Patent Publication Number: US-10770063-B2

Title: Real-time speaker-dependent neural vocoder

Description:
FIELD OF THE DISCLOSURE 
     This disclosure relates to techniques for performing real-time speech synthesis. In particular, this disclosure relates to for performing real-time speech synthesis in the voice of a particular person. 
     BACKGROUND 
     Synthesizing realistic sound human speech in real-time based upon linguistic features and F0 is a challenging problem. The application of deep learning to speech synthesis such as the WaveNet project has produced promising results. Deep-learning approaches to speech synthesis such as WaveNet have many applications, including the classical text-to-speech (“TTS”) problem. While initially WaveNet and others addressed TTS starting from linguistic features, ensuing work showed that speech could be synthesized directly from input text. The approach has also been adapted to other problems, including voice conversion, speech enhancement, and musical instrument synthesis. 
     Despite the impressive quality of the synthesized waveform, deep learning techniques such as WaveNet still suffer from several drawbacks. In particular, these approaches require substantial training corpus (roughly 30 hours), the synthesis process is slow (about 40 minutes to produce a second of audio), and the result contains audible noise. 
     More recent work showed that WaveNet could also be used as a vocoder, which generates a waveform from acoustic features, rather than linguistic features. Working from acoustic features, the training process is effective with a substantially smaller corpus (roughly one hour) while still producing higher quality speech than baseline vocoders like mel-log spectrum approximation (MLSA). Several research efforts have addressed the problem of computational cost including algorithmic improvements for the same architecture called Fast WaveNet, which can synthesize a second of audio in roughly a minute. Other efforts have been able to achieve real-time synthesis by reducing the WaveNet model size significantly, but at the expense of noticeably worse voice quality. Other efforts have facilitated parallelization of WaveNet for GPU computing allowing real-time operation with some GPU clusters. However, this method does not reduce actual computational costs, but instead demands a far costlier hardware solution. 
     In general, deep-learning techniques for performing speech synthesis such as WaveNet suffer from significant drawbacks, namely requiring a large training corpus and having slow synthesis time, and therefore new approaches are necessary. Further, known methods such as the WaveNet model suffer from high computational complexity due to the employment of a dilated convolution and gated filter structure. Thus, deep-learning techniques for performing speech synthesis achieving a large receptive field for correlating audio samples far in the past with a current input sample that do not impose significant computational penalties are required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart depicting a feedforward operation of a Fast Fourier Transform neural network (generally referred to herein as FFTNet) according to one embodiment of the present disclosure. 
         FIG. 2 a    is a block diagram of an FFTNet block, which may be utilized as part of an FFTNet for performing speech synthesis according to one embodiment of the present disclosure. 
         FIG. 2 b    is a high-level block diagram of an FFTNet according to one embodiment of the present disclosure. 
         FIG. 2 c    is a block diagram depicting an operation of a portion of an FFTNet according to one embodiment of the present disclosure. 
         FIG. 3  is a number of plots showing the posterior distribution for conditional sampling based upon voiced and unvoiced sounds according to one embodiment of the present disclosure. 
         FIG. 4  shows a number of bar charts for the MOS test with an error bar indicating standard deviation across utterances. 
         FIG. 5 a    illustrates an example computing system that executes an FFTNet according to one embodiment of the present disclosure. 
         FIG. 5 b    illustrates an example integration of an FFTNet into a network environment according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes a deep-learning approach for performing speech synthesis herein referred to as a Fast Fourier Transform (“FFT”) neural network, or “FFTNet” for short. According to one embodiment of the present disclosure, FFTNet may be employed as a synthesizer that transforms audio features to a speech audio signal. The audio features may be inferred from other processes. For example, FFTNet may be used to synthesize speech audio with high quality. 
     FFTNet may also be employed to perform signal compression. Because speech may be synthesized from low-dimensional features, it is possible to transmit the audio features instead of the signal for audio transmission. Thus, for example, 10 kbps audio features may be transmitted to a receiver where they are decoded using FFTNet, which achieves better perceptual quality than 32 kbps MP3. 
     According to one embodiment of the present disclosure, an FFTNet may be trained using backpropagation and gradient descent. In particular, according to one embodiment of the present disclosure training sets may be generated using audio samples of the same speaker. Then, acoustic features such as F0 and MCC may be extracted and interpolated to match the audio samples. 
     FFTNet, provides an alternative deep learning architecture, coupled with several improved techniques for training and synthesis. In contrast to conventional approaches that down-sample audio via dilated convolution in a process that resembles wavelet analysis, the FFTNet architecture resembles a classical FFT, and achieves far greater computational efficiency and uses substantially fewer parameters than the WaveNet model. According to one embodiment of the present disclosure, a deep-learning speech synthesis technique is performed utilizing a recursive algorithm that splits each successive input by a factor of 2. A 1×1 convolution is applied to each ½ of the input block whereupon the convolved portions are summed. The model architecture of FFTNet substantially reduces the computational complexity of known deep-learning speech synthesis methods such as WaveNet that rely upon a dilated convolution and gated filter structure. According to one such embodiment, the recursive structure of the FFT is utilized. Further, the FFT kernel e i2πkn/N  is replaced with a small network structure that learns a non-linear transformation and employs a 1×1 convolution. In other words, the FFT may be understood as performing a linear transformation with respect to each point due to the multiplication of each time point by the FFT kernel. According to one such embodiment, the FFT kernel is replaced by a 1×1 convolution and a non-linear network herein referred to as a post-processing block. By utilizing the FFT kernel with a small network structure as described herein, the computational complexity of the WaveNet model, which requires a gated filter structure, skip layers and other architectural details is significantly reduced. This allows, for example, the generation of speech synthesis in real-time or near real-time. 
     According to some such embodiments, FFTNet models produce audio more quickly (&gt;70× faster) than the Fast WaveNet formulation thereby enabling real-time synthesis applications. Moreover, when used as a vocoder, FFTNet produces higher quality synthetic voices, as measured by a “mean opinion score” test than conventional approaches. The FFTNet training and synthesis techniques can improve the original WaveNet approach such that the quality of the synthesized voice is on par with that of the FFTNet architecture (albeit much slower to synthesize). The FFTNet architecture may also be leveraged in a variety of other deep learning problems such as classification tasks and autoencoders. Numerous other embodiments and variations will be appreciated in light of this disclosure. 
     General Overview 
     Existing techniques for speech synthesis using deep learning such as WaveNet model the probability of a speech waveform as follows: 
               p   ⁡     (   x   )       =       ∏     t   =   1     T     ⁢     p   ⁡     (         x   t     |     x   1       ,   …   ⁢           ,     x     t   -   1         )               
That is, the joint probability of a waveform x={x 1 , . . . , x T } is factorized as a product of conditional probabilities as shown above. Each audio speech sample x t  is conditioned on the samples at all previous timesteps. Similar to PixelCNNs, the conditional probability distribution is modeled by a stack of convolutional layers. The model outputs a categorical distribution over the next value x t  with a softmax layer and is optimized to maximize the log-likelihood of the data with respect to the parameters. A dilated causal convolution structure is utilized that allows for a larger receptive field. Further, similar to PixelCNN, in order to simulate the behavior a LSTMs (“Long Short Term Memory”) networks, gated activation functions are utilized. Further, residual and skip connections are utilized.
 
