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Title: MelNet: A Generative Model for Audio in the Frequency Domain

Paper content:
Capturing high-level structure in audio waveforms is challenging because a single second of audio spans tens of thousands of timesteps.
While long-range dependencies are difficult to model directly in the time domain, we show that they can be more tractably modelled in two-dimensional time-frequency representations such as spectrograms.
By leveraging this representational advantage, in conjunction with a highly expressive probabilistic model and a multiscale generation procedure, we design a model capable of generating high-fidelity audio samples which capture structure at timescales which time-domain models have yet to achieve.
We demonstrate that our model captures longer-range dependencies than time-domain models such as WaveNet across a diverse set of unconditional generation tasks, including single-speaker speech generation, multi-speaker speech generation, and music generation.
Audio waveforms have complex structure at drastically varying timescales, which presents a challenge for generative models.
Local structure must be captured to produce high-fidelity audio, while longrange dependencies spanning tens of thousands of timesteps must be captured to generate audio which is globally consistent.
Existing generative models of waveforms such as WaveNet (van den Oord et al., 2016a) and SampleRNN (Mehri et al., 2016) are well-adapted to model local dependencies, but as these models typically only backpropagate through a fraction of a second, they are unable to capture high-level structure that emerges on the scale of several seconds.
We introduce a generative model for audio which captures longer-range dependencies than existing end-to-end models.
We primarily achieve this by modelling 2D time-frequency representations such as spectrograms rather than 1D time-domain waveforms ( Figure 1 ).
The temporal axis of a spectrogram is orders of magnitude more compact than that of a waveform, meaning dependencies that span tens of thousands of timesteps in waveforms only span hundreds of timesteps in spectrograms.
In practice, this enables our spectrogram models to generate unconditional speech and music samples with consistency over multiple seconds whereas time-domain models must be conditioned on intermediate features to capture structure at similar timescales.
Modelling spectrograms can simplify the task of capturing global structure, but can weaken a model's ability to capture local characteristics that correlate with audio fidelity.
Producing high-fidelity audio has been challenging for existing spectrogram models, which we attribute to the lossy nature of spectrograms and oversmoothing artifacts which result from insufficiently expressive models.
To reduce information loss, we model high-resolution spectrograms which have the same dimensionality as their corresponding time-domain signals.
To limit oversmoothing, we use a highly expressive autoregressive model which factorizes the distribution over both the time and frequency dimensions.
Modelling both fine-grained details and high-level structure in high-dimensional distributions is known to be challenging for autoregressive models.
To capture both local and global structure in spectrograms with hundreds of thousands of dimensions, we employ a multiscale approach which generates spectrograms in a coarse-to-fine manner.
A low-resolution, subsampled spectrogram that captures high-level structure is generated initially, followed by an iterative upsampling procedure that adds high-resolution details.
(1x, 5x, 25x, 125x)
Figure 1: Spectrogram and waveform representations of the same 4 second audio signal.
The waveform spans nearly 100,000 timesteps whereas the temporal axis of the spectrogram spans roughly 400.
Complex structure is nested within the temporal axis of the waveform at various timescales, whereas the spectrogram has structure which is smoothly spread across the time-frequency plane.
Combining these representational and modelling techniques yields a highly expressive and broadly applicable generative model of audio.
Our contributions are are as follows:
• We introduce MelNet, a generative model for spectrograms which couples a fine-grained autoregressive model and a multiscale generation procedure to jointly capture local and global structure.
• We show that MelNet is able to model longer-range dependencies than existing time-domain models.
Additionally, we include an ablation to demonstrate that multiscale modelling is essential for modelling long-range dependencies.
• We demonstrate that MelNet is broadly applicable to a variety of audio generation tasks, including unconditional speech and music generation.
Furthermore, MelNet is able to model highly multimodal data such as multi-speaker and multilingual speech.
We have introduced MelNet, a generative model for spectral representations of audio.
MelNet combines a highly expressive autoregressive model with a multiscale modelling scheme to generate high-resolution spectrograms with realistic structure on both local and global scales.
In comparison to previous works which model time-domain signals directly, MelNet is particularly well-suited to model long-range temporal dependencies.
Experiments show promising results across a diverse set of audio generation tasks.
Furthermore, we believe MelNet provides a foundation for various directions of future work.
Two particularly promising directions are text-to-speech synthesis and representation learning:
• Text-to-Speech Synthesis: MelNet utilizes a more flexible probabilistic model than existing end-to-end text-to-speech models, making it well-suited to model expressive, multi-modal speech data.
• Representation Learning: MelNet is able to uncover salient structure from large quantities of unlabelled audio.
Large-scale, pre-trained autoregressive models for language modelling have demonstrated significant benefits when fine-tuned for downstream tasks.
Likewise, representations learned by MelNet could potentially aid downstream tasks such as speech recognition.

assistant: text: We introduce an autoregressive generative model for spectrograms and demonstrate applications to speech and music generation