Questions about Sync, SDI, and Genlock

This topic has 1 reply, 4 voices, and was last updated 2 years, 9 months ago by AndersonSoundRecording.

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- June 10, 2013 at 9:04 AM #66017
  AndersonSoundRecording
  Participant
  I'm trying to wrap my head around some sync issues.
  
  In audio, if you have two digital recorders running for any length of time, if their clocks are not locked to a single source there will be a certain amount of drift that occurs between them.
  
  So I wonder: if you have multiple digital video cameras running, but they are not locked up somehow, will there be a similar "drift" over time?
  
  Which leads me to another round of questions:
  Say you have a camera with an SDI out, but no Genlock in (e.g. the Sony EX-1). Let's say you wanted to record two of these cameras to a central source (like a Pix 240).
  
  Can you record the two independent SDI streams to this single box without worrying about the fact that the cameras are not referencing a central sync source? Or do they both have to be receiving Genlock for this to work?
  
  Different, but related scenario:
  Let's say you are using one camera with SDI out, but no Genlock in, and another camera that has both SDI out and Genlock in (say a Sony EX-1 and a Canon XF-105).
  
  Is it possible to somehow derive Genlock from the SDI signal coming from the EX-1, so that the XF-105 could be in sync?
  
  By extension, if this is possible, is there a box that could get Genlock from this SDI signal and turn it into word clock for a double-system audio rig?
  
  I'm new to the whole video thing, so forgive my ignorance if this is not remotely possible. In the audio world, there actually is a way to do this sort of thing, so I am hoping that there is a similar workaround in video.
- June 10, 2013 at 9:42 AM #207721
  Mike Wilhelm
  Keymaster
  Hi Anderson-
  If you're not shooting video for broadcast, you generally souldn't have to worry about this. All you'll need to do is make sure your video is the same frame rate. Secondly, make sure the timecode for both is set to either "drop frame" or "non drop frame," and make sure all video sources are the same. If they're different, you'll get that drift. Here's a video talking about the difference between the two:
  
  http://www.youtube.com/watch?v=ykjyNeuQROU
- June 10, 2013 at 10:27 AM #207722
  AndersonSoundRecording
  Participant
  Thanks Mike.
  
  So drift between cameras is not an issue so long as the frame rates are set correctly.
  
  What of clock drift from the double-system audio? Since that's referencing samples per second and not frames per second, I imagine there would be some loss of sync over the course of an hour.
  
  Is there a way to derive sync from an SDI signal? Methinks if I could get genlock from this and turn it around to word clock, I'd be able to have my audio be sample-accurate with the video.
- June 10, 2013 at 11:34 AM #207724
  Mike Wilhelm
  Keymaster
  I've never had a problem with drift as long as the audio is captured along with the video in camera. If you are using external audio recorders, that's a different story.
- June 10, 2013 at 6:18 PM #207741
  gldnears
  Member
  Browse Markertek.com. They have a whole lotta little boxes . . .
- June 11, 2013 at 7:19 AM #207752
  AndersonSoundRecording
  Participant
  I guess the Ambient Lockit is sort-of the device I am looking for:
  http://www.ambient.de/en/products/ambient-recording/clockit-timecode/lockit-sync-box.html
  
  Looks like it creates some sort of wireless network and can output genlock or word.
- June 11, 2013 at 7:36 AM #207753
  gldnears
  Member
  For all the mention of " double system " audio ( running a separate audio recorder ) in these forums, I'm surprised that there is so little discussion of lip-sync drift. In general, I suspect that the higher the quality of the devices used ( and supposedly the more stabile internal clocks ), the less drift . . . . . but my history includes film cameras and Nagra tape recorders which HAD to be referenced to a common source, or at least to very accurate crystal oscillators in order to maintain sync. As a test some time ago I shot a small concert with my HVX-200A and a Korg MR-1000 digital audio recorder. In Post I was impressed that the drift was so small over an hour of running time. I recall that I had to make two 1-frame adjustments . I don't have any experience with the Zoom recorders and their ilk, but I'm skeptical that they have any provision for external word clock reference, so what ya get is what ya get for sync.
- June 10, 2013 at 11:48 AM #207726
  AndersonSoundRecording
  Participant
  That's exactly the story I'm talking about 🙂
  
  Camera is getting scratch audio, but multi-track capture is happening on an independent audio rig, to be sync'ed to picture later.
  
  If I can figure out a way to rig up a poor-man's Genlock to Work Clock signal using an SDI from the A camera, there should not be any drift.
  
  The real question is: is this possible? Are there any boxes that can take SDI and generate black burst from it?
  
  and then I need to find a small box that can take black burst in and generate word clock out.
  
  I have some big old rackmount boxes that would do the latter, but I want something a little more compact (if it exists).
- June 10, 2013 at 3:45 PM #207734
  Mike Wilhelm
  Keymaster
  Hm… I can't say that I've heard of such a device.
  
  Actually, you might want to check out the Atomos Samurai.
  http://www.bhphotovideo.com/c/product/767073-REG/ATOMOS_ATOMSAM001_Samurai_HD_SDI_Hard_Disk.html
- June 10, 2013 at 5:40 PM #207740
  rs170a
  Participant
  If you haven;t already done so, browse the forums on http://jwsoundgroup.net as this is a site dedicated to pro audio people who do what you're doing for a living. I've seen questions like yours come up a lot and there are numerous ways of dealing with it so check it out.
  
  Mike
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