A discussion of the DCP would not be complete without mention of the two types of distribution package used in production: Interop DCP and SMPTE DCP. They are functionally similar in that the DCP definitions provided earlier in this chapter apply to both types of packaging formats. But they are substantially different in that they are not interoperable.
Interop DCP is based on an early draft proposal for SMPTE DCP. It was put into practice in 2004, in preparation for the rollout of digital cinema. SMPTE DCP was not finalized until 2009, four years after the rollout began. Interop DCP was intended as a temporary format until the standardized version came into existence. However, the lack of backwards compatibility, as well as the requirement for functionality not available in legacy equipment, has hampered the transition to SMPTE DCP.
Digital cinema owes its success to Interop DCP, which continues to be the primary distribution format at the time of this writing. Despite its success, Interop DCP was not formally standardized, but the documentation is available through Cinepedia at Interop DCP. In contrast, SMPTE DCP is well-defined, and published as a suite of SMPTE standards. Maintenance on Interop DCP continued up until 2012, when it was decided to limit new development to only SMPTE DCP.
There are several differences in SMPTE DCP that prevent backwards compatibility and interoperability. Both formats utilize a Composition Playlist (CPL), but with different, non-interoperable XML structures. There are also structural differences in Subtitle and Audio track files. SMPTE DCP Subtitle track files are encrypted, a valued feature to distributors. But the structure of the subtitle track file is significantly different from its Interop counterpart, as there was an intellectual property issue with the Interop version at the time the standard was created. The result is that DLP Series 1 projectors, the largest concentration of which are in the United States, are incapable of rendering the SMPTE DCP subtitle track file. This incompatibility can be overcome by rendering SMPTE subtitles in the server, a feature which is now commonplace in newer servers and media blocks. Another difference is a reliance on audio channel routing in the server, which also is not supported in many older systems. As a result, practical SMPTE DCP distributions are constrained to bypass the new audio features, falling back to Interop-style audio packaging. This workaround requires the use of “Channel Configuration 4” described in Annex A of SMPTE ST 429-2 DCP Operational Constraints, which was originally included for test purposes. More explanation of digital cinema audio is available in the chapter on Sound.
The DCP type can be most easily recognized by the namespace root called out in the XML-based Composition Playlist (CPL). SMPTE DCP uses the SMPTE-RA.ORG namespace root, while Interop DCP uses DIGICINE.COM. The top-level document that defines SMPTE DCP is SMPTE ST 429-2 DCP Operational Constraints. Interop DCP documentation is available here.
There is also an excellent presentation by Jim Whittlesey on the differences between Interop DCP and SMPTE DCP.