     Existing methods such as WaveNet rely upon a dilated convolution structure such that an n-layer network has a receptive field of 2 n  meaning that as many as 2 n  previous samples can influence the synthesis of the current sample, which leads to superior synthesis quality. However, with these types of techniques such as WaveNet, only one sample is generated per iteration and thus to generate one second of audio sampled at 16 KHz, the causal dilated network needs to be applied 16,000 times. Faster methods have been proposed, which can produce 200 samples per second, but the performance is still far from real-time on personal computers. With dilated convolution, the nodes that influence the prediction of a new sample may be represented as an inverted binary tree structure. Thus, dilated convolution resembles wavelet analysis in that each filtering step is followed by down-sampling. The causal dilated convolutional structure, gated activation functions and skip connections of known techniques such as WaveNet introduce significant computational complexity. 
     In contrast, and according to one embodiment of the present disclosure, the recursive structure of the Cooley-Tukey Fast Fourier Transform provides an alternative model for providing the effect of a dilated convolution by increasing the receptive field. A number of benefits flow from using an FFT-based alternative structure, as will be appreciated in light of this disclosure. Given an input sequence x 1 , x 2 , . . . , x n , the FFT computes the k-th frequency component f k  from the time-domain series x 0  . . . x N-1  as follows: 
               f   k     =         ∑     n   =   0       N   -   1       ⁢       x   n     ⁢     e       -   2     ⁢   π   ⁢           ⁢     ink   /   N             =           ∑     n   =   0         N   /   2     -   1       ⁢       x     2   ⁢   n       ⁢     e       -   2     ⁢   π   ⁢           ⁢     i   ⁡     (     2   ⁢   n     )       ⁢     k   /   N             +       ∑     n   =   0         N   /   2     -   1       ⁢       x       2   ⁢   n     +   1       ⁢     e       -   2     ⁢   π   ⁢           ⁢     i   ⁡     (       2   ⁢   n     +   1     )       ⁢     k   /   N               =   Denote                           ⁢       ∑     n   =   0       N   -   1       ⁢       x   n     ⁢     e       -   2     ⁢   π   ⁢           ⁢     ink   /   N         ⁢           ⁢   as   ⁢           ⁢     f   ⁡     (     n   ,   N     )                 
and the above equation can be simplified as:
 
     
       
         
           
             
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     According to some such embodiments, x n  may be interpreted as a node with K channels corresponding to quantization levels (e.g., 256 quantization channels). The FFT kernel e −2πi(2n)k/N  may be interpreted as a transformation function. In this context, each term 
               f   ⁢     (     n   ,   N     )       =       f   ⁡     (       2   ⁢   n     ,     N   2       )       +     f   ⁡     (         2   ⁢   n     +   1     ,     N   2       )               
is analogous to applying a transformation to previous nodes x 2n  and x 2n+1  and summing up the results. In the classical FFT the FFT kernel operates as a linear transformation on the input samples. According to one embodiment of the present disclosure, the classical FFT kernel is replaced by a small network structure that performs 1×1 convolution in conjunction with a post-processing block that may perform a non-linear transformation.
 
     According to one such embodiment, given an input x 0:N  defined as the 1D series (x 0 , x 1 , . . . x N-1 ) a series of layers or blocks herein referred to as FFTNet blocks, clip or segments the input into two halves (herein referred to as a right half and a left half) as follows:
 
( x   L   =x   0:N/2  and  x   R   =x   N/2:N )
 
and then sums up the results:
 
 z=W   L   *x   L   +W   R   *x   R  
 
where W L  and W R  are 1-D convolution weights for x L  and x R . Each FFTNet block further incorporates a non-linear activation function, which may be a ReLU activation function followed by a 1-D convolution to produce inputs for the next layer according to the relation:
 
 x =ReLU(conv1×1(ReLU( z )))
 
Replacing the classical FFT kernel with this FFTNet block achieves the same increase in receptive field as with conventional techniques such as WaveNet, but at the same time, obviates the need for gated activation functions and skip layers, which would otherwise increase the computational complexity.
 
     According to some such embodiments, auxiliary conditions such as linguistic features are transformed by the 1-D convolution and added to z, as follows
 
 z =( W   L   *+W   R   *x   R )+( V   L   *h   L   +V   R   *h   R ),
 
where h L  and h R  are the two halves of the condition vector h and V L  and V R  are 1-D convolution weights. In some such cases, note that if the condition information is stationary along the time axis the condition information becomes V*h N , instead of (V L *h L +V R *h R ).
 
