows an illustrative embodiment of another system  200  for performing speech recognition and synchronizing text to the recognized speech. Many of the components of the system  200  are similar to components of the system  100 , and may be embodied as computer hardware or software. For example, the system  200  includes an audio engine  202  that receives an audio signal, suppresses instrumentals of the audio signal, and outputs vocals of the audio signal. The audio engine  202  may output the vocals in a forward (direct) form and in a reverse form. The forward form is the vocals as spoken naturally in a forward direction, the reverse form is the vocals reversed in a backwards or opposite direction. To output the vocals in the reverse form, the audio engine  202  may playback the audio signal in an opposite direction, for example. The reverse form of the vocals may not be intelligible or understandable by a listener; however, the reverse form of the vocals can be used to further analyze the audio signal, for example. In one example, the audio engine  202  may use the Sox software from Sound eXchange to reverse input audio signals. 
     The system also includes an ASR decoder  204  to receive the forward and reverse audio signals from the audio engine  202 , and to perform speech recognition and lyric synchronization of the audio signals. 
     A filter  206  receives lyrics text that corresponds to lyrics of the audio signal, and the filter  206  cleans and normalizes the lyrics text to output the text in a direct or forward direction and in a reverse or backwards direction. The forward words output from the filter  206  are the words of the lyrics written from left to right in a standard forward direction (as words as written in this disclosure). The reverse words output from the filter  206  are the words of the lyrics written/read from right to left in a backwards direction, and thus, only the order of the words may be reversed, for example. 
     A grammar processor  208  receives the words of the lyrics in the forward and reverse direction, and outputs “grammars” corresponding to words in the forward and reverse directions. 
     The ASR decoder  204  receives the forward and reverse grammars from the grammar processor  208 , as well as forward and reverse dictionary word to phoneme mappings for the forward and reverse grammars from a dictionary database  210  to map words to phonetic transcriptions, for example. The ASR decoder  204  further receives statistical models of forward and reverse phonemes (e.g., small units of speech or sound that distinguish one utterance from another) from an HMM database  212 . Acoustic (HMM) models for the reverse path will be trained on a training set of songs that were reversed, for example. Either or both of the dictionary database  210  and the HMM database  212  may be components of the system  200 , or may be contained within components of the system  200 , in other examples. 
     The ASR decoder  204  may perform mapping or synchronization of the audio signal to the lyrics text in the forward direction and in the reverse direction, for example. When performing the synchronization, the ASR decoder  204  may further output timing information as described above. Example methods of the forward synchronization are described above with reference to Tables 1 and 2. 
     To perform a reverse synchronization, the ASR decoder  204  uses the reverse audio, reverse grammar, reverse phonetic dictionary (e.g., the word “asleep” is phonetically transcribed as ‘P IY L SH AH’ in the reverse phonetic dictionary), and reverse HMMs (e.g., each phoneme will be trained on reversed audio data, and thus, a model for phoneme ‘ah’ in forward and reverse HMM set would be different). Table 3 below illustrates reverse input lyrics and reverse phonetic transcriptions of the lyrics in Table 1. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Lyric 
                 Reverse Input Lyrics Text 
               
               
                 Line 
                 (words and corresponding phonetic transcription) 
               
               
                   
               
             
            
               
                 N-2 
                 World The For Light Produce 
               
               
                   
                 D L RE W . HA HD . RUO F . T YA L . ECOOD HORP 
               
               
                 N-1 
                 Fireflies If Asleep Fell I As 
               
               
                   
                 Z YA L F RE YA F . F HI . P YI L S HA . L HE F. YA. Z EA 
               
               
                 N 
                 Eyes Your Believe You Would 
               
               
                   
                 Z YA. R OA Y . V YIL HIB . WUY . D HU W 
               
               
                   
               
            
           
         
       
     
