RE: [EXTERNAL] Possible addition to SAUR on minimal frame rates

I suggest we add this draft text to a branch and edit it from there. Also, Steve, where is it meant to be inserted?

From: Noble, Stephen <steve.noble@pearson.com>
Sent: Wednesday, 2 March 2022 16:40
To: RQTF <public-rqtf@w3.org>
Subject: [EXTERNAL] Possible addition to SAUR on minimal frame rates

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In thinking of adding some possible recommendations to the SAUR, I wonder if the text below would be a helpful addition?  It may also have some implications for the RAUR. Or maybe we don't really need it. The issue of the frame rate on accessibility is not explicit in the SAUR thus far, though we do mention it in passing. There is a potential negative accessibility impact on media synchronization when video is compressed to save space. This is also true in a remote meeting (or a recording of the meeting when viewed later) when the meeting platform drops the frame rate in the video below a certain level. Some aspects of this interchange are not in the control of the meeting platform provider, of course. If you try to connect to a remote meeting through a dial-up connection, there's nothing the platform provider can do to overcome that limitation on your end. However, there are some aspects the provider can control, such as the minimum supported frame rate they use on their end to host meetings and to encode meeting recordings, as well as how the sync captions in for a low frame rate scenario.

Thus, I would appreciate any comments on the text below. It probably needs a good bit of editing and some of the text may be unneeded. I am a technology historian at heart, and I often get caught up in explaining where certain standards came from.

<begin suggested text>

One issue impacting the ability to properly synchronize audiovisual media elements is the video frame rate. Generally speaking, nominal video frame rates supported in most current video technologies have their roots in legacy film industry and analog television broadcast specifications used in the U.S. and Europe. These differing specifications adopted 24 fps for film, and either 30 fps (or 29.97 fps) and 25 fps as regional base standards for broadcast video, due in part to the need to accommodate legacy television receivers and synchronize frame rates with local electrical power frequencies (60 Hz in the U.S., and 50 Hz in Europe). However, it is not uncommon for authors and/or hosting platforms to use video codecs which compress digital video files and/or transmissions in order to save server storage space or to speed up streaming. Since such video compression tends to reduce the video frame rate on the user end, there then becomes a potential for lower frame rates to negatively impact the accessibility of digital multimedia.

In recognition of this potentially adverse impact, as far back as 1999 the United Nations Specialized Agency “ITU” recommended a minimum frame rate of 20 fps be supported in video transmission to better support lip reading and speech intelligibility [ITU-T 1999]. The current Harmonised European Standard [EN 301 549 V3.2.1 (2021-03)] specifies that two-way voice communication technologies which includes real-time video functionality “shall support a frame rate of at least 20 Frames Per Second” and further “should preferably support a frame rate of at least 30 Frames Per Second.” As previously discussed under Section 2.1, at 25 fps, there will be one frame every 40 ms. Since we also know that intelligibility can decline appreciably for even the shortest asynchrony of 40 ms, it is therefore reasonable to suggest that 25 fps be considered the lowest acceptable video frame rate to support the accessibility of audiovisual content.

This suggests a number of accessibility implications for content authors and user agents. In particular, when hosting and streaming audiovisual content containing human speakers, digital video files should be encoded and transmitted with a minimal frame rate of 25 fps, other than legacy film versions recorded at the 24 fps frame rate for cinema viewing.

There are also implications for remote meetings. Remote meeting platforms such as Microsoft Teams and Zoom generally support a frame rate of up to 30 fps. However, various factors such as limited CPU power or low bandwidth sometimes reduce the nominal frame rate below this level. Meeting recordings may be further limited to even lower nominal frame rates in efforts to save data storage space on hosting servers. This could negatively impact accessibility of live meetings and especially meeting recordings. This negative impact will be greatest in the use case of a meeting attendee who is lip reading, but also to a lesser degree for someone reading captions. It is therefore similarly recommended that meeting platform providers support a minimal rate of at least 25 fps during live meetings which will be carried over into meeting recordings. Additionally, in the case of caption synchronization, when bandwidth issues and other technical limitations cause frame rates to drop below this level, it is recommended that the meeting platform providers should match the caption timing as referenced by the audio rather than the video to reduce temporal display errors.

</end suggested text>

Let me know your thoughts.

--Steve



Steve Noble
Instructional Designer, Accessibility
Psychometrics & Testing Services

Pearson
502 969 3088
steve.noble@pearson.com<mailto:steve.noble@pearson.com>
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