     Various uses of the FFTNet architecture as provided herein will be apparent. For example, according to one embodiment of the present disclosure an FFTNet may be utilized as a vocoder. In particular, h t  may be F0 (pitch) and MCC (“Mel-Cepstral Coefficient”) features at time t. To generate the current sample x t , the previously generated samples x t−N:t  and auxiliary condition h t−N+1:t+1  (shifted forward by 1) are utilized as the network input. According to one specific example such embodiment, the auxiliary condition is obtained as follows. An analysis window of size 400 is performed every 160 samples. The MCC and F0 features are extracted for each overlapping window. For the h t  corresponding to the window centers, the computed MCC and F0 values (26 dimensions in total) are assigned. For the h t  that are not located at the window centers, linear interpolation is utilized to obtain values based on the assigned h t  in the last step. Numerous other use cases and applications will be appreciated in light of this disclosure, and this disclosure is not intended to be limited to specific details of any such illustrative examples. 
     According to one further specific example embodiment, and as discussed in more detail below, an FFTNet utilizes a fully connected layer followed by a softmax layer (size 1 with K=256 channels) as the last two layers to produce a posterior distribution of the new sample&#39;s quantized values. To determine the final value of the current sample, either an argmax or random sampling may be performed on the posterior distribution. 
     FFTNet Methodology and Architecture 
       FIG. 1  is a flowchart depicting an operation of an FFTNet according to one embodiment of the present disclosure. It will be appreciated that the process shown in  FIG. 1  may comprise one component of a composite system to perform text-to-speech conversion, voice manipulation, pitch bending, etc. Exemplary embodiments describing the operation of an FFTNet in conjunction with a linguistic features model will be described below. 
     The process is initiated in  102 . In  103 , a 1×1 convolution is performed. This 1×1 convolution layer transforms an input (e.g., 256-channel one-hot encoded u-law-quantized signal) into the right number of channels for FFTNet (e.g., 128) before FFTNet operation starts. 
     As will be described in detail below, according to one embodiment of the present disclosure, an FFTNet may comprise a plurality of layers of a repeatable structure that are successively computed to generate voice output samples. According to one embodiment, the number of layers comprising an FFTNet may be log 2 (N), where N is the size of a tensor dimension for an audio block input. In  104 , it is determined whether all layers have been computed. If so (‘Yes’ branch of  104 ), in  120  a fully connected layer  250  is applied to the current output. In  122 , a softmax classifier is applied to the output of the fully connected layer to generate an output sample. The process ends in  116 . 
     If all layers have not been computed (No′ branch of  104 ), flow continues with  124  whereby the current input is set either to set to the previous layer&#39;s output. That is, if this is the first layer, the input is set to the original input to the FFTNet, while if, instead, the current layer is not the first layer, the layer input is set to the previous layer&#39;s output. In  106 , the layer&#39;s input is split evenly into right and left halves. Thus, if the input to the layer is of size N, the left and right halves are of size N/2. In  108 , a 1×1 convolution is performed separately on the right and left halves. A method for generating a convolution kernel is described below. In  110 , the convolved right and left halves are summed to generate a composite tensor. Thus, after the summing operation, the resulting tensor has a dimension of size N/2. 
     In  112 , a first activation function is applied to the composite tensor. In  114 , a 1×1 convolution is applied. In  118 , a second activation function is applied. Flow then continues with  124 . According to one embodiment of the present disclosure and as discussed in more detail below, the first and second activation functions may be ReLU (“Rectified Linear Unit”) functions. 
     The operation of an FFTNet block or layer will now be described. For purposes of the present discussion the terms FFTNet block and FFTNet layer will be utilized interchangeably. As previously mentioned, each FFTNet block/layer may comprise a repeatable structure that receives an input tensor and generates an output tensor. 
     According to one embodiment of the present disclosure a skip-layer implementation may be utilized. In this case, according to one embodiment of the present disclosure, the input is summed with the output at each iteration. 
       FIG. 2 a    is a block diagram of an FFTNet block, which may be utilized as part of an FFTNet for performing speech processing according to one embodiment of the present disclosure. As will be described with respect to  FIGS. 2 b -2 c   , a series of FFTNet blocks  220  may be arranged in a sequential fashion so that the output of one FFTNet block  220  is provided as input to a next FFTNet block. As shown in  FIG. 2 a   , FFTNet block  220  receives input tensor  230 ( 1 ) and generates output tensor  230 ( 2 ). As will be appreciated, input tensor  230 ( 1 ) and output tensor  230 ( 2 ) may comprise tensor objects as understood in the field of deep learning and deep neural networks. In particular, tensor objects may comprise multidimensional arrays. Example tensor dimensions for input tensor  230 ( 1 ) and output tensor  230 ( 2 ) are described below. However, in general, as shown in  FIG. 2 a   , input tensor  230 ( 1 ) may comprise a tensor with one of its dimension of size N while output tensor  230 ( 2 ) may comprise a tensor with a corresponding dimension of size N/2. Thus, when FFTNet blocks  220  are arranged in a series as described below with respect to  FIGS. 2 b -2 c   , the output of each FFTNet block  220  is a tensor in which one dimension is ½ of the corresponding input dimension. In this fashion, a sequential chain of FFTNet blocks  220  of length log 2(N) where N is the size of the corresponding input dimension will generate a final output of size 1 in the corresponding tensor dimension. In particular, a series of FFTNet blocks  220  may perform a recursive operation as each block comprises a repeatable structure. 
     As described in detail below, input tensor  230 ( 1 ) and output tensor  230 ( 2 ) may also comprise a first dimension that encodes the value of audio samples in a quantized fashion. This quantized dimension is referred to as “channels”, which is a term well understood in the context of deep neural networks. In particular, in the context of image data, the channels typically encode the red, blue and green components of a pixel, and therefore there are typically 3 channels, which may vary across a convolutional neural network structure. In the context of the present disclosure, however, the channels encode the quantization level of an audio sample. In particular, according to one embodiment of the present disclosure, the values of input tensor  230 ( 1 ) may be quantized into an arbitrary number of bins (e.g., 256). In this case, input tensor  230 ( 1 ) and output tensor  230 ( 2 ) may include a channel dimension of size 256. According to one embodiment of the present disclosure, the input data (real value audio samples) is quantized into a particular number of channels. The channel size may then be reduced to accommodate the number of channels used in the FFTNET using a 1×1 convolutional layer. 
     For example, assume the input data audio samples is of size [8000, 1]. The audio samples may be quantized using u-law to obtain quantized audio samples with size [8000, 256], for example. Suppose for purposes of this example that the FFTNet utilizes 128 channels instead of 256. In order to accommodate the 128 channels, a 1×1 convolutional layer may be utilized to transform the 256-channel audio samples into 128-channel audio samples with a resulting data size of [8000, 12]. In this example, the kernel dimension for the 1×1 convolution is [1, 1,256, 128] with the first two dimensions the convolution size (1×1) and the last two a fully connected network that transforms 256 channels into 128 channels. 
     A second tensor dimension encoded in input tensor  230 ( 1 ) comprises a block size, which is an even number. For purposes of the present discussion, this dimension will be referred to as the block size dimension. In particular, because, as will be described below, each FFTNet block divides its input tensor  230 ( 1 ) into a left and right half and generates an output tensor  230 ( 2 ) having a block size dimension that is ½ the size of the block size dimension of input tensor  230 ( 1 ), it must be an even number. 
     The operation of an FFTNet block  220  will now be described. Referring now to the operation of FFTNet block  220 , input tensor  230 ( 1 ) is received by splitter  260 , which splits input tensor  230 ( 1 ) into a left input tensor  240 ( 1 ) and a right input tensor  240 ( 2 ) with respect to one of its tensor dimensions. Left input tensor  240 ( 1 ) and right input tensor  240 ( 2 ) are then provided to respective 1-D convolvers  222 , which respectively perform convolution on left input tensor  240 ( 1 ) and right input tensor  240 ( 2 ) to generate respective left convolved tensor  246 ( 1 ) and right convolved tensor  246 ( 2 ). Although  FIG. 2 a    shows two separate 1-D convolvers  222 , it will be understood that according to alternative embodiments, a single convolver  22  may be utilized to perform convolution of left input tensor  240 ( 1 ) and right input tensor  240 ( 2 ). Left convolved tensor  246 ( 1 ) and right convolved tensor  246 ( 2 ) are then provided to summer  242 , where they are summed to generate summer composite tensor  244 . In particular, according to one embodiment of the present disclosure, summer  242  performs a standard tensor summation to generate composite tensor  244 . Composite tensor  244  is then provided to post-processing block  236 , which performs post-processing on composite tensor  244  to generate output tensor  230 ( 2 ). 
     According to one embodiment of the present disclosure, post-processing block  236  may further comprise a first ReLU (“Rectified Linear Unit”), 1×1 convolution block and a second ReLU. As will be appreciated, according to one embodiment of the present disclosure, first and second ReLU may implement an activation function according to the following relationship:
 
