The MainSound Track File audio characteristics of bit depth, sample rate, channel count, and reference level are defined in SMPTE ST428-2 DCDM Audio Characteristics, which are applicable to both Interop DCP and SMPTE DCP. The maximum number of audio channels carried in the Sound Track File is 16.
Audio channels in the Composition map to the audio outputs of the media block or server in a one-to-one manner. An audio signal carried in channel 1 of the track file will output at channel 1 of the media block or server. Audio signals are typically paired in AES3-formatted streams for output. The pairing takes place in numerical order, i.e, audio channels 1 and 2 are output in the 1st AES3 signal, and so on. The one-to-one mapping of channel to output occurs for both Interop DCP and SMPTE DCP, although SMPTE DCP has exceptions discussed further below.
The ordering of audio and data in the MainSound Track File is called out in ISDCF Doc4 – 16 Channel Audio Packaging Guide. The table below summarizes the ordering of the MainSound Track File:
|Channel in Package||5.1||7.1||Description|
|4||LFE||LFE||Low Frequency Effects|
|5||Ls||Lss||Left Surround / Left Side Surround|
|6||Rs||Rss||Right Surround / Right Side Surround|
|8||VI-N||VI-N||Visually Impaired Narrative|
|9||not used||not used||Reserved for SDDS 7.1|
|10||not used||not used||Reserved for SDDS 7.1|
|11||not used||Lrs||Left Rear Surround|
|12||not used||Rrs||Right Rear Surround|
|13||Motion Data||Motion Data||Motion Seats|
|14||Sync||Sync||FSK Sync Signal for Immersive Sound|
|15||Sign Language||Sign Language||Sign Language Video|
|16||not used||not used|
Table S-1. MainSound Track File Signal Order
It was noted earlier that the signal order described in the table above also describes the ordering of audio outputs from the media block or server. If Left audio is to be placed differently in the track file, then the playback device must route the Left signal to the output defined in the table for interoperability.
SMPTE DCP prescribes several incompatible methods for packaging 5.1 and 7.1 audio. Of these multiple methods, it is recommended that only the channel configuration defined in SMPTE 429-2 DCP Operational Constraints, Annex A, Channel Configuration 4, be used in commercial distributions. In this channel configuration, the ordering of audio and data channels defined in Table S-1 above is recommended.
However, SMPTE 429-2 also offers a few other methods for packaging 5.1 and 7.1 audio. SMPTE 429-2 defines alternative packaging channel configurations for 5.1 and 7.1, as well as configurations for other sound formats. In addition, it is possible to label each audio channel in accordance with the SMPTE ST 377-4 MXF Multichannel Audio Labeling Framework. These alternative methods for packaging multi-channel audio order the channels differently, and as discussed, require signal routing in the media block or server to output the signal to the proper speaker or device. However, the alternative methods in SMPTE DCP are not available in Interop DCP, and not all media blocks and servers are configured to properly route signals. For SMPTE DCP, it is recommended that Channel Configuration 4 and the signal ordering described in the table above be used for compatibility.