Subject: another note about time granularity

From: Eric Fleischman <[email-masked]>
Date: Thu, 6 Mar 1997 10:02:44 -0800

As you know, indexes permit the rapid seeking to a given point within a 
video stream. These points must be accurately "seeked to" and rarely occur 
within second boundaries. This is doubtlessly why RTSP has proposed a 
timestamp with a sub-second "frame" reference.

In our product, many authors have repeatedly bludgeoned us to provide a 
similar capability for non-video data. This has compelled us to invent and 
support the concept we call a "marker". A marker functions as a video index 
and is available for every media type. Many authors seem to use markers to 
put "paragraphs" or "chapters" within their creation. Markers permit the 
user to "jump" to that paragraph or chapter. Markers require sub-second 
granularities for the same reasons frames do. Unfortunately, unlike frames, 
markers are not subject to isochronous-based predictabilities thus they 
have a need for much finer time granularities than do indexes.

While synchronization is independent from markers, markers are often placed 
where content is explicitly synchronized. As you can well imagine, 
synchronization demands a fairly fine grained clock if those references are 
to become "public" via markers.

These are reasons why a need exists for the RTSP clock to have a very fine 
time granularity in the microsecond or millisecond range.

---

From: Karl Auerbach <[email-masked]>
Date: Thu, 6 Mar 1997 13:16:44 -0800 (PST)

> time granularity in the microsecond or millisecond range.

NTP timestamps do have that kind of granularity, and more.

However, to add a dose of reality --

Sound travels at about one foot per millisecond.  Thus a person sitting 2'
from a rendering computer will hear the sound 2 milliseconds after seeing
the video.  A person 10' away will hear the sound 10 milliseconds after
the video.  In an auditorium setting, the people in the back row may hear
the sound 75 to a 100 milliseconds later. 

Humans seem well adapted to sound-after-sight delays of dozens of
milliseconds.

And a typical TV or computer monitor can't render changes (and hence
synchronization) of portions of the image faster than 1/30th (33
milliseconds - TV) to 1/75 (13 milliseconds - pc monitor) of a second.

And even high performance film systems (IMAX) are on the same order of
magnitide.

These considerations tend to mitigate the need for sub-microsecond
inter-track synchronization (especially on computers with clocks that
drift by a couple of percent, with operating systems with 5 to 10
millisecond clock granularity, and with sound rendering hardware with
rather "interesting" delay characteristics.) 

		--karl--