 f ( x )= x   + =max( x, 0)
 
According to alternative embodiments, post-processing block  236  may implement any type of activation function(s) including a sigmoid function or tan h function. 1×1 block may perform a 1-D convolution operation.
 
     Thus, as previously described, FFTNet block  220  receives an input tensor  230 ( 1 ) having a block size dimension of size N and outputs output tensor  230 ( 2 ) with the corresponding block size dimension of size N/2. A series of FFTNet blocks  220  may be arranged to perform a recursive operation in which an input tensor with block size dimension of N is processed repeatedly until the block size dimension is of size 1. 
       FIG. 2 b    is a high-level block diagram of an FFTNet according to one embodiment of the present disclosure. FFTNet  200  may be utilized to generate synthetic speech audio samples and according to some embodiments may be utilized in conjunction with a linguistic model and linguistic features that may be utilized as auxiliary information as described above. As shown in  FIG. 2 b   , an FFTNet  200  may further comprise FFTNet blocks  220 ( 1 )- 220 (N), fully connected layer  250  and softmax classifier  224 . For clarity, the intermediate results generated by FFTNet blocks  220 ( 1 )- 220 (N) are omitted in  FIG. 2 b   . As shown in  FIG. 2 b    and as previously described with respect to  FIG. 2 a   , each FFTNet block  220 ( 1 )- 220 (N) is a repeatable structure and further comprises splitter  260 , first and second 1D convolvers  222  and output block  236 , summer  242  and output block  236 . Output block  236  further comprises first and second ReLU blocks and 1×1 convolver. 
     As shown in  FIG. 2 b   , input sample block  228  is provided as input to the FFTNet block. Input sample block  228  may encode a block of audio samples as a tensor of arbitrary dimensionality. According to one embodiment of the present disclosure, and as described below, during inference time, input sample block  228  may be a tensor of the following dimensionality:
         [1, BLOCK_SIZE, QUANTIZATION_LEVELS]
 
where BLOCK_SIZE is the number of samples processed during each iteration and QUANTIZATION_LEVELS is a number of quantization levels for quantizing audio samples. Although,  FIG. 2 b    does not depict the conversion of input sample block  228  into a tensor of the dimensionality described, it will be understood that such a conversion may take place and according to other embodiments any other arbitrary tensor dimension may be utilized to encode input sample block.
 
FFTNet block  220 ( 1 ) generates output tensor  230 ( 1 ), which is then provided as input to FFTNet block  220 ( 2 ), which generates output tensor  230 ( 2 ) as previously described with respect to  FIG. 2 a   . A similar operation will occur with respect to each succeeding FFTNet block  220 ( i ). Thus, each FFTNet block  220 ( i ) receives as input the output tensor  230 ( i −1) of a previous FFTNet block  220 ( i −1) and generates output tensor  230 ( i ). Final FFTNet block  220 (N) receives output tensor  230 (N−1) from FFTNet block  220 (N−1) (not shown in  FIG. 2 b   ) and processes this to generate output tensor  230 (N).
       