     The reverse input lyrics shown in Table 3 are the reverse input lyrics of Table 1. As shown Table 3, a first line of the audio signal is the last line of the audio signal in Table 1. Thus, the lines of the lyrics are in reverse order, and also, the words in the lines are in reverse order (e.g., reversed from the order in Table 1). Further, the corresponding phonetic transcription of lyrics, mapped via the reverse dictionary database  10 , are also in reverse order (e.g., read from right to left in reverse order). Note that for simplicity only the last 3 lines of lyrics (out of N total) are displayed in the example. 
       FIG. 3  illustrates a conceptual diagram showing the reversing of the input lyrics. As shown, for the reverse lyrics, Line N in the forward direction becomes a first line in the reverse direction (Line  1   R ), Line N−1 in the forward direction becomes a second line in the reverse direction (Line  2   R ), and so forth until Line  1  in the forward direction becomes a the last line in the reverse direction (Line N R ), for example. 
     Table 4a below indicates output lyrics with corresponding output timing information. In the same manner as described above for the forward direction, timing information may be output with the output lyrics in the reverse direction that may indicate an elapsed amount of time from a beginning of the received reversed audio signal. The timing information may be output as an elapsed amount of time from a beginning of the audio signal to a beginning of the line of lyrics (line start time and an elapsed amount of time from a beginning of the audio signal to an end of the line of lyrics (line end time). 
     As shown in Table 4a, a first line of the reverse output lyrics may have a start time of 197.8 seconds and an end time of 200.6 seconds. The start and end times are an elapsed amount of time from a beginning of the reversed audio signal, for example. A second line of reverse output lyrics is shown in Table 4a to have a start and end time of 202.5 and 203.3 seconds, and a third line of reverse output lyrics is shown in Table 4a to have a start and end time of 203.6 and 206 seconds. 
     To determine the timing information, the ASR decoder  204  identifies an elapsed amount of time from a beginning of the reverse audio signal to a time when vocals of the reverse audio signal begin when the audio signal is played in a reverse direction. Note that in the above example, timing information is specified at the line level, so the line N−2 starts at 197.8 seconds and ends at 200.6 seconds. However, timing information may also be provided at a word level as well. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4a 
               
               
                   
               
               
                   
                   
                   
                 End 
               
               
                 Lyric 
                 Start 
                 Output lyrics text  
                 Time 
               
               
                 Line 
                 [sec] 
                 (words and corresponding phonetic transcription) 
                 [sec] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 N-2 
                 197.8 
                 World The For Light Produce 
                 200.6 
               
               
                   
                   
                 D L RE W . HA HD . RUO F . T YA L .  
                   
               
               
                   
                   
                 ECOOD HORP 
                   
               
               
                 N-1 
                 202.5 
                 Fireflies If Asleep Fell I As 
                 203.3 
               
               
                   
                   
                 Z YA L F RE YA F . F HI . P YI L S HA .  
                   
               
               
                   
                   
                 L HE F. YA. Z EA 
                   
               
               
                 N 
                 203.6 
                 Eyes Your Believe You Would 
                 206  
               
               
                   
                   
                 Z YA. R OA Y . V YIL HIB . WUY . D HU W 
               
               
                   
               
            
           
         
       
     
     The ASR decoder  204  outputs the reverse output lyrics to a word and time reverter  214 . The outputs of the reverse lyrics are W N-i   R  that indicates the reversed lines/words and T N-i   R  that indicates the corresponding mapped timing of the lines/words. The word and time reverter  214  will reverse or put the lines/words from the reverse output back to a forward direction according to Equation (1) below.
 
 W   i   RR   =W   N-i   R   , i= 1 :N   Equation (1)
 
The output of the word and time reverter  214  is W i   RR  which indicates reversed output text of the reverse alignment.
 
     The timing information for start of a (or word), i, can be computed as:
 
 T   i   RR   =T   total   −T   N-i   R   Equation (2)
 
where T total  is a duration of the song or audio signal and T i   R  is an end time of the line i in reversed synchronized lyrics.
 
     In the example described herein, a total duration of the song, T total , is 228 seconds. Table 4b below shows example data as the output of the word and time reverter  214 . 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4b 
               
               
                   
               
               
                   
                 Start 
                   
                 End 
               
               
                 Lyric 
                 Time 
                 Output lyrics text  
                 Time 
               
               
                 Line 
                 [sec] 
                 (words and corresponding phonetic transcription) 
                 [sec] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 22 
                 Would You Believe Your Eyes 
                 24.4 
               
               
                   
                   
                 W UH D . Y UW . B IH L IY V . Y AO R . AY Z 
                   
               
               
                 2 
                 24.7 
                 As I Fell Asleep If Fireflies 
                 25.5 
               
               
                   
                   
                 AE Z . AY . F EH L . AH S L IY P . IH F .  
                   
