Neural Speech Coding for Real-time Communications using Constant Bitrate Scalar Quantization

TL;DR

This paper introduces a simple scalar quantization approach for neural speech codecs, improving real-time communication at low bitrates with reduced complexity and training overhead.

Contribution

It proposes a scalar quantization method as an alternative to vector quantization, simplifying training and enhancing low-bitrate, real-time neural speech coding.

Findings

Performs well at very low bitrates

Reduces training complexity by avoiding codebook storage

Operates efficiently in real-time with low latency

Abstract

Neural audio coding has emerged as a vivid research direction by promising good audio quality at very low bitrates unachievable by classical coding techniques. Here, end-to-end trainable autoencoder-like models represent the state of the art, where a discrete representation in the bottleneck of the autoencoder is learned. This allows for efficient transmission of the input audio signal. The learned discrete representation of neural codecs is typically generated by applying a quantizer to the output of the neural encoder. In almost all state-of-the-art neural audio coding approaches, this quantizer is realized as a Vector Quantizer (VQ) and a lot of effort has been spent to alleviate drawbacks of this quantization technique when used together with a neural audio coder. In this paper, we propose and analyze simple alternatives to VQ, which are based on projected Scalar Quantization (SQ). These quantization techniques do not need any additional losses, scheduling parameters or codebook storage thereby simplifying the training of neural audio codecs. For real-time speech communication applications, these neural codecs are required to operate at low complexity, low latency and at low bitrates. We address those challenges by proposing a new causal network architecture that is based on SQ and a Short-Time Fourier Transform (STFT) representation. The proposed method performs particularly well in the very low complexity and low bitrate regime.