     Output tensor  230 (N) from the final FFTNet block  220 (N) is provided to fully connected layer  250 , which may comprise a single fully connected layer  250  of artificial neural network nodes. Fully connected layer  250  generates fully connected layer output  234 , which is provided to softmax classifier  224 . Softmax classifier  224  processes fully connected layer output  234  to generate final output  232 , which, according to one embodiment of the present disclosure, may comprise a single audio sample. As shown in  FIG. 2 b   , according to one embodiment of the present disclosure, final output  232  may be provided as input to FFTNet  200  via sample input block  228 . That is, as shown in  FIG. 2 b   , according to some embodiments of the present disclosure the output of FFTNet  200  is utilized as input. A method for providing the final output  232  as input to FFTNet  200  is described below 
     As previously described, input/output tensors  230 ( 1 )- 230 (N) of FFTNet blocks  220 ( 1 )- 220 (N), fully connected layer output  234  and softmax classifier output  232  may comprise tensors of a particular dimension. As will be appreciated in the field of deep learning and deep neural networks, a tensor may comprise a multidimensional array. Example tensor dimensions for sample input sample block  228 , output tensors  230 ( 1 )- 230 (N), fully connected layer output  234  and final output  232  are described below. 
       FIG. 2 c    is a block diagram depicting an operation of a portion of an FFTNet according to one embodiment of the present disclosure. In particular,  FIG. 2 c    shows the operation of two FFTNet blocks  220 ( 1 ),  220 ( 2 ).  FIG. 2 c    does not show fully connected layer  250  or softmax classifier  224 . As shown in  FIG. 2 c   , input tensor  230 ( 1 ) is received by FFTNet block  220 ( 1 ). Input tensor  230 ( 1 ) may be a sample block input or may be the output of upstream FFTNet blocks  220 ( i ). In the example depicted in  FIG. 2 c   , input tensor  230 ( 1 ) has a block size tensor dimension of size 8. Input tensor  230 ( 1 ) is split into a left tensor and right tensor having a block size dimension of size 4, which are respectively processed by 1-D convolvers  222  to generate left convolved tensor  246 ( 1 ) and right convolved tensor  246 ( 2 ). Left and right convolved tensors  246 ( 1 )- 246 ( 2 ) are summed by summer  242  to generate a composite tensor (not shown in  FIG. 2 c   ), which is then provided to post-processing block  236 . Post-processing block  236  performs a first ReLU operation, a 1-D convolution and a second ReLU operation to generate output tensor  230 ( 2 ), which is provided as input to FFTNet block  220 ( 2 ). 
     Similar to the operation of FFTNet block  220 ( 2 ), input tensor  230 ( 2 ) is received by FFTNet block  220 ( 2 ). In this case, input tensor  230 ( 2 ) has a block size dimension of size 4. Input tensor  230 ( 2 ) is split into a left tensor and right tensor having a block size dimension of size 2, which are respectively processed by 1-D convolvers  222  to generate left convolved tensor  246 ( 3 ) and right convolved tensor  246 ( 4 ). Left and right convolved tensors  246 ( 3 )- 246 ( 4 ) are summed by summer  242  and the composite tensor (not shown in  FIG. 2 c   ) is then provided to post-processing block  236 . Post-processing block  236  performs a first ReLU operation, a 1-D convolution and a second ReLU operation to generate output tensor  230 ( 3 ), which may be provided as input to additional FFTNet blocks in the chain. 
     Training 
     According to one embodiment of the present disclosure, an FFTNet may be trained using backpropagation and gradient descent using an Adam optimizer in conjunction with mini batches. In particular, according to one embodiment of the present disclosure training sets may be generated using audio samples of the same speaker. The acoustic features F0 and MCC are extracted and interpolated to match the audio samples. At training time, batches of size [6, 4000] may be utilized in which 6 utterances are randomly selected. For each utterance, a length of 4000 audio samples is selected together with the corresponding F0 and MFCC as input data. According to one embodiment of the present disclosure, the training data size is [6, 4000] for the audio samples, [6, 4000, 1] for pitch and [6, 4000, 26] for MCC. 
     Further, according to one embodiment of the present disclosure, 10 FFTNet blocks are utilized, resulting in a receptive field of 2048. To perform efficient training, the split-summation structure of FFTNet is utilized in conjunction with zero-padding 
     Tensor Dimensions 
     According to one embodiment of the present disclosure, an identical operation is applied for each batch at training time. According to one embodiment of the present disclosure, Input sample block  228  may comprise previously generated samples of dimensions [batch_size, 1024, 1] in floating point format, where 1024 is the block size. According to one embodiment of the present disclosure, the input samples are quantized as floating-point data to [batch_size, 1024, 256] (i.e., 256 quantization bins), where the third dimension is the channel dimension of size 256. 
     For example, assume the utilization of 128 channels. Prior to the application of the first FFTNet block  220 , a 1×1 convolver  222  transforms 256 bins into 128 channels:
         [batch_size, 1024, 128]       

     Each FFTNet block  220  reduces the length by a factor of two, so after each FFTNet block the tensor dimensions appear as follow:
         [batch_size, 512, 128]   [batch_size, 256, 128]   [batch_size, 128, 128]   . . .   [batch_size, 4, 128]   [batch_size, 2, 128]   [batch_size, 1, 128]
 
Now, the 2nd dimension can be extracted such that the following tensor dimensions are:
   [batch_size, 128]       

     Fully connected layer  250  may be then be applied. According to one embodiment of the present disclosure, fully connected layer  250  may be equivalent to a 1×1 convolution. Fully connected layer  250  may transform FFTNet Block output into 256 channels because the output is the posterior distribution of 256 quantized value bins.
         [batch_size, 256]
 
The final output (after another fully connected layer) may be of dimension:
   [batch_size, 256]       

     According to one embodiment of the present disclosure, output samples are fed back as input to the FFTNet  200  in input sample block  228 . For example, assuming a sample input block  228  of size 1024, [1, 2, 3, . . . , 1024] for input and output sample [1025]. In the next step, the input [2, 3, . . . , 1025] are utilized to produce sample [1026]. 
     According to one embodiment of the present disclosure, softmax classifier  224  may utilize a cross-entropy loss function for training. According to one embodiment of the present disclosure, a loss function may be a cross-entropy loss function. The cross-entropy loss function may be expressed as: 
                 E   t     ⁡     (       y   t     ,     )       =       -     y   t       ⁢   log   ⁢           ⁢                     E   t     ⁡     (     y   ,     y   ^       )       =         ∑   t     ⁢       E   t     ⁡     (       y   t     ,     )         =     -       ∑   t     ⁢       y   t     ⁢   log   ⁢           ⁢                 
where y t  is the target (correct) word at each time step t and ŷ t  is the prediction. Typically, the full sequence may be treated as a single training example so that the total error is the sum of errors at each time step.
 
     According to one embodiment of the present disclosure, softmax classifier  224  may be represented as: 
               σ   ⁢     :     ⁢           ⁢     R   K       -&gt;       [     0   ,   1     ]     K                       σ   ⁡     (   z   )       j     =           e     z   j           ∑     k   =   1     K     ⁢     e     z   k           ⁢           ⁢   for   ⁢           ⁢   j     =   1       ,   …   ⁢           ,   K         
Effectively, softmax classifier  224  maps a K-dimensional vector k to a K-dimensional vector σ(z) of real values in the range [0, 1] that add up to 1 so that σ(z) exhibits properties of a probability mass function.
 