               
               
                   
                   
                 F AY ER F L AY Z 
                   
               
               
                 3 
                 27.4 
                 Produce Light For The World 
                 30.2 
               
               
                   
                   
                 PROH DOOCE . L AY T . F OUR . DH AH .  
                   
               
               
                   
                   
                 W ER L D 
               
               
                   
               
            
           
         
       
     
     The ASR decoder  204  may output the forward synchronized lyrics and corresponding timing information, and the “reversed” reverse synchronized lyrics and timing information to a confidence score engine  216 . The confidence score engine  216  computes confidence flags or scores for the timing information using a mismatch between the forward and reverse alignment. 
     To determine a mismatch between the forward and reverse alignment, the confidence score engine  216  compares a difference between the forward and reverse timing information to a predefined threshold, and marks the line as a low or high confidence line in accordance with the comparison. Line timing information may be defined as T n   BP  where n is the line index, B defines a boundary type (S for start time, E end time) and P defines pass type (F for forward, R for reverse), then a start mismatch for line n is defined as:
 
 MM   n   S   =abs ( T   n   SF   −T   n   SR )  Equation (3)
 
and an end mismatch for line n is defined as:
 
 MM   n   E   =abs ( T   n   EF   −T   n   ER )  Equation (4)
 
The mismatch metrics can then be compared to a predefined threshold to determine if the line should be flagged as a low or high confidence line.
 
       FIG. 4  is a conceptual illustration of an example of determining mismatches between the forward and reverse alignments. Using the above example, start and end mismatch metrics would have values of zero for line boundaries of the first and last lines. The start mismatch metric for the second line would have a value of zero, however, the end mismatch metric would have a value of 1.5 seconds (MM n   E =abs(T n   EF   =T   n   ER ), T 2   EF =27, T 2   ER =25.5, and MM 2   E =abs(27−25.5)=1.5). The value of MM 2   E  would be compared to a threshold value, and if 1.5 seconds exceeds the threshold value, then the second line of the lyrics would be flagged as a low confidence line. The second line of the forward and/or reversed aligned lyrics could be flagged. 
     The threshold value may be any value, for example such as about one second, and may depend to some extent on a type of the audio signal. For example, the threshold may be dynamic, such that for faster songs where lines of lyrics may be shorter in length, the threshold may be decreased. The threshold for the confidence flag may be determined using techniques that minimize classification errors based on an example training set. For example, a number of false positives and or false negatives (i.e., where a line has correct boundaries but has been marked with low confidence, or has incorrect boundaries and has been marked with a high confidence) may be used as a training set. 
     In addition, a cost function may used be when determining the threshold to minimize errors that may be more relevant for a specific application, for example, to minimize a number of bad boundaries that are flagged as good (in a case where accuracy is desired) or to minimize a number of good boundaries that are flagged as bad (in a case where minimizing additional processing cost is desired). 
     The above example uses lines of lyrics, however, the mismatch metrics may also be used at any granularity level of content, such as words or phonemes. 
     The confidence score engine  216  may also analyze forward (or reverse) recognition results and determine a probability metric of line duration given a distribution of durations of all lines in the song or audio signal. This metric leverages the symmetric notion of modern western songs and computes a probability that a duration of a specific line fits a line duration model for a song or audio signal, for example. Given the duration of each line as determined in the automated alignment process (e.g., taken from the forward and/or reverse alignment), a parametric model of line duration can be estimated by calculating a mean and standard deviation of line duration. Then, for each line, if a distance from the mean duration is larger than a threshold, e.g., two standard deviations, the line is flagged as a low-confidence line. A value of the threshold may differ, and may be dynamic, based on an application or desired level of accuracy of the timing boundary information, for example. 
     Table 5 below illustrates computing line duration, mean, and standard deviation using the examples above in Tables 1-2 for the forward alignment. In the example in Table 5, a line is marked as a low confidence line if the distance to the mean (or difference between the line duration and the mean) is greater than one standard deviation. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Forward Time 
                 Distance to  
                   
                   
                 Standard 
               
               
                   
                 Duration 
                 mean 
                 Confidence 
                 Mean 
                 Deviation 
               
