# Psychaocoustic Models for Perceptual Audio Coding--A Tutorial Review
## 6 Summary and Conclusions  
The art of perceptual audio coding allows substantial bitrate savings in the representation of high quality audio material. A major part of the bitrate reduction can be attributed to the elaborated exploitation of psychoacoustic effects and appropriate signal processing methods, including:  
* noise shaping of the quantization error in frequency and time dimension to achieve best possible masking of the error signal, and thus optimal subjective quality;
* controlling the use of optional coding tools for improved codec performance.  
Within an audio codec, the psychoacoustic model is of central importance in implementing these functions. This article provides a tutorial overview of common psychoacoustic models for use in perceptual audio coding. The core functionality of a basic psychoacoustic model consists of a masking threshold calculation, including aspects such as the non-uniform layout of the frequency perceptual scale, masking within and between frequency bands, the impact of tonality on masking, and masking over time. In addition to modeling these effects, practical models often have to adapt the masking model output to the rest of the codec architecture, such that the overall system achieves optimum coding performance (i.e., highest possible subjective quality at a given bitrate) at moderate computational complexity.  
Beyond simple monophonic masking, spatial masking and BMLD-related effects are typically considered in psychoacoustic models for coding of stereo, surround, or 3D signals.  
Over time, optimized psychoacoustic models have been developed that are not meant to operate as stand-alone modules but are tightly integrated into the codec structure to provide excellent system quality with very high computational efficiency.  
Beyond calculation of masking effects, the proper control of coding tools also frequently requires a perceptual model. This includes tools for joint stereo processing, tools for parametric coding options (bandwidth extension and parametric multi-channel or 3D coding), and may include on/off switching decisions, as well as the choice of optimal coding parameters.  
Historically, there have been numerous advancements over the years in both audio coding and its perceptual models. With the availability of more computational resources, next generations of perceptual models have been evolving, including novel aspects such as modulation perception, and are on their way to proving their usefulness.  
Future generations of perceptual models may face new challenges in the context of compressed audio for virtual reality (VR) applications with 6 Degrees of Freedom (6 DoF) [84; 85], where beyond spatial audio, also visual representation and user movement become very relevant and need to be understood and integrated as part of a multi-modal experience. The authors look forward to the further evolution in the understanding of the relevant perceptual aspects of audio coding, rendering, and processing in such highly immersive and holistic perceptual scenarios.  
Conceptualization, [PERSON] and S.D.; methodology, [PERSON] and [PERSON]; software, [PERSON]; validation, [PERSON] and [PERSON]; formal analysis, [PERSON] and [PERSON]; investigation, [PERSON] and [PERSON]; resources, [PERSON]; data curation, [PERSON]; writing--original draft preparation, [PERSON] and [PERSON]; writing--review and editing, [PERSON] and S.D.; visualization, [PERSON] and [PERSON]; supervision, [PERSON]; project administration, [PERSON]; funding acquisition, [PERSON].  
This research received no external funding.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l}
3D & Three Dimensional \\
3 GPP & 3 rd Generation Partnership Project \\
6 DoF & Six Degrees of Freedom \\ AAC & Advanced Audio Coding \\ \end{tabular}