Presention timing and playing

Notation convention for handling presentation timestamps (PTS)

File streams

The underscore "_" tags "stream time". Stream timestamps are timestamps on each recorded frame: they can be far in the past or in the future.

The following equation holds:

(t_-t_0) = (t-t0)

where:

Symbol	Explanation
t_	the file stream time
t	the wallclock time
t0_	file stream reference time
t0	wallclock reference time

t0 is measured at the time instant when t0_ is set. This is done at seek.

We define a "reference time":

reftime = (t0 - t0_)

The we get a frame's wallclock time like this:

t = t_ + reftime

Where t_ is the frame's timestamp in the stream. You can think it as we're correcting a recorded frame's timestamp to present time.

[ check: t = t_ + reftime <=> t = t_ + t0 - t0_ <=> t - t0 <=> t_ - t0_ ]

Different actions for file streams

For file streams, a target_time_ is set. Frames are consumed until t_>=target_time_

Action	What it does
open	Open file, do seek(0_)
seek	set target_time_, once t_>=target_time_, stop
play	set target_time_, keep consuming frames with t_>=target_time_. Update target_time_.
stop	Stop
close	Deregister file

Realtime streams

Symbol	Explanation
t_	frame's timestamp
t	the wallclock time
tb	buffering time

Define "relative timestamp" :

trel = t_ - t + tb = t_ - (t-tb) = t_ - delta

*             [fifo]
* => in                       => out
* [young                        old]
* 
* absolute timestamps
* 90  80  70  60  50  40  30  20  10
*
* relative timestamps trel:
* .. with delta=(t-tb)=45
*                   |
* 45  35  25  15  05 -15 -25 -35 -35
* 
* negative values == older frames, positive values == younger frames
*

• negative frames are late: "presentation edge" is at 0
• increasing buffering time moves the "presentation edge" to the right == less late frames
• remember: large buftime will flood the fifo and you'll run out of frames in the stacks
• 50-100 milliseconds is a nice buftime value

extreme cases:

• all negative == all frames too old (i.e. due)
  
• all positive == all frames too young (i.e. in the future)