               
                   
               
             
            
               
                 Line 1 
                 2.4 
                 0.1 
                 High 
                 2.5 
                 0.216 
               
               
                 Line 2 
                 2.3 
                 0.2 
                 High 
                   
                   
               
               
                 Line 3 
                 2.8 
                 0.3 
                 Low 
               
               
                   
               
            
           
         
       
     
     A confidence score may also be computed and output from the confidence score engine  216  on a word level, in addition to or rather than on a line level, for example. 
     In other embodiments, the confidence score engine  216  may create a model of a line duration, and estimate a probability that the line is an outlier from the model based on a comparison of line durations. An outlier may indicate that the line was incorrectly processed during speech recognition, for example. The HMM models are generally not trained on the exact input audio signal, but rather are trained on training data. Thus, input audio signals may differ from those used to train the HMM models, which can result in errors during speech recognition or force-alignment. 
     Thus, methods are provided for computing confidence scores or metrics that include performing a comparison of alignment in forward and reverse directions, and performing line-duration confidence measures, for example. 
       FIG. 5  is a conceptual illustration of an example of determining outliers of synchronized or mapped lines using either the forward and reverse alignments. As shown, Lines  1 ,  2 , N−1, and N each have substantially equal timing information. However, Line  3  has timing information T 3  (or length) that may differ by more than a threshold amount from the length of Line  1 , T 1 , or from the length of Line  2 , T 2 . Thus, Line  3  may be marked as an outlier using the line duration comparison. 
     In one example, estimation of line duration distribution may be constrained to lines of lyrics that belong to a same type of music segment (e.g., chorus only) as the line for which confidence is being estimated. For example, a song may be divided based on segments of the song (verse, chorus, bridge), and a value used for line duration, and thus, values of mean and standard deviation used to determine a confidence score, can be taken from a respective segment. For instance, when determining a confidence score of a line from the chorus, line durations values of lyrics corresponding to the chorus may be used. 
     The system  200  thus may output synchronized audio/lyrics in a forward and reverse direction, timing boundary information of words or lines of the lyrics in relation to the audio signal, and a confidence score/flag indicating how confident or reliable that the timing boundary information or content of the lyrics may be considered. The confidence score may be determined in a number of ways, for example, based on comparison of forward and reverse timing boundary information, using line duration comparisons, using comparisons of multiple alignments performed with multiple HMMs, etc. The system  200  may include or output the data to a database, and thus, the system  200  may process songs or audio signals in a batch mode to create a set of timed-annotated lyrics from a set of music and lyric files. 
     The system  200  may further use speech recognition techniques to map expected textual transcriptions of the audio signal to the audio signal. Alternatively, correct lyrics are received and are taken as the textual transcriptions of the vocal elements in the audio signal (so that speech recognition is not needed to determine the textual transcriptions), and a forced alignment of the lyrics can be performed to the audio signal to generate timing boundary information, for example. 
       FIG. 6  shows a flowchart of an illustrative embodiment of a method  600  for processing audio signals. It should be understood that for this and other processes and methods disclosed herein, the flowchart shows functionality and operation of one possible implementation of present embodiments. In this regard, each block may represent a module, a segment, or a portion of program code, which includes one or more instructions executable by a processor for implementing specific logical functions or steps in the process. The program code may be stored on any type of computer readable medium, for example, such as a storage device including a disk or hard drive. The computer readable medium may include non-transitory computer readable medium, for example, such as computer-readable media that stores data for short periods of time like register memory, processor cache and Random Access Memory (RAM). The computer readable medium may also include non-transitory media, such as secondary or persistent long term storage, like read only memory (ROM), optical or magnetic disks, compact-disc read only memory (CD-ROM), for example. The computer readable media may also be any other volatile or non-volatile storage systems, or other computer readable storage mediums. 
     In addition, each block in  FIG. 6  may represent circuitry that is wired to perform the specific logical functions in the process. Alternative implementations are included within the scope of the example embodiments of the present disclosure in which functions may be executed out of order from that shown or discussed, including substantially concurrent or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art. 