     Zero Padding 
     According to one embodiment of the present disclosure, an FFTNet  200  may employ zero-padding, which achieves dilated convolution. In particular, given a sequence of length M, the input x 1:M  is shifted to the right by N samples with zero padding. The N padded zeros are denoted as x −N:0  where ∀j&lt;0, x j =0. The equation describing each FFTNet block then becomes:
 
 z   0:M   =W   L   *x   −N:M−N   +W   R   *x   0:M  
 
     According to some embodiments, experimental results demonstrate that without zero padding, an FFTNet  200  tends to produce noise or gets stuck (outputting zeros) when the inputs are near silence. Zero-padding during training allows the network to generalize to partial input. According to some embodiments, training sequences of length between 2N and 3N are utilized so that a significant number (33%-50%) of training samples are partial sequences. 
     Conditional Sampling 
     As FFTNet  200  includes a softmax classifier  224  as the final processing element, the prediction error comes from two sources: training error and true error. The true error corresponds to noise mostly resides in unvoiced signal. According to one embodiment of the present disclosure, to synthesize noise, an FFTNet  200  may learn the noise&#39;s distribution by the output posterior distribution on which random sampling may be employed to obtain the sample&#39;s value. Training error comes from the model itself. The prediction strategy that provides the minimal training error is argmax. However, argmax is not suitable for simulating signals that contain true noise, since it always chooses the center of a noise distribution leading to zero noise in the synthesis output. Instead of using argmax universally, according to some embodiment of the present disclosure, different prediction strategies are utilized for unvoiced and voiced sounds. In particular, a different strategy may be employed for voiced and unvoiced sounds. 
       FIG. 3  is a plot showing the posterior distribution for conditional sampling based upon voiced and unvoiced sounds according to one embodiment of the present disclosure. For unvoiced sounds, the posterior distribution is randomly sampled. For voiced sounds, the normalized logits (the input values before softmax classifier  224 ) is multiplied it by a constant c&gt;1 and passed through softmax classifier  224  to obtain a posterior distribution where random sampling is performed. In this way, the posterior distribution will look steeper while the original noise distribution is preserved. According to one embodiment of the present disclosure the variable c=2 is used. 
     Injected Noise 
     Because of training error, the synthesized samples always contain some amount of noise; during synthesis, the network will generate samples that get noisier over time. The output samples serve as network input to generate the next sample, adding more and more randomness to the network. When the noise builds up, the output sample might drift leading to clicking artifacts. According to one embodiment of the present disclosure, to avoid such drift, an FFTNet  220  may be modified to be robust to noisy input samples. In particular, this is achieved by injecting random noise to the input x 0,M  during training. According to one embodiment of the present disclosure, the amount of noise to inject into the input is based on the amount of noise the network is likely to produce. According to one embodiment of the present disclosure, based upon the observation that the prediction is often one category (out of 256) higher or lower than the ground-truth category Gaussian noise centered at 0 with a standard deviation of 1/256 (based on 8-bit quantization) is injected. 
     Post-Synthesis Denoising 
     Experiments show that injected noise eliminates clicking artifacts almost perfectly for but introduces a small amount of random noise to voiced samples. According to one embodiment of the present disclosure, a spectral subtraction noise reduction is employed to reduce the injected noise for the voice samples. The amount of reduction is proportional to the amount of noise injected during training. It is possible to apply noise reduction to the unvoiced samples too, but it may result in artifacts. 
     Inference 
     Once trained, the FFTNet may be utilized in an inferencing application, such as voice synthesis, according to some embodiments. 
     Tensor Dimensions 
     According to one embodiment of the present disclosure, at inference time, the above tensor dimensions described with respect to training time are preserved except the batch size is 1. 
     Experimental Results 
     According to one embodiment of the present disclosure, four voices, two male (BDL,RMS) and two female (SLT,CLB), from the CMU Arctic dataset were used in experiments. The first 1032 utterances (out of 1132) were used for training and the remaining were used for evaluation. The waveforms were quantized to 256 categorical values based on μ-law. 25-coefficient Mel Cepstral Coefficients (with energy) and F0 were extracted from the original samples. 
     Four networks were constructed for each voice, 2 WaveNets and 2 FFTNets  200 . For each type of network, two training strategies were employed: 
     Strategy On: Zero Padding Only 
     Strategy Two Applies All Training Techniques (described above) 
     For comparison, a WaveNet was implemented containing two stacks of 10-layer dilated convolution (d=20, 21, . . . , 29) with 256 dilation and 128 skip channels. The total receptive field was 2048 samples. Varying numbers of channels were tested and an optimal configuration for performing vocoding was determined. 
     According to one embodiment of the present disclosure, an FFTNet implementation  200  utilizing 11 FFT-layers with 256 channels and a receptive field of 2048 was utilized. Such an FFTNet configuration has less than 1M parameters and with proper caching, the computation cost for generating one second of audio (16 kHz) is approximately 16GFLOPs. This means that a modern CPU could generate audio samples in real-time. In each training step, a minibatch of 5×5000-sample sequences was fed to the network, optimized by Adam algorithm with a training rate of 0.001. The variance of injected noise was set to be 1/256. In each minibatch, all sequences were determined from different utterances. 
     A WaveNet was trained using 200,000 steps. A FFTNet was trained with 100,000 steps to ensure convergence. Based upon experiments, synthesis using FFTNet resulted in more than 70 times faster performance than Fast WaveNet, requiring only 0.81 second to generate 1 second of audio on a laptop CPU (2.5 GHz Intel Core i7). 
     Subjective Evaluation 
     A Mean Opinion Score (MOS) test that asks subjects to rate the quality of the synthetic utterances was performed. Participants from United States who have an approval rate over 90% were recruited to ensure the reliability of the study results. A validation test to ensure a subject was paying attention was also performed. Six conditions were established for each utterance as follows: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 MLSA 
                 MLSA Filter 
               
               
                   
                 WN 
                 WaveNet with only zero-padding 
               
               
                   
