1. Reference distribution
SMPTE proposal
(View the Notes pane to see comments by Rob Silfvast and Willem-Jan Dirks)

2. Introduction
For background on the approach supported within SMPTE:
Google search [“Paul Briscoe” and “the new genlock”]
First years there will be a mixed environment. Feedback of customers is to keep a long transition period in mind.
Traditional gen-lock sources will source the reference in this mixed environment.
Receiving signals that some “Big players” in the broadcast domain are aiming for the SMPTE 33TS reference solution.
We don’t want to inherit the current situation where references need to be tuned for different sources.
[RS] Do “traditional gen-lock sources” typically operate on their own independent clock, or are they locked to TAI time? (the 4th bullet above seems to suggest that they have independent clock, not locked to TAI time). Once a system is “mixed” I would expect it to use a clock that is TAI-derived, otherwise the proposed SMPTE method of calculating sync from network distributed time would not be able to be used (all nodes would have to slave to the traditional genlock source).
[WJD] most of the time the SPG’s (sink pulse generators) are locked to TAI. However this is not necessary. The SPG can run on its own internal clock.

3. Mixed environment
[RS] The 2msec phase shift can easily be eliminated. The Talker merely needs to send a “clock stream” that is phase aligned to the legacy genlock source which it receives. Once the talker has synchronized itself to Genlock, it knows the exact frequency and phase of that signal, and can transmit clock stream packets 2msec before each Vsync instant (with presentation time stamp matching the next Genlock edge time). The listeners of this clock stream can then reproduce Vsync without any delay (only minor error due to internal clock resolution) relative to the master Genlock.
[RS] If the application involves low-latency video switching between endpoints, then we may have to assume a buffering delay on the order of one (or maybe 2) video line(s) for packet-based video streams (APVF or ). This would impose a delay in the data stream path, meaning that a receiver must operate on a delayed house sync, or that a complete frame buffer must be accommodated in the path through the switch (which would then cause it to be a high-latency switch). Question: Do SDI switches (routers) provide small enough delay such that downstream receiver can operate on the main house sync, and the SDI video input is not arriving too late? (perhaps the delay can be absorbed by “back porch” margin in the video frame timing?)

4. Pro:
Doesn’t increase bandwidth usage. Calculations are made in each endpoint using the timestamps already distributed by the clock master. Only a single control packet to tell the endpoint which reference to take.
SMPTE defined standard which is respected in the Video Broadcast (VB) domain.
Ready for future reference standards.
Multiple reference standards distribution in a single network (not used today in VB).
[RS] Regarding the first bullet, what happens when multiple nodes change from one reference to another? They may all receive the “single control packet” at approximately the same time, but each might take a different amount of time to process it. Thus, this benefit only applies when there is no chance of changing between one reference and another “on the fly.”
[WJD] Answer: In current installations, changing from reference will (and expected) result in video artifacts. Changing reference in a operational environment is not allowed. When powering up a system, a reference is chosen and that will remain for the whole production.

5. Cons: Calculations are not straight forward.
No backup master as when using streams, this could result in a “reference jump” when switching from Master to a new Master.
A relation to TAI (via GPS) is needed for synchronizing “Traditional” infrastructures to an AVB network.
[RS] I would tend to agree that the calculations are complex, and require a high amount of numerical precision. I would like to know more details about this. In the February F2F (1722a) we heard from SMPTE that these calculations can be handled on silicon costing only “a few pennies” but that was not backed up by concrete information. I have strong doubts about that.
[WJD] Answer: Remark 1: Paul Briscoe indicted at NAB that he made calculations for implementing the SMPTE standard. I think we have to contact him for more detail.
[RS] Regarding the third bullet, if we have a mixed system, is the traditional genlock always slaving to network/TAI sync, or vice versa, or can it be either way? This sounds like an area that can cause big problems and/or confusion.
[WJD] Answer: All options are possible and yes tis could cause problems. However we will have a mixed environment for a long time. So a Clock-Master can be sourced by a SPG which is also sourcing traditional (non AVB) nodes (see cam1). An AVB node, which is slave on a AVB network, can be a Reference source for traditional sources (see cam2).

6. To Do before deciding:
Investigating the “cost” of a SMPTE implementation
Most complex implementation is for the 59,94 reference, since this has a 1,001 relation to a second. Each 1001 second there is an aligned frame.
Find solution for “ref jump” when master changes
Automatic master switching is under discussion for broadcast, since customers are afraid of this principle.
[RS] We must point out very clearly to SMPTE, that in the IEEE we have a strong requirement that the media clock be independent and orthogonal to 802.1AS time, one of the primary reasons for this is to support multiple asynchronous media clocks (that can drift relative to each other) on the same network. A SMPTE-based facility should strongly consider whether they might ever want to support this case, especially as network infrastructure becomes more and more shared, it’s more likely for two different “sessions” to be operating on the same LAN.
[WJD] Answer: In Broadcast all signals are synchronized or use no reference. The use case where 2 sessions are active on a single network with independent reference is very unlikely.