     Initially in the method  600 , an input audio signal and corresponding lyrics text are received, as block  602 . The input audio signal may include both vocal elements and non-vocal elements, and may be a musical track or song, for example, or only a portion of a musical track or song. Following, as an optional step, instrumentals (or non-vocals) may be suppressed, as shown at block  604 . 
     Then, an alignment of the vocal elements with the corresponding textual transcriptions of the vocal elements is performed, as shown at block  606 . Timing boundary information can then be determined that is associated with an elapsed amount of time for a duration of a portion of the vocal elements, as shown at block  608 . 
     A confidence metric may then be output that indicates a level of certainty for the timing boundary information for the duration of the portion of the vocal elements, as shown at block  610 . The confidence metric may be determined in any number of ways, for example, such as by comparing line durations of the vocal elements to search for outliers, by comparing a forward and reverse alignment output, by comparing alignments performed in parallel or serial and using different HMMs. Other examples are possible as well. 
       FIG. 7  shows a flowchart of another illustrative embodiment of a method  700  for processing audio signals. Initially in the method  700 , an input audio signal and corresponding lyrics text are received, as block  702 . The input audio signal may include both vocal elements and non-vocal elements, and may be a musical track or song, for example, or only a portion of a musical track or song. Following, as an optional step, instrumentals (or non-vocals) may be suppressed, as shown at block  704 . Then forward and reverse grammars are determined from the lyrics text, as shown at block  706 . 
     Next, a forward alignment of the grammars for the lyrics text processed in a forward direction with corresponding phonetic transcriptions of the vocal elements is performed, as shown at block  708 . As part of the forward alignment, at the same time, or subsequently, a duration of a line, word, or phoneme of the grammars corresponding to the lyrics text is determined. The duration may indicate an elapsed amount of time from a beginning of the input audio signal to an end of the line of grammars, or an elapsed amount of time from a beginning of the line of grammars to the end of the line of grammars, for example. 
     In addition, a reverse alignment of the grammars for the lyrics text processed in a reverse direction with corresponding phonetic transcriptions of the vocal elements is performed, as shown at block  710 . As part of the reverse alignment, at the same time, or subsequently, a duration of a line, word, or phoneme of the reverse grammars corresponding to the reverse lyrics text is determined. The forward and reverse alignment may be performed in parallel (at the same time or substantially same time) or in a serial manner, for example. 
     The forward and reverse line boundaries are then compared to compute mismatches for each line/word of the lyrics, at block  712 . As one example, the start and end mismatch metrics described in Equations (2)-(3) are computed and compared to a threshold value. 
     Based on the comparison performed, a determination is made whether the metric is within a given threshold, at block  714 . If the metric is within the threshold, the line of lyrics is marked as a high confidence line, at block  716 . A high confidence line has a high reliability, certainty, or probability that the start and end time of the line highly or reliably corresponds to the vocal elements in the input audio signal. If the metric is not within the threshold, the line of lyrics is marked as a low confidence line, at block  718 . A low confidence line has a low reliability, certainty, or probability that the line of grammars reliably corresponds to the vocal elements in the input audio signal. 
     As another example, at block  720 , a probability metric of line duration can be computed and compared to a threshold (e.g., two standard deviations of line duration), at block  722 . If the metric is within the threshold, the line of lyrics is marked as a high confidence line, at block  716 . If the metric is not within the threshold, the line of lyrics is marked as a low confidence line, at block  724 . 
     Following, audio synchronized with corresponding text, timing information, and/or confidence scores of each line of text are output, at block  726 . The audio synchronized with corresponding text may also include time-annotations indicating a duration of a line of the text, for example. The confidence scores may indicate values of any one of the metrics described herein, or may include a high or low confidence value, for example. 
     The information output from the method  700  may be used in many different applications. Examples of such applications are described below. 
     In one example, in the system  100  of  FIG. 1  or the system  200  of  FIG. 2 , Hidden Markov models are used for automated speech recognition, and the HMMs may be trained on a large corpus of data that aims to provide a good coverage of acoustic space, as well as generalization such that models work well on unseen speech. 