                 FFTN 
                 FFTNet with only zero-padding 
               
               
                   
                 WN+ 
                 WaveNet with zero padding, conditional 
               
               
                   
                   
                 sampling, injected noise and post-synthesis 
               
               
                   
                   
                 denoising 
               
               
                   
                 FFTN+ 
                 FFTNet with zero padding, conditional 
               
               
                   
                   
                 sampling, injected noise and post-synthesis 
               
               
                   
                   
                 denoising 
               
               
                   
                 Real 
                 Actual recording 
               
               
                   
                   
               
            
           
         
       
     
     In each task (called a HIT), a subject was presented with 32 different sentences in which 24 of them were composed of 4 instances from each of the above 6 conditions. From a held-out set of sentences, 4 more instances of the “Real” condition and 4 more cases of badly edited “Fake” (3-bit A-law encoded) condition to validate that the subject was paying attention and not guessing randomly were determined. For the data to be retained, the subject was allowed to make at most one mistake on these validation tests, by either rating &lt;3 on “Real” examples or &gt;3 on “Fake” examples. 480 HITs (120 per voice) were launched and 446 after validation were retained. 
       FIG. 4  shows a bar chart for the MOS test with an error bar indicating standard deviation across utterances. The proposed training technique improved both WaveNet and FFTNet significantly with an ANOVA test p-value less than 10 9 for both networks. The proposed network FFT+ also improved on WN with a p-value of &lt;10 20. Both WN+ and FFT+ had significant overlap with the real examples in MOS scores. The proposed method FFT+ had a slightly lower MOS than WaveNet WN+(with an insignificant p-value); but it is much faster, as noted above. FFT had a similar quality to the baseline method MLSA (insignificant pvalue) due to noisy artifacts. 
     Objective Evaluation 
     A distortion between the original and the synthesized speech using RMSE and MCD was performed. RMSE measures frequency domain difference between two signals; and MCD measures the difference in the cepstral domain, which reflects whether the synthesized speech can capture the characteristics of the original speech. Both measurements are in dB. The result is shown in the following table: 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                 MCD (dB) 
                 RMSE (dB) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 voice 
                 slt 
                 clb 
                 rms 
                 bdl 
                 slt 
                 clb 
                 rms 
                 bdl 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 mlsa 
                 2.76 
                 3.03 
                 3.62 
                 3.28 
                 8.05 
                 9.14 
                 8.80 
                 8.25 
               
               
                 WN 
                 4.47 
                 4.04 
                 4.60 
                 3.05 
                 9.71 
                 9.65 
                 9.38 
                 8.29 
               
               
                 WN+ 
                 4.57 
                 4.13 
                 4.41 
                 3.28 
                 9.80 
                 8.95 
                 9.74 
                 8.67 
               
               
                 FFT 
                 5.24 
                 5.07 
                 4.82 
                 4.23 
                 10.39 
                 9.77 
                 10.33 
                 10.13 
               
               
                 FFT+ 
                 4.73 
                 4.69 
                 4.41 
                 3.82 
                 9.88 
                 9.58 
                 9.89 
                 9.64 
               
               
                   
               
            
           
         
       
     
