XML is the markup language used in many digital cinema messages. With XML, one can pass data from one device to the next in a manner that can also be read by human beings. Note that this isn’t the kind of reading you might do at night lying in bed with your Kindle, but it’s a huge improvement over the binary transfer of data, which is not readable by humans at all.
So that devices know how to interpret the text that is handed them by the XML message, a “schema” is needed. The schema for a particular message is standardized as a unique application of XML. For this process to be useful, the XML message must identify which schema is to be used to interpret it. Imagine a processor that holds 100 different schemas. The processor will have no idea how to interpret a newly received XML message without some kind of pointer that says which schema to use.
In XML lingo, this pointer is called a “namespace.” In SMPTE digital cinema standards, the namespace looks like the URL in your browser. It has the form http://some_domain.com/some_other_name. The choice of the web browser URL is historical. Some engineers believe that these names should point to useful things on the web. But no one actually creates such web pages, and so the choice of web browser-type URL has absolutely no significance in digital cinema.
In SMPTE standards, these namespaces were not meant to end with a forward slash (“/”). There are some words in W3C, the standards body behind XML, that disapprove of such trailing slashes. But a few namespaces that end with a slash unintentionally snuck their way into approved standards: S430-4 Log Record Format and S433 Data Types. This poses a problem to programmers and standards makers. No one expects the slash to be there, and many product implementations of digital cinema standards don’t incorporate it. Some published standards require use of the 430-4 and 433 namespaces, sans slash. But the slash is normative in 430-4 and 433, and Cinecert tests for its presence in its implementation of the DCI Compliance Test Plan.
In the February SMPTE 21DC standards meetings, it was decided to move forward with no changes to documents 430-4 and 433. In turn, this requires changes in several other documents that incorrectly reference the problem namespaces by omitting the trailing slash. But even the smartest of experts get confused, and it seems some committee members are only now realizing that we are allowing improperly formed namespaces to be standardized. As trivial as this matter may sound, it is blocking the publication of one of the CSP/RPL protocol standards for closed captions, and impacts the use of several other digital cinema standards. It has earned the name “the ugly slash.” The next round of SMPTE 21DC meetings is coming up early June. We will revisit the ugly slash again.