     Hidden Markov Models may be trained on a large set of training data with the goal that all variations of multiple speakers are captured. Such a type of HMM is referred to as speaker independent. Alternative HMMs can be obtained when models are trained on data that corresponds to a specific speaker, and such HMMs are referred to as speaker dependent systems. Speaker dependent systems may require that a large amount of training data for a specific speaker be collected for training purposes. However, instead of training speaker dependent models, adaptation techniques can be used. For example, using a small amount of data from the speaker, the HMM can be transformed to better fit characteristics of the speaker&#39;s voice. High-quality results can be achieved when using data with known transcriptions (e.g., supervised adaptation) and with a batch of data available for adaptation (e.g., static adaptation) opposed to incremental adaptation where models are adapted as more data is available. Linear transformations can be used to adapt the models, in which a set of transformations is computed using a Maximum Likelihood Linear Regression that reduces a mismatch between the adaptation data and an initial model set. Alternatively, a Maximum a Posteriori (MAP) technique can also be used to adapt HMMs, in which prior knowledge about model parameters distribution is used. 
     In an example embodiment, the methods of  FIG. 6  or  FIG. 7  may be performed in an iterative manner. The methods  600  or  700  may be performed in a first iteration, and lines (or words) of the speech or lyrics that have high-confidence scores can be selected and stored. The HMMs may then be adapted using the high-confidence data of the lines (or words) of the lyrics that have high-confidence scores using supervised adaptation techniques. For example, the methods  600  or  700  may be performed in a second iteration using the retrained HMM to attempt to acquire a larger number of high-confidence scores on the lines of lyrics. The HMMs may be retrained again with resulting high-confidence data, and an iterative synchronization process may continue by enhancing the HMMs via adaption using high-confidence lines output from the methods  600  or  700 , for example. 
       FIG. 8  shows a flowchart of an illustrative embodiment of a method  800  for processing audio signals in an iterative manner. Initially, audio and lyrics are aligned using any of the methods described herein, at block  802 . Time-annotated audio information is output as well as confidence scores or metric values indicating a number of high-confidence lines. Next, if the audio alignment process resulted in a number of high confidence lines greater than a threshold value, at block  804  (e.g., N which may be based on amount of data needed to perform supervised adaptation, e.g., more than 1 minute of audio data), then the HMMs are adapted and retrained using the data from the high confidence lines, at block  806 . The audio and lyrics may then be realigned using the retrained HMMs, for example. 
     An output of the realignment process during the second iteration may be compared to an output of the alignment process of the first iteration, and if a number of high confidence lines in the second iteration is higher, the output of the second iteration may be stored as the time-annotated audio signal. 
     In another example embodiment, methods described herein may be used to train data-specific HMMs to be used to recognize corresponding audio signals. For example, rather than using a general HMM for a given song, selection of a most appropriate model for a given song can be made. Multiple Hidden Markov models can be trained on subsets of training data using song metadata information (e.g., genre, singer, gender, tempo, etc.) as a selection criteria.  FIG. 9  is a block diagram illustrating a hierarchical HMM training and model selection. An initial HMM training set  902  may be further adapted using genre information to generate separate models trained for a hip-hop genre  904 , a pop genre  906 , a rock genre  908 , and a dance genre  910 . The genre HMMs may be further adapted to a specific tempo, such as slow hip-hop songs  912 , fast hip-hop songs  914 , slow dance songs  916 , and fast dance songs  918 . Still further, these HMMs may be adapted based on a gender of a performer, such as a slow dance song with female performer  920  and slow dance song with male performer  922 . Corresponding reverse models could also be trained using the training sets with reversed audio, for example. 
     A result of a one-time training process is a database of different Hidden Markov Models each of which may include metadata specifying a specific genre, tempo, gender of the trained data, for example. 
     Still further, in another example,  FIG. 10  shows a flowchart of an illustrative embodiment of a method  1000  for adapting HMMs using existing synchronized-lyrics data from a specific performer. An input audio signal may include information (e.g., metadata) indicating a name of the song, a name of the artist of the song, etc. A system (such as system  100  or  200 , for example) may search a database of synchronized lyrics to determine if there exists synchronized audio and lyrics for songs by the artist of the input audio signal, at block  1002 . If there exists synchronized lyrics for a song or audio sample by the artist of the input signal, then an HMM model is retrained and adapted to the audio sample of the artist, at block  1004 . If there are no synchronized lyrics for a song or audio sample by the artist of the input signal, then a standard HMM is used, at block  1006 , and the audio and lyric alignment is performed at block  1008  with the appropriate HMM. Using the method of 1000, HMMs may be enhanced by using synchronized lyrics metadata from songs that have already been processed for a specific performer (e.g., singer). If such data already exists in the system, the data may be used to perform adaptation of the HMMs before synchronization process is performed. In this manner, a speaker independent HMM can be adapted to better model characteristics of a specific speaker. 
