# A Psychoacoustic-Based Multiple Audio Object Coding Approach via Intra-Object Sparsity
## 1 Introduction  
With the development of multimedia video/audio signal processing, multi-channel 3D audio has been widely employed for applications, such as cinemas and home theatre systems, since it can provide excellent spatial realism of the original sound field, as compared to the traditional mono/stereo audio format. There are multiple formats for rendering 3D audio, which contain channel-based, object-based and HOA-based audio formats. In traditional spatial sound rendering approach, the channel-based format is adopted in the early stage. For example, the 5.1 surround audio format [1] provides a horizontal soundfield and it has been widely employed for applications, such as the cinema and home theater. Furthermore, typical '3D' formats include a varying number of height channels, such as 7.1 audio format (with two height channels). As the channel number increases, the audio data will raise dramatically. Due to the bandwidth constrained usage scenarios, the spatial audio coding technique has become an ongoing research topic in recent decades. In 1997, ISO / MPEG (Moving Picture Experts Group) designed the first commercially-used multi-channel audio coder MPEG-2 Advanced Audio Coding (MPEG-2 AAC) [2]. It could compress multi-channel audio by adding a number of advanced coding tools to MPEG-1 audio codecs, delivering European Broadcasting Union (EBU) broadcast quality at a bitrate of 320 kbps for a 5.1 signal. In 2006, MPEG Surround (MPS) [3,4] was created for highly transmission of multi-channel sound by downmixing the multi-channel signals into mono/stereo signal and extracting Interaural Level Differences (ILD), ITD (Interaural Time Differences) and IC (Interaural Coherence) as side information. Spatially Squeezed Surround Audio Coding (S\({}^{3}\)AC) [5, 6, 7], as a new method instead of original \"downmix plus spatial parameters\" model, exploited spatial direction of virtual sound source and mapping the soundfield from 360\({}^{\circ}\) into 60\({}^{\circ}\). At the receiver, the decoded signals can be achieved by inverse mapping the 60\({}^{\circ}\) stereo soundfield into 360\({}^{\circ}\). However, such channel-based audio format has its limitation on flexibility, i.e., each channel is designated to feed a loudspeaker in a known prescribed position and cannot be adjusted for different reproduction needs by the users. Alternatively, a spatial sound scene can be described by a number of sound objects, each positioned at a certain target object position in space, which can be totally independent from the locations of available loudspeakers [8]. In order to fulfill the demand of interactive audio elements, object-based (a.k.a. object-oriented) audio format enables users to control audio content or sense of direction in application scenarios where the number of sound sources varies, sources move are commonly encountered. Hence, object signals generally need to be rendered to their target positions by appropriate rendering algorithms, e.g., Vector Base Amplitude Panning (VBAP) [9]. Therefore, object-based audio format can personalize customer's listening experience and make surround sound more realistic. By now, object-based audio has been commercialized in many acoustic field, e.g., Dolby ATMOS for cinemas [10]. To facilitate high-quality bitrate-efficient distribution of audio objects, several methods have been developed, one of these techniques is MPEG Spatial Audio Object Coding (SAOC) [11, 12], SAOC encodes audio objects into a mono/stereo downlink signal plus side information via Quadrature Mirror Filter (QMF) and extract the parameters that stand for the energy relationship between different audio objects. Additionally, Directional Audio Coding (DirAC) [13,14] compress a spatial scene by calculating a direction vector representing spatial location information of the virtual sources. At the decoder side, the virtual sources are created from the downmixed signal at positions given by the direction vectors and they are panned by combining different loudspeakers through VBAP.