Why is 2 frame gap between subtitles recommended?

[overlookmotel]

When subtitles from an SRT contain short gaps between them (e.g. 1 subtitle ends, and next subtitle starts 1 frame later), DCP-o-matic warns:

"At least one of your subtitles starts less than 2 frames after the previous one. It is advisable to make the gap between subtitles at least 2 frames."

What is the reason for a 2 frame gap? We've seen glitching with display of closed (CCAP) captions on closed captioning devices when there's no gap at all between captions (and a vague unconfirmed report of also with 1-frame gap). But I wasn't aware of any problem with 1-frame gaps in open subtitles/captions.

Subtitles received from subtitling companies often only contain a single frame gap between subtitles. Should we be editing these to increase the length of the gaps before making DCPs from them?

Can anyone illuminate?

[carl]

Hey, that's my interpretation of the Bv2.1 statement "All Subtitle instances as defined In SMPTE ST 428-7 should have a minimum duration of 15 frames
with a minimum duration of 2 frames between Subtitle instances."

The cynic in me would hazard a guess that this comes from a bug in some playback server, but I don't know.

[overlookmotel]

Thanks for coming back. Ah ha that makes sense. I wasn't aware of that being specified.

As I understand it, Bv2.1 is an attempt to specify what's required for a DCP to play correctly *anywhere*, so in that sense a known bug in a playback server would be a good justification for its inclusion, even though it's a pain.

[IoannisSyrogiannis]

No bug there. Neither on server nor on Texas Instruments.

Frames between sequential subtitles, number or characters per line, subtitles not overlapping scene changes, number of lines, minimum and maximum duration of screening, characters per second etc. etc. are all factors that make the difference between the audience reading comfortably the subtitles and missing the image for the sake of translation.

You may -if you want- have overlapping subtitles as well, they will play out.

[Kewl]

The Netflix guide for subtitles is useful. https://partnerhelp.netflixstudios.com/ ... Guidelines

[overlookmotel]

Thanks both for your thoughts and for sharing the Netflix doc.

The Netflix guide is atypically comprehensible for this kind of document. Nice work Netflix! However, it seems to be a "style guide" for what they feel makes for a good viewing experience on their platform. i.e. it's aimed to make subtitles work stylistically, not technically.

It does mandate 2-frame gaps between subtitles, but I'm not clear if that's a technical limitation, or more a matter of "this is our taste". So unfortunately I'm not sure if it's a definitive answer to whether DCPs with 1-frame gaps between subs will play correctly on all DCI equipment or not.

[Carsten]

I am sure that the SMPTE spec is not aimed at comfortable reading. That's not what SMPTE does. There is probably some known issue with some machines. Now, that could be some outdated server that now is hardly ever used. Unfortunately, the ISDCF people who established Bv2.1 were not too open about the reasons for some decisions that made it into Bv2.1. I attended one of their meetings were it was discovered very reluctantly that the spec 'one reel has captions - all reels need captions', incl. the need to create empty captions files (which earned it's own SMPTE document), had become necessary because one prominent server had shown issues. Actually, we don't know wether that bug has already been ironed out. But the SMPTE document will remain.

[IoannisSyrogiannis]

The Bv2.1 is not a standard, recommended practice or engineering guideline.

7.2.5 Spot Duration
All Subtitle instances as defined In SMPTE ST 428-7 should have a minimum duration of 15 frames with a minimum duration of 2 frames between Subtitle instances.

Note: It is advised to avoid using persistent subtitles where possible, instead opting for a repetitive Subtitle instance within the SubtitleList as defined In SMPTE ST 428-7.

2 Conformance Notation
Normative text is text that describes elements of the design that are indispensable or contains the conformance language keywords: "shall", "should", or "may". [...]

All text in this document is, by default, normative, except: the Introduction, any section explicitly labeled as "Informative" or individual paragraphs that start with "Note:”

The keywords "shall" and "shall not" indicate requirements strictly to be followed in order to conform to the document and from which no deviation is permitted.
The keywords, "should" and "should not" indicate that, among several possibilities, one is recommended as particularly suitable, without mentioning or excluding others; or that a certain course of action is preferred but not necessarily required; or that (in the negative form) a certain possibility or course of action is deprecated but not prohibited.

SMPTE ST 428-7
Subtitle elements [...] may have overlapping temporal regions.

"Mandate" is not the word I would use, as far as SMPTE Bv2.1 goes. Nor the restrictions on the functionality of the subtitles a mater of taste.

The entitlement of opinions is undisputed. Sharing of experiences is the best I can hope for in this forum and most others. I will reiterate that you might as well have overlapping subtitles. I can't say with certainty about .png subtitles, though.

[overlookmotel]

"the SMPTE spec is not aimed at comfortable reading". Indeed! But, even compared to other broadcaster specs, Netflix's doc is very clearly written. It's the kind of document we could point even non-technical filmmakers to, and they'd be able to understand it, which is really useful.

Personally, I'm fine with a spec/guidance including elements which are purely to work around a bug in some specific server/projector. That's very much aligned with my aim when working on film festivals - I want to know that we can send a DCP to any venue, without knowing anything about what equipment it has in advance, and be confident it will play without any problems no matter what.

I am less fine with secrecy about the reasoning behind any particular decision. What's to be ashamed of?

"The Bv2.1 is not a standard, recommended practice or engineering guideline." Ha! What is it then?

I think at this point I need to sit down and read Bv2.1 in full myself, rather than bugging other people for answers.

But thanks for helping with this. It's reassuring that so far no-one has said "I happen to know that anything less than 2 frame gaps will definitely produce faulty playback on X server" though, so it sounds like it's not a particularly risky move to have 1-frame gaps, even if it'd fail a Bv2.1 validation.

[IoannisSyrogiannis]

SMPTE DCP Bv2.1 Application Profile is a defined feature set of and constraints on Digital Cinema (DCinema) Packages and DCP elements within current standards, applicable to theatrical distribution, which have been integrated, globally agreed upon, tested, and rolled out by various industry experts, vendors, manufacturers, content owners, and industry bodies such as Inter-Society Digital Cinema Forum (ISDCF) and European Digital Cinema Forum (EDCF).

You may read it your self, freely: https://ieeexplore.ieee.org/document/9161348