Production Technology Seminar 2009 - EBU Technical

Is the label to be based on time or frame count?
Frames (or other media unit) Time
(+) Obvious way to index material, like pages in a book (+) Same for all material types. Very good e.g. for the multicamera
capture – the label will match regardless of the
different frame rates or type of application.
(-) But different for different material types: audio and video
media units different… and establishing the correspondence
between labels may work not so well
(-) But numbers don‟t increment simply – the frame rate may
not be exactly locked to your Time Code.
You may think it does not matter to choose time or frame count, because you can convert one to the
other world, but it is not so straightforward as one may think.
The phase of the essence signal which has been labelled makes a difference. If your decision boundary
for whether this time matches to one frame count or to the next is close to the actual frame boundary,
there can be difficulties.
And you must rely on a constant frame rate exactly related to the time.
To address these questions of frame versus time, it was decided to basically include both types of
labelling, depending on the application, in 2 proposals.
TRL Type 1
Includes: a timestamp to high precision fraction of a second (960 Hz resolution = ~1.0416 ms), a
sufficient size to count up to AD 2117, information about time zone, leap seconds etc. It also includes the
media unit rate (nominal rate).
Use: e.g. acquisition time, for variable rate, over/undercranking or all speed cameras.
TRL Type 2
It includes: a media unit number which is basically an incrementing count (e.g; frames), the media unit
rate (can only be the nominal rate), plus the time stamp of the 1 st unit you have labelled or phase datum
(allowing to calculate the current timestamp, assuming your media unit is locked to time and constant).
Use: e.g. postproduction EDL.
Binding
There is no use defining a label if we can‟t store it or carry it throughout the system without it being lost.
It is not too difficult to include one additional field, or to add a bit of extra information to files formats, to
packet data. It just become more difficult with synchronous streams (such as SDI video, or AES3 audio,
etc.) with a constrained data size and with plenty of devices which will strip off the information.
2.8 How can you possibly synchronise a TV plant using Ethernet?
Bob Edge, Manger, Standards and technology, Thomson Grass Valley
The computer industry has been working on IP network time synchronisation for decades. Most IP
network solutions (protocols) have accuracies measured in milliseconds, digital TV plants can operate
with 50 nanoseconds or more of jitter. Why is IP network jitter so difficult to control? How does
IEEE 1588 21 solve the problems?
In an ISO layered network [4] with a master clock on one device, timing protocol messages start at the
application layer, then are then sent through the layered software network stack, they are then
transferred on a physical network as a packet which is received by the layered software network stack
on the receiver side, and finally delivered to the application layer in the receiver. The network timing
protocol manages these packets at the application layer on each device. In an ideal world application
21 IEEE standard for a Precision Clock Synchronisation Protocol for Networked Measurement and Control. 2002 / July 2008
© EBU 2009 / Production Technology seminar / January 27 - 29, 2009
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