Media servers and genlock

Hello,

I was reading lately about the use of timecode in movie production. I was surprised to learn that the external timecode does not actually synchronize cameras; it only gives the starting time. Once the camera is rolling, frames are counted up from that starting point onwards; purely based on the recording speed of the camera, without any influence of the timecode. So, on longer takes, cameras may drift apart from each other. The same goes for external audio recording, where the starting timecode is put as metadata into the file; but once its recording, its free running.

To solve this issue, movie productions do not just use timecode for longer takes, they also genlock the cameras to be sure they run at exactly the same speed. Audio recorders are then feed a wordclock signal coming from the same source, so that everything stays in sync.

That got me thinking about media servers. I have the impression, that the same basic problem exists. So, a media server can start a video clip at exactly the right timecode, but once it is running, it comes down to the playback speed of the graphics card. For longer video clips, you would probably have to genlock the media servers and the timecode generator to insure sync. Is this ever done? So far, I have only heard about genlocking/framelocking media servers when multiple outputs are used to feed the same led wall, but never in the context of keeping in sync with the timecode or with audio.
 
Hello,

I was reading lately about the use of timecode in movie production. I was surprised to learn that the external timecode does not actually synchronize cameras; it only gives the starting time. Once the camera is rolling, frames are counted up from that starting point onwards; purely based on the recording speed of the camera, without any influence of the timecode. So, on longer takes, cameras may drift apart from each other. The same goes for external audio recording, where the starting timecode is put as metadata into the file; but once its recording, its free running.

To solve this issue, movie productions do not just use timecode for longer takes, they also genlock the cameras to be sure they run at exactly the same speed. Audio recorders are then feed a wordclock signal coming from the same source, so that everything stays in sync.

That got me thinking about media servers. I have the impression, that the same basic problem exists. So, a media server can start a video clip at exactly the right timecode, but once it is running, it comes down to the playback speed of the graphics card. For longer video clips, you would probably have to genlock the media servers and the timecode generator to insure sync. Is this ever done? So far, I have only heard about genlocking/framelocking media servers when multiple outputs are used to feed the same led wall, but never in the context of keeping in sync with the timecode or with audio.
@Pyrotech In the mid 1970's, Super Tramp came to our city with their "Crime Of The Century" tour. They were travelling by rail and the load in & out took FOREVER. Rock solid synch was achieved interestingly. On the front side of the projector was a 35 mm print. On the rear side of the projector, driven by the same shaft, was a sprocket driven 8 track magnetic recording. One track of the mag synchronized their lighting and pyro. Definitely rock solid synch.
Toodleoo!
Ron Hebbard
 
Your assessment is correct. All devices that only start synchronized will "free run" and eventually drift apart. Things like temperature, line voltage and frequency, as well as hundreds of other factors all play into how fast stuff drifts. I know PRG's Mbox media server has a timecode lock that accepts LTC and maintains framelock. I suspect most of the higher end ones do. Mbox also can accept genlock, but it just synchs the video output signal (vs the actual media playback)... Again, I suspect most higher end servers function similarly.
 
Here's a good post from Dataton as to why you want to use it with media servers.
A lot of modern servers use timing via Ethernet
 
@Pyrotech In the mid 1970's, Super Tramp came to our city with their "Crime Of The Century" tour. They were travelling by rail and the load in & out took FOREVER. Rock solid synch was achieved interestingly. On the front side of the projector was a 35 mm print. On the rear side of the projector, driven by the same shaft, was a sprocket driven 8 track magnetic recording. One track of the mag synchronized their lighting and pyro. Definitely rock solid synch.
Toodleoo!
Ron Hebbard
When I was working for Associates and Ferren we built time-code controllable film projectors. I just posted some photos here: https://www.controlgeek.net/visual-resume We had tried to have the projector drive audio sync but there was too much wow and flutter. So we did it the other way, put timecode on an audio track and the projector would chase time code. The projectors all had synchronous motors to stay in sync electromechanically.
 
I had the time to do some more research, and I found a very interesting document from 7th Sense that is all about sync:

Synchronising Delta Media Systems

Here are my key takeaways:

  • The problem is real whenever an external timecode source is used to synchronize media servers. The servers may drift away from their timecode over time if the timecode source is not in sync with the media servers
  • On timeline based media servers, it is the length of the entire timeline that counts. So, on long timelines, drift may occur even when all individual clips on that timeline are short
  • You do not necessarily need a genlocked timecode generator. When the timecode is output by an audio device that gets a wordclock signal from the same sync source that the media servers get their genlock from, no drift will occur.

The document also states that their media servers can synchronize themselves to a leader using leader follower timing with a accuracy of ±1 frame with no genlock, and frame accurate with. I suppose other vendors offer a similar performance. This leaves two possible ways of working with media servers on longer projects:

  • Have all video and audio media come from the media servers, and have the media servers output the timecode too. Cue based devices like DMX lighting control desks or firing systems will have no trouble keeping up with that timecode. Genlock is in this scenario only necessary if multiple media servers feed the same continuous led wall or if projectors are edge blended.
  • Whenever audio or timecode come from outside the media servers, an external sync source is necessary. This sync source will have to provide genlock to media servers and timecode generators and wordclock to audio devices.

Thank you for all the replies in this thread.
 
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I'm still not clear on your use case, but I think a look at digital signage products and software might be a source of inspiration.
 

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