     The result shows that MLSA tends to preserve most of the cepstral and spectral structure while the MOS test puts it in a significantly lower tier as it generates audible over-smoothing artifacts. The training techniques described above do not reduce distortion in WaveNet, but they significantly improve FFTNet in both metrics. WaveNet with the proposed techniques performs significantly better in subjective evaluation than the one without. 
     Integration in Computing System and Network Environment 
       FIG. 5 a    illustrates an example computing system that executes an FFTNet  200  according to one embodiment of the present disclosure. As depicted in  FIG. 5 a   , computing device  500  may include CPU  504  that executes one or more processes to perform FFTNet  200 . In particular, CPU  504  may be further configured via programmatic instructions to execute FFTNet  200  (as variously described herein). Other componentry and modules typical of a typical computing system, such as, for example a co-processor, a processing core, a graphics processing unit, a mouse, a touch pad, a touch screen, display, etc., are not shown but will be readily apparent. Numerous computing environment variations will be apparent in light of this disclosure. Computing device  500  can be any stand-alone computing platform, such as a desk top or work station computer, laptop computer, tablet computer, smart phone or personal digital assistant, game console, set-top box, or other suitable computing platform. According to alternative embodiments CPU  504  may be replaced with a GPU. 
       FIG. 5 b    illustrates an example integration of an FFTNet  200  into a network environment according to one embodiment of the present disclosure. As depicted in  FIG. 5 b   , computing device  500  may be collocated in a cloud environment, data center, local area network (“LAN”) etc. Computing device  500  shown in  FIG. 5 b    is structured identically to the example embodiment described with respect to  FIG. 5 a   . As shown in  FIG. 5 b   , client application  512  may interact with computing device  500  via network  510 . In particular, client application  512  may make requests and receive responses via API calls received at API server  506 , which are transmitted via network  510  and network interface  508 . 
     It will be understood that network  510  may comprise any type of public or private network including the Internet or LAN. It will be further readily understood that network  510  may comprise any type of public and/or private network including the Internet, LANs, WAN, or some combination of such networks. In this example case, computing device  500  is a server computer, and client application  512  may be any typical personal computing platform 
     As will be further appreciated, computing device  500 , whether the one shown in  FIG. 5 a    or  65 , includes and/or otherwise has access to one or more non-transitory computer-readable media or storage devices having encoded thereon one or more computer-executable instructions or software for implementing techniques as variously described in this disclosure. The storage devices may include any number of durable storage devices (e.g., any electronic, optical, and/or magnetic storage device, including RAM, ROM, Flash, USB drive, on-board CPU cache, hard-drive, server storage, magnetic tape, CD-ROM, or other physical computer readable storage media, for storing data and computer-readable instructions and/or software that implement various embodiments provided herein. Any combination of memories can be used, and the various storage components may be located in a single computing device or distributed across multiple computing devices. In addition, and as previously explained, the one or more storage devices may be provided separately or remotely from the one or more computing devices. Numerous configurations are possible. 
     In some example embodiments of the present disclosure, the various functional modules described herein and specifically training and/or testing of network  340 , may be implemented in software, such as a set of instructions (e.g., HTML, XML, C, C++, object-oriented C, JavaScript, Java, BASIC, etc.) encoded on any non-transitory computer readable medium or computer program product (e.g., hard drive, server, disc, or other suitable non-transitory memory or set of memories), that when executed by one or more processors, cause the various creator recommendation methodologies provided herein to be carried out. 
     In still other embodiments, the techniques provided herein are implemented using software-based engines. In such embodiments, an engine is a functional unit including one or more processors programmed or otherwise configured with instructions encoding a creator recommendation process as variously provided herein. In this way, a software-based engine is a functional circuit. 
     In still other embodiments, the techniques provided herein are implemented with hardware circuits, such as gate level logic (FPGA) or a purpose-built semiconductor (e.g., application specific integrated circuit, or ASIC). Still other embodiments are implemented with a microcontroller having a processor, a number of input/output ports for receiving and outputting data, and a number of embedded routines by the processor for carrying out the functionality provided herein. In a more general sense, any suitable combination of hardware, software, and firmware can be used, as will be apparent. As used herein, a circuit is one or more physical components and is functional to carry out a task. For instance, a circuit may be one or more processors programmed or otherwise configured with a software module, or a logic-based hardware circuit that provides a set of outputs in response to a certain set of input stimuli. Numerous configurations will be apparent. 
     The foregoing description of example embodiments of the disclosure has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the disclosure be limited not by this detailed description, but rather by the claims appended hereto. 
     Further Example Embodiments 
     The following examples pertain to further embodiments, from which numerous permutations and configurations will be apparent. 
     Example 1 is a method for generating speech samples, the method comprising receiving an input tensor, splitting said received input tensor into a first portion and a second portion, performing a 1×1 convolution respectively on said first portion and said second portion to generate a respective first intermediate result and a second intermediate result, summing said first intermediate result and said second intermediate result to generate a third intermediate result, applying a post-processing function on said third intermediate result to generate a fourth intermediate result, computing an output tensor by summing said received input tensor with said fourth intermediate result, recursing by setting said input tensor to said output tensor until said output tensor is of size one in a pre-determined dimension, and, performing a prediction of a speech sample using said output tensor of size one in a pre-determined dimension. 
     Example 2 includes the subject matter of Example 1, wherein performing a prediction of a speech sample using said output tensor of size one in a pre-determined dimension further comprises processing said output tensor by a fully connected neural network layer to generate a fifth intermediate result, and, applying a softmax classifier to said fifth intermediate result to generate a speech sample. 
     Example 3 includes the subject matter of Example 1 or 2, wherein said input tensor comprises a one-hot vector comprising a plurality of channels, wherein each channel is set to 0 except for a single channel corresponding to a quantization value of an audio signal. 
     Example 4 includes the subject matter of Example 1, 2 or 3 wherein said post-processing function comprises a first non-linear activation function followed by a 1×1 convolution followed by a second non-linear activation function. 
     Example 5 includes the subject matter of Example 4, wherein said first and second non-linear activation functions are ReLU (“Rectified Linear Unit”) activation functions. 
     Example 6 includes the subject matter of Example 1, 2, 3, 4 or 5 further comprising during a training operation performing a zero-padding operation. 
     Example 7 includes the subject matter of Example 6, wherein said zero-padding operation comprises shifting said input tensor to the right by N samples, wherein said N samples are set to 0. 
     Example 8 is a system for generating speech samples comprising a plurality of FFTNet blocks arranged in series, wherein each FFTNet block includes a splitter module, a convolution module, a summation block, and a post-processing module, wherein said post-processing module generates an output based upon said composite tensor, a fully connected layer, wherein said fully connected layer is coupled to a last FFTNet block in said series; and, a softmax classifier coupled to an output of said fully connected layer. 
     Example 9 includes the subject matter of Example 8, wherein said post-processing module comprises a first activation function block followed by a 1×1 convolution block followed by a second activation block. 
     Example 10 includes the subject matter of Example 9, wherein said first and second activation blocks implement a ReLU activation function. 
     Example 11 includes the subject matter of Example 8, wherein said convolution module performs a 1×1 convolution. 
     Example 12 includes the subject matter of Example 8, wherein said splitter module splits an input tensor into a left tensor and a right tensor; 
     Example 13 includes the subject matter of Example 12 wherein said convolution module performs a convolution upon said left tensor and said right tensor to generate a respective convolved left tensor and a convolved right tensor. 
     Example 14 includes the subject matter of Example 13, wherein said summation block generates a composite tensor based upon the convolved left tensor and the convolved right tensor. 
     Example 15 is a computer program product including one or more non-transitory machine-readable mediums encoded with instructions that when executed by one or more processors cause a process to be carried out for generating speech samples, the process comprising receiving an input tensor, splitting said received input tensor into a first portion and a second portion, performing a 1×1 convolution respectively on said first portion and said second portion to generate a respective first intermediate result and a second intermediate result, summing said first intermediate result and said second intermediate result to generate a third intermediate result, applying a post-processing function on said third intermediate result to generate a fourth intermediate result, computing an output tensor by summing said received input tensor with said fourth intermediate result, recursing by setting said input tensor to said output tensor until said output tensor is of size one in a pre-determined dimension, and, performing a prediction of a speech sample using said output tensor of size one in a pre-determined dimension. 
     Example 16 includes the subject matter of Example 15, wherein performing a prediction of a speech sample using said output tensor of size one in a pre-determined dimension further comprises processing said output tensor by a fully connected neural network layer to generate a fifth intermediate result, and, applying a softmax classifier to said fifth intermediate result to generate a speech sample. 
     Example 17 includes the subject matter of Example 15 or 16, wherein said input tensor comprises a one-hot vector comprising a plurality of channels, wherein each channel is set to 0 except for a single channel corresponding to a quantization value of an audio signal. 
     Example 18 includes the subject matter of Example 15, 16, 17 or 18 wherein said post-processing function comprises a first non-linear activation function followed by a 1×1 convolution followed by a second non-linear activation function. 
     Example 19 includes the subject matter of Example 18, wherein said first and second non-linear activation functions are ReLU (“Rectified Linear Unit”) activation functions. 
     Example 20 includes the subject matter of Example 15, 16, 17, 18 or 19 further comprising during a training operation performing a zero-padding operation.