Synthetic Mel Spectrograms

• Synthetic mel spectrograms are compact, mel-scaled acoustic representations generated by neural networks to mimic natural speech and music.
• They are central to applications in neural TTS, speech enhancement, and audio coding by enabling efficient dimensionality reduction and perceptual alignment.
• Advancements in generative modeling—including sequence-to-sequence, hierarchical, contrastive learning, and inversion techniques—drive improvements in synthesis quality and real-time performance.

Synthetic mel spectrograms are time–frequency acoustic representations produced artificially—typically by machine learning models—in place of real, measured spectrograms used in speech, singing, and sound synthesis pipelines. As condensed but perceptually aligned representations, mel spectrograms play a central role in neural text-to-speech (TTS), speech enhancement, audio coding, and related domains. Synthetic mel spectrograms are generated as intermediate features, enhancement targets, or compression codes. Their properties, modeling strategies, and downstream applications have shaped advances in modern speech and audio generation.

1. Definition, Motivation, and Calculations

Synthetic mel spectrograms are sequences of mel-scaled spectral frames generated by neural networks as stand-ins for natural acoustic measurements. Unlike raw waveforms or full linear-frequency spectrograms, mel spectrograms provide a compact, perceptually meaningful summary. Their calculation, as used in Tacotron 2 and many other systems (Shen et al., 2017), follows:

• Compute the short-time Fourier transform (STFT) on the input audio $x$, typically using a Hann window and prescribed frame/hop sizes.
• Apply an 80-channel mel filterbank $H(i, f)$, resulting in

$$S_{\mathrm{mel}}(t, i) = \log \left( \max \left( \sum_f H(i, f) \cdot |\mathrm{STFT}_x(t, f)|,\ 0.01 \right ) \right )$$

where dynamic range compression (log and clipping) is applied after filtering.

In synthesis, the process is reversed: text, lyrics, or other structured inputs are mapped to synthetic mel spectrograms, often in low dimension (e.g., 80 bands), which are subsequently converted to the time-domain waveform using a vocoder. Advantages include dimensionality reduction, better alignment with human auditory perception, and smoother structure that supports reliable prediction and prosody control.

2. Generative Modeling Approaches

Multiple architectures exist for generating synthetic mel spectrograms, differing in their neural modeling strategies and learning objectives:

2.1 Sequence-to-Sequence Predictors

End-to-end TTS systems map textual input through encoders and autoregressive decoders to sequences of mel spectrogram frames. In Tacotron 2 (Shen et al., 2017), the process involves a stack of convolutional, recurrent, and attention modules, trained to minimize framewise MSE:

$$L = \sum_t || S_{\mathrm{mel}}(t) - S_{\mathrm{mel}}^*(t) ||^2$$

An additional post-net refines predictions via a residual CNN.

2.2 Multi-Scale and Hierarchical Modeling

Hierarchical approaches, such as Multi-Scale Spectrogram (MSS) modeling (Abbas et al., 2021), predict mel spectrograms at different linguistic granularity—sentence-, word-, and phoneme-level—propagating coarse-scale prosody to finer resolutions through conditional modeling. This improves prosodic consistency and alignment.

2.3 Contrastive and Multimodal Learning

Contrastive learning, exemplified by Clip-TTS (Liu, 26 Feb 2025), jointly embeds text and mel spectrograms into a common space during training via cosine similarity of projected representations. Duration predictors align phoneme representations to mel frame lengths, enabling semantically meaningful and context-aware spectrogram synthesis.

2.4 Compression and Tokenization

Spectral codecs based on quantization, such as those using Finite Scalar Quantization (FSQ) (Langman et al., 7 Jun 2024), convert mel spectrograms into low-bitrate discrete token streams. FSQ independently quantizes latent features, yielding representations that are easier for non-autoregressive models to predict than traditional Residual Vector Quantization (RVQ) on waveforms.

2.5 Super-Resolution, Enhancement, and Restoration

Super-resolution and enhancement models, such as GAN-based frameworks combining Pix2PixHD and ResUnet (Sheng et al., 2019) or diffusion-enhanced models (Tian et al., 2023), refine coarse or degraded synthetic mel-spectrograms. These models exploit adversarial, perceptual, and structural loss functions (MSE, SSIM, GAN losses) to maximize both perceived quality and metric similarity.

3. Inversion and Vocoding

Synthesizing waveforms from synthetic mel spectrograms requires inversion—estimating both magnitude and phase across linear frequency bins—to reconstruct the time-domain signal:

3.1 Neural Vocoders

WaveNet (Shen et al., 2017) and its variants use dilated convolutional stacks, often trained with mixture-of-logistics outputs, as highly expressive, high-fidelity neural vocoders. Conditioning on mel spectrograms enables parameter reduction and smaller receptive fields compared to earlier systems.

3.2 Analytical and Hybrid Methods

iSTFTNet (Kaneko et al., 2022) introduces a hybrid approach, using upsampling and explicit inverse short-time Fourier transform (iSTFT) modules to reconstruct waveforms efficiently after dimension reduction.

