Sound in digital cinema could be divided in two parts :
Main Sound : Introduced at the very beginning of the SMPTE standards and DCI specifications.
Immersive Sound (OBAE / IAB) : standardized since 2010 and enhanced though the late 2010s
The Main Sound was introduced in DCI specifications and SMPTE standards since the beginning.
Under the CPL tag named MainSound, you have an MXF that includes up to 16 channels of data 1 with the possibility of dispatching.
You always encounter this type of Sound MXF alongside an Picture MXF.
| Encoding | PCM WAVE/RIFF 2 uncompressed and interlaced channels |
|---|---|
| Sample Bitdepth | 24 bits / sample |
| Sample Rate | 48 kHz or 96 kHz |
| Maximum Channels | 16 channels |
| Minimal Bandwidth 3 | 19 Mb/s (48 kHz) 37 Mb/s (96 kHz) |
You will find this informations about the CPL MainSound and MXF MainSound in their respective chapters.
The Immersive Sound was introduced in the early 2010s 4, under the name Object-Based Audio Essence (OBAE) and is based on the SMPTE standards Immersive Audio Bitstream (IAB).
Under the CPL tag named AuxData, you have an MXF that includes additional sounds elements based on the standard Immersive Audio Bitstream (IAB). It's from the IAB standards that we have Dolby Atmos, DTS-X, and Barco-Auro.
You will sometimes encounter this type of IAB MXF in addition to MXF Picture and MXF Sound.
The OBAE (or IAB) is a special type of sound asset because it is based on sound objects with a bunch of metadata, including spatialisation and temporal information.
Notice that we are mainly referring to the Immersive Audio Bitstream (IAB) standards. So don't be surprised to find few references to the acronym OBAE, and even fewer to Enhanced Audio (EA).
| - | Technical capacity | D-Cinema Constraints |
|---|---|---|
| Encoding | PCM WAVE ou DLC Encoding | Only DLC |
| Sample Bitdepth | 24 bits by sample | |
| Sample Rate | 48 kHz or 96 kHz | Only 48 kHz |
| Maximum channels | 128 channels |
You will find all information about Immersive Audio in the respective chapters, including Dolby Atmos chapter and its MXF Immersive Audio, as well as Barco Auro and DTS-X which are also "Immersive Audio".
Of course, I'm talking about data because while the main channels carry useful audio assets for the viewer, there're also technical channels used for synchronization - or data - used by external devices, such as Dolby Atmos or even DBOX. ↩
The DCI specifications and SMPTE standards talk a lot of about BWF (Broadcast Wave Format) which is a kind of evolution of the WAVE format with more metadata. But these metadata aren't used for the audio asset. See the MXF Sound Essence chapter and especially the notes in footer for more information about the WAVE/BWF format. ↩
Few examples of differents calculations of the audio bandwith : ↩
Integrated into the DCI specifications in 2013. ↩
