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Issue 29 - SMPTE tests

From: Sean Hayes <Sean.Hayes@microsoft.com>
Date: Sat, 11 Apr 2009 16:10:38 +0100
To: Public TTWG List <public-tt@w3.org>
Message-ID: <AB3FC8E280628440B366A29DABB6B6E8046B7ECBDC@EA-EXMSG-C334.europe.corp.microsoft.com>
In respect of issue 29 regarding SMPTE mode.

Rather than try and work from David's tests, I took an approach of starting from scratch. Short summary, new tests are attached; to replace existing tests:

The longer discussion now follows:

Rather than try and test each attribute individually, these tests are designed to work for the range of expected scenarios, as the attributes are not fully independent.

The following SMPTE time codes are in standard use:
29.97 fps drop frame
29.97 fps non-drop frame
25 fps (EBU derived from SMPTE).
24 fps and 23.96 (IVTC) for film work.

I have created one test for each of these 6. The mechanics of clockMode, smpteMode, frameRate and frameRateMultiplier are somewhat confusing and we should probably have simply defined the 6 modes as an enumeration; as it means that certain things we can express in Timed text don't seem to make much sense in a SMPTE world, or at least are not adequately documented.
For example:
frameRate="30" smpteMode="NTSCDrop" - is this to be interpreted as 29.97 drop frame or an error?

The tickrate and tick rate multiplier mechanics are perhaps useful in the media clock mode, where you are trying to match an unknown video rate; and clock mode expressions don't make much sense without knowing the exact frame rate, however in such a case milliseconds or ticks may be more useful.

Sync007 is an example of how one might sync to MPEG-2 PCR using ticks. I'm not sure this is what tick rate was intended for, but it's an important use case which otherwise might be  missed.

With these tests, we can consider Issue 29 closed, however I'm wondering whether we ought to define that tickRate and tickRateMultiplier are only defined in clock mode="media", and that for clockmode="smpte" the smpteMode attribute only is defined, with the values :



Sean Hayes
Media Accessibility Strategist
Accessibility Business Unit

Received on Saturday, 11 April 2009 15:08:43 UTC

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