Sinusoidal modeling (Natsiou et al., 2022) reconstructs phase and magnitude by estimating sinusoid parameters from the mel-spectrogram, using amplitude and frequency propagation tailored to harmonic signals.

3.3 Optimization-Based Inversion

Optimization-based methods such as ADMM-joint estimation (Masuyama et al., 9 Jan 2025) formulate mel-spectrogram inversion as a constrained minimization over STFT magnitude and phase. The augmented Lagrangian is:

$$\mathcal{L}_{\lambda, \rho}(X, Y, Z, W, V, U) = \mathcal{J}(X, Y) + \lambda \mathcal{J}_I(W) + \text{(constraint and dual terms)}$$

Efficiently splitting updates improves convergence and quality over cascaded or plain Griffin-Lim approaches.

3.4 Specialized Music Vocoding

For musical audio, shift-invariant target spaces (magnitude spectrum + phase gradient) can address pitch instability not handled by speech-centric vocoders (Giorgi et al., 2022), enabling more stable synthesis of sustained and polyphonic notes.

4. Enhancement, Denoising, and Super-Resolution

Mel-spectrogram enhancement can be performed either as an upstream processing step or jointly with synthesis/training:

• Mel-FullSubNet (Zhou et al., 21 Feb 2024) and CleanMel (Shao et al., 27 Feb 2025) employ interleaved full-band and sub-band neural architectures in the Mel domain for speech denoising and dereverberation, demonstrating superior results over linear-frequency or time-domain methods in terms of both perceptual speech quality (PESQ, DNSMOS) and ASR error rates.
• Diffusion-based models (Tian et al., 2023) enhance Mel-spectrograms degraded by noise, room acoustics, clipping, or band limiting, directly in the log-Mel domain, using conditional score-matching and SDE-driven reverse diffusion; text alignment further guides the enhancement for improved ASR and synthesis.

Super-resolution networks (Sheng et al., 2019) formulate the Mel-spectrogram refinement as a conditional image-to-image translation task, with GANs and residual UNets enhancing spectral detail, harmonic structure, and producing higher evaluation scores (e.g., SSIM, MOS).

Wavelet-based enhancement (Hu et al., 18 Jun 2024) adds a CWT-based fine spectrogram prediction as an auxiliary task for AR and NAR TTS, yielding higher subjective clarity in the output.

5. Real-Time and Streaming Synthesis

To reduce latency for online applications and interactive systems, single-stage architectures have emerged that generate continuous Mel-spectrograms on-the-fly:

• StreamMel (Wang et al., 14 Jun 2025) interleaves text tokens with mel-spectrogram frames in a unified sequence, enabling frame-synchronous, left-to-right, autoregressive synthesis with low first-packet latency. Each mel frame $y_t$ is generated as

$$p(y | x; \theta) = \prod_{t=0}^{T-1} p(y_t | z_{<t'}; \theta)$$

where $z_{<t'}$ represents all prior interleaved tokens.

• Architectures like Clip-TTS (Liu, 26 Feb 2025) leverage efficient, parallel Transformer blocks and joint contrastive embedding for high-quality, rapid synthesis.

These systems balance streaming, low-latency inference with speaker similarity and naturalness, offering a path for integration with real-time speech LLMs and interactive agents.

6. Adaptation and Compatibility

Because Mel-spectrograms may differ in extraction parameters across systems, adaptors have been proposed for interoperability:

• The Universal Adaptor (Wang et al., 2022) provides a two-stage pipeline, using pseudo-inverse filtering, Griffin–Lim-based inversion, and a configuration-conditioned U-Net. This allows conversion of Mel-spectrograms between arbitrary parameterizations while maintaining synthesis quality, facilitating the decoupling of synthesizer and vocoder training.

Such adaptation is essential for flexible TTS and VC pipelines, rapid prototyping, and combining independently developed models.

7. Applications and Evaluation

Synthetic mel spectrograms are foundational in TTS, voice conversion, emotional speech generation, singing voice synthesis, enhancement for ASR and denoising, forensic analysis, and audio restoration:

• Emotional TTS using synthetic mel spectrograms (with disentangled speaker/emotion embeddings) improves both the realism of generated speech and the performance of downstream SER systems (Shahid et al., 2023).
• Forensic systems detect and attribute synthetic speech via CNNs operating on log-Mel spectrograms (Rahman et al., 2023).
• Spectral codecs using FSQ-based mel-spectrogram quantization demonstrate improved quality and efficiency for non-autoregressive speech synthesis relative to waveform token-based codecs (Langman et al., 7 Jun 2024).

Objective evaluation metrics include MOS, SSIM, STOI, DNSMOS, PESQ, SCM, WER, CER, and harmonic error. Continuous advances in modeling approaches, representation learning, inversion, and enhancement reinforce the importance and versatility of synthetic mel spectrograms in state-of-the-art speech and audio technologies.