Is Hierarchical Quantization Essential for Optimal Reconstruction?

TL;DR

This paper investigates whether hierarchical quantization in VQ-VAEs is necessary for optimal reconstruction, finding that single-level models can match hierarchical ones when properly trained and regularized.

Contribution

The study demonstrates that with proper training techniques and hyperparameter tuning, single-level VQ-VAEs can achieve reconstruction quality comparable to hierarchical models, challenging common assumptions.

Findings

Single-level VQ-VAEs can match hierarchical models in reconstruction fidelity.

Codebook collapse can be mitigated with initialization, resets, and hyperparameter tuning.

Proper training reduces the perceived advantage of hierarchical quantization.

Abstract

Vector-quantized variational autoencoders (VQ-VAEs) are central to models that rely on high reconstruction fidelity, from neural compression to generative pipelines. Hierarchical extensions, such as VQ-VAE2, are often credited with superior reconstruction performance because they split global and local features across multiple levels. However, since higher levels derive all their information from lower levels, they should not carry additional reconstructive content beyond what the lower-level already encodes. Combined with recent advances in training objectives and quantization mechanisms, this leads us to ask whether a single-level VQ-VAE, with matched representational budget and no codebook collapse, can equal the reconstruction fidelity of its hierarchical counterpart. Although the multi-scale structure of hierarchical models may improve perceptual quality in downstream tasks, the effect of hierarchy on reconstruction accuracy, isolated from codebook utilization and overall representational capacity, remains empirically underexamined. We revisit this question by comparing a two-level VQ-VAE and a capacity-matched single-level model on high-resolution ImageNet images. Consistent with prior observations, we confirm that inadequate codebook utilization limits single-level VQ-VAEs and that overly high-dimensional embeddings destabilize quantization and increase codebook collapse. We show that lightweight interventions such as initialization from data, periodic reset of inactive codebook vectors, and systematic tuning of codebook hyperparameters significantly reduce collapse. Our results demonstrate that when representational budgets are matched, and codebook collapse is mitigated, single-level VQ-VAEs can match the reconstruction fidelity of hierarchical variants, challenging the assumption that hierarchical quantization is inherently superior for high-quality reconstructions.