[M4IF Technotes] confusion about modulo_time_base, vop_time_increment, ...

[Arcin Bozkurt]

I will be answering your questions in a different order and hopefully will
be clear. I will write my interpretation and how I am doing it.
You are right, modulo_time_base in normal circumstances will never be two.
You can create a setup where it will be, but it would not be very realistic
I believe.
The only time modulo_time_base is non-zero, is when the real time completes
a one full second from the previous time a modulo_time_base was non-zero.
Assume the simple fixed rate case: 30 fps. And the
vop_time_increment_resolution is 300. 300 means each tick is 1/300 seconds
long and the vop_time_increment is given as number of ticks in this
resolution.
Your display times will be 0, 1/30, 2/30, 3/30, .... 29/30, 1, 31/30, etc.
In this case the values for modulo_time_base will be
0, 0, (repeat 29 times), 1, 0 repeat 30 times), 1, 0 (repeat 30 tiems)
and the Vop_time_increment will be
0 10, 20, ... 290, 0 (this corresponds to a "1" in modulo_time_base), 10,
20, ... 290, 0 , etc...
The confusing part is "local time base" related parts I believe. The
standard says:
modulo_time_base: This value represents the local time base in one second
resolution units (1000 milliseconds).
vop_time_increment: The local time base in the units of seconds is recovered
by dividing this value by the vop_time_increment_resolution.
Therefore, local time base in resolution of seconds, is given by
modulo_time_base (as an increment from the previous) and local time base in
resolution of "1/vop_time_increment_resolution" is given by
vop_time_increment. (as an increment from the last modulo_time_base)
If you look at this "division" as a floating point operation you will get
(increment/increment_resoultion) (in seconds) which is exactly what we want.
I hope this clarifies a bit.
Once again, this is my understanding.
-arcin
-----Original Message-----
From: technotes-admin   lists.m4if.org
[mailto:technotes-admin   lists.m4if.org]On Behalf Of Christoph Lampert
Sent: Wednesday, August 07, 2023 4:50 AM
To: technotes   lists.m4if.org
Subject: [M4IF Technotes] confusion about modulo_time_base,
vop_time_increment, ...
Hi,
I still have problem understanding the correct specifications
for writing timecodes to the VOP header.
First there is modulo_time_base, it consists of a number of
1s followed by a 0, one 1 for every second that passed since the last
GOV header or last module_time_base of the previously decoded I-, S- or
P-VOP.
Am I right, that this does not mean elapsed "full seconds", but is some
kind of "carry" flag, when the ticks-counter "overflows"?
So if we talked minutes and hours, it would be 1 when the clock jumps from
0:59 to 1:00, right?
In real life, the chances of this value ever getting 2 is very small,
correct?
And then, there is vop_time_increment:
It has a value in [0,vop_time_increment_resolution) and the last comment
on it is "The local time base in the units of seconds is recovered by
dividing this value by the vop_time_increment_resolution."
That's simply wrong, isn't it? You will always end up with 0 when dividing
this value by vop_time_increment_resolution... but what _is_ really meant?
Thank you for your help,
Christoph
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