     In a specific example of an application of methods in  FIGS. 8-10 , an input audio sample of a particular song by The Beatles may be received along with corresponding lyrics text. If a system has performed audio-lyric synchronization of ten different songs for The Beatles, the system may first adapt a generic pop type-HMM using the previously audio-lyric synchronized data. The system may then use the adapted HMM for the audio-lyric synchronization process, for example. 
     In one embodiment, during any of the methods described herein, any of the data specific HMMs (e.g., as shown in  FIG. 9  or enhanced as described in  FIG. 10 ) may be used. In one example, a parallel audio and lyric synchronization process can be performed using each of the different HMMs. Using the resulting confidence information, a best result (e.g., result with a least number of low confidence lines) among all the different outputs can be selected as a final result. 
       FIG. 11  is a block diagram illustrating a parallel audio and lyric synchronization system  1100 . The system  1100  includes a number of aligners (1, 2, . . . , N), each of which receives a copy of an input audio signal and corresponding lyrics text. The aligners operate to output time-annotated synchronized audio and lyrics, and may be or include any of the components as described above in system  100  of  FIG. 1  or system  200  of  FIG. 2 . Each of the aligners may operate using different HMMs models (such as the different HMMs described in  FIG. 9 ), and there may a number of aligners equal to a number of different possible HMMs. 
     Outputs of the aligners will include synchronized lyrics (SL 1 , SL 2 , . . . , SL N ), timing boundary information, and a corresponding confidence score (N 1   LowConf , N 2   LowConf , . . . , N N   LowConf ). The confidence score may be or include any of the metrics discussed above, and may also indicate a number of low confidence lines in the synchronized lyrics. A selector  1102  may receive the outputs of the aligners and select the output that has a best result, such as an output that has a lowest number of low confidence lines, for example. 
     In another example, a best HMM model may be selected based on criteria used to assign data to a training set, and the selected HMM model may be used to align the audio and lyrics. For example, an input audio signal may include metadata indicating a type of song, genre, tempo, performer&#39;s gender, etc., and such information may be used to select a specific HMM (as described in  FIG. 9 ) to be used during speech recognition.  FIG. 12  is a block diagram of an example system  1200  for selecting an appropriate HMM. An aligner  1202  may receive an input audio signal and lyrics text. The aligner  1202  may be or include any of the components of the system  100  in  FIG. 1  or the system  200  in  FIG. 2 . The aligner  1202  may also receive a selected HMM from an HMM selector  1204 . The HMM selector  1204  may also receive the input audio signal or may receive only metadata of the input audio signal (either from the aligner  1202  or independently) and can use the metadata information to select an appropriate HMM from an HMM database  1206 . For example, if the audio signal that is being processed is a slow rock song, the metadata data may indicate such information and an HMM trained on slow rock songs would be selected and provided to the aligner to be used during speech recognition. To select an appropriate HMM, a back-off technique can be used in which a most specific model is sought first, and if such a model does not exist, a less specific model will be sought, etc. If no metadata about the song is known, or if no model matches the metadata, a generic HMM would be used for the synchronization. 
     Thus, using the examples shown in  FIGS. 8-12 , criteria can be defined to segment types of songs (e.g., genre), and HMM can be generated for specific type of song, and can subsequently be appropriately selected for using during speech recognition. 
       FIG. 13  is a system  1300  for hybrid synchronization of audio and lyrics. The system  1300  includes an aligner  1302 , which may be or include any components of the system  100  in  FIG. 1  or the system  200  in  FIG. 2 , to perform audio-lyric synchronization. The aligner  1302  outputs to a user interface  1304 , which may enable a user to perform manual correction of lyrics that have errors in the lyrics text or timing information, for example. Thus, the system  1300  enables automated synchronization of audio and lyrics and provides for manual corrections to be made. In one embodiment, the aligner  1302  may output lines of the lyrics that have been marked with low confidence (or highlight low confidence lines) to the user interface  1304  for review or correction by a user, for example. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims.