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Re: Revisiting Bug 22148 - adding jitter to video quality metrics

From: Mark Watson <watsonm@netflix.com>
Date: Fri, 30 Aug 2013 17:25:57 -0700
Message-ID: <CAEnTvdDn3AoPU6peXaN_He8QnLzFjOJaj0X+yXVGwqTpOWJ_Gw@mail.gmail.com>
To: David Singer <singer@apple.com>
Cc: "public-html-media@w3.org" <public-html-media@w3.org>
Hi David,

See inline...


On Fri, Aug 30, 2013 at 4:02 PM, David Singer <singer@apple.com> wrote:

> <https://www.w3.org/Bugs/Public/show_bug.cgi?id=22148>
>
> We are concerned about the new definition of the displayed frame delay,
> and the use of this value to accumulate a jitter value in totalFrameDelay.
>
>
> > Displayed Frame Delay
> > The delay, to the nearest microsecond, between a frame's presentation
> time and the actual time it was displayed. This delay is always greater
> than or equal to zero since frames must never be displayed before their
> presentation time. Non-zero delays are a sign of playback jitter and
> possible loss of A/V sync.
> >
> and
> > totalFrameDelay
> > The sum of all displayed frame delays for all displayed frames. (i.e.,
> Frames included in the totalVideoFrames count, but not in the
> droppedVideoFrames count.
> >
>
> [by the way, editors, you have a missing ")" there]
>
> Here are our concerns:
>
> 1.  The use of microseconds may be misleading.  There is an implied
> precision here which is rarely (if ever) achievable; by no means everyone
> can time 'to the nearest microsecond' and sometimes the measurement has to
> be done 'before the photons emerge from the display', at a point in the
> pipeline where the rest of it is not completely jitter-free.


> 2.  In any case, frames are actually displayed at the refresh times of the
> display;  display times are actually quantized to the nearest refresh time.
>  So, if I was slightly late in pushing a frame down the display pipeline,
> but it hit the same refresh as if I had been on time, there is no
> perceptible effect at all.


> 3.  Thus, ideally, we'd ask for the measurement system to be aware of
> which display refresh the frame hit, and all results would be quantized to
> the refresh rate. However, in some (many?) circumstances, though the
> average or expected pipeline delay is known or can be estimated, the
> provision of frames for display is not tightly linked to the display
> refresh, i.e. at the place of measurement, we don't know when the refreshes
> happen.
>

These are fair points. It's probably not correct to require "to the
nearest microsecond" - the whole thing will always be approximate.

The downstream delay due to frame refresh will be on average half the
refresh interval. So, on average, this could be accounted for.


>
> 4.  There is a big difference in jitter between presenting 2000 frames all
> 5ms late (consistently), and in presenting 50 of them 200ms late and the
> rest on time, though for both we'd report 10,000ms totalFrameDelay. The 5ms
> late may not matter at all (see above), whereas 200ms is noticeable
> (lipsync will probably be perceptibly off).  There is nothing in the
> accumulation of values, today, that takes into account *variation*, which
> is really the heart of what jitter is about.
>

The user of this property should have some notion of the number of frames
displayed, or at least elapsed time which will suffice so long as the frame
rate is roughly constant. The intention is that they would sample this on a
regular basis and evaluate the rate of change. A rate of change below some
threshold is in the noise or indicates perfect rendering. If the rate of
change is above some threshold then this indicates consistent late
rendering.

The application is to detect CPU overload, which in some system manifests
as dropped frames but in other systems manifests first as late rendering
before frames are dropped in order to "catch up" (if things don't get back
on track). An app can track the severity of such events over time and
decide to stream at a lower rate on this device.

...Mark



>
> I don't have a proposal right now for something better, but felt it was
> worth surfacing these concerns.  Do others have similar, or other,
> concerns, about these measurements?  Or indeed, suggestions for something
> that might alleviate these or other concerns (and hence, be better)?
>
> I guess a big question is:  what are the expected *uses* of these two
> values?
>
>
>
>
>
> David Singer
> Multimedia and Software Standards, Apple Inc.
>
>
>
Received on Saturday, 31 August 2013 00:26:25 UTC

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