Topics

syncing witness cameras to Alexa

Jake Pollock
 

Currently prepping a feature that will involve a scene with face replacements. VFX is requesting their two witness cameras be able to sync up with my Alexa. 

My my question is if it’s possible to sync two non-Alexa cameras with my two Alexas. And if it’s possible, what camera models can you recommend?
Usually VFX use fairly low grade witness cameras, but they feel that it will be critical to have frame-exact sync.

My equipment vendor tells me it is only doable with all cameras of the same model. Was hoping to use the Alexa LF for added resolution over the Minis we’ll use for the principal package. However, I can’t imagine using those for VFX. It’s not only a budget issue, as well as a minimal footprint on set. The two scenes will be shot on location and I feel that studio-grade witness cameras will box us in and seriously limit our angles.

Any input or suggestions would be greatly appreciated! 

Cheers,
Jake Pollock 
Based in Taipei, currently in Hong Kong 

Video Assist Hungary
 

Any good camera should have a timecode input. If they use low grade cameras, you can still record the SMPTE timecode to their second audio track (with the first used for scratch track for added sync)


-----------------------------

Balazs Rozgonyi IMDb
CEO - technical director / VA - DIT -  3D

0-24 central phone: +36 70 626 2354

Video Assist Hungary - Technology for your vision IMDb IMDbPro
The only video rental place in Hungary!





On 2018. Mar 9., at 1:50, jakehpollock via Cml.News <jakehpollock=yahoo.com@...> wrote:

Currently prepping a feature that will involve a scene with face replacements. VFX is requesting their two witness cameras be able to sync up with my Alexa. 

My my question is if it’s possible to sync two non-Alexa cameras with my two Alexas. And if it’s possible, what camera models can you recommend?
Usually VFX use fairly low grade witness cameras, but they feel that it will be critical to have frame-exact sync.

My equipment vendor tells me it is only doable with all cameras of the same model. Was hoping to use the Alexa LF for added resolution over the Minis we’ll use for the principal package. However, I can’t imagine using those for VFX. It’s not only a budget issue, as well as a minimal footprint on set. The two scenes will be shot on location and I feel that studio-grade witness cameras will box us in and seriously limit our angles.

Any input or suggestions would be greatly appreciated! 

Cheers,
Jake Pollock 
Based in Taipei, currently in Hong Kong 

Jake Pollock
 

Balazs,

The issue isn’t time code. What I need is to slave-sync our Alexas with the VFX witness camera. 

VFX need exact frame references (ie in a frontal shot, the witness cameras will shoot profile angles) so they have frame by frame info for face positions/expression.

I’m told by my rental guy, I can only do this if all cameras are the same model, but hoping to find out if it’s possible for VFX to use something other than Alexa. 

The last VFX intense show, they used Blackmagics without slaving to my cameras (3-D rigged Dragons and 2D Alexa) but we weren’t dealing with face replacement on that one 

Cheers,

Jake Pollock 
Cinematographer
Currently in Hong Kong 


On 9 Mar 2018, at 17:35, Video Assist Hungary <info@...> wrote:

Any good camera should have a timecode input. If they use low grade cameras, you can still record the SMPTE timecode to their second audio track (with the first used for scratch track for added sync)


-----------------------------

Balazs Rozgonyi IMDb
CEO - technical director / VA - DIT -  3D

0-24 central phone: +36 70 626 2354

Video Assist Hungary - Technology for your vision IMDb IMDbPro
The only video rental place in Hungary!





On 2018. Mar 9., at 1:50, jakehpollock via Cml.News <jakehpollock=yahoo.com@...> wrote:

Currently prepping a feature that will involve a scene with face replacements. VFX is requesting their two witness cameras be able to sync up with my Alexa. 

My my question is if it’s possible to sync two non-Alexa cameras with my two Alexas. And if it’s possible, what camera models can you recommend?
Usually VFX use fairly low grade witness cameras, but they feel that it will be critical to have frame-exact sync.

My equipment vendor tells me it is only doable with all cameras of the same model. Was hoping to use the Alexa LF for added resolution over the Minis we’ll use for the principal package. However, I can’t imagine using those for VFX. It’s not only a budget issue, as well as a minimal footprint on set. The two scenes will be shot on location and I feel that studio-grade witness cameras will box us in and seriously limit our angles.

Any input or suggestions would be greatly appreciated! 

Cheers,
Jake Pollock 
Based in Taipei, currently in Hong Kong 

Nick Shaw
 

On 9 Mar 2018, at 10:01, jakehpollock via Cml.News <jakehpollock=yahoo.com@...> wrote:

I’m told by my rental guy, I can only do this if all cameras are the same model, but hoping to find out if it’s possible for VFX to use something other than Alexa. 

I've not done it with an ALEXA, but I have set up VFX witness cameras where the main camera was a RED EPIC, and the witness cameras were Sony prosumer camcorders. But in that case, both the EPIC and Sony camera could take a tri-level sync through the genlock input, so I just used an AJA Gen10 sync generator. I believe the ALEXA SXT uses a different sync method, and cannot take a standard tri-level sync, so you would need a master sync generator which can produce a sync signal in the appropriate format for all the cameras. I don't have a suggestion of a device which does this, I'm afraid. I believe the ALEXA MINI can take tri-level sync, but it is an option which has to be specially enabled on the camera by ARRI Service.

Another thing to watch out for is if you are shooting at true 24fps. Many cheaper cameras (most Sony prosumer ones, for example) can only do 23.976 fps, so cannot be shutter synced to a camera running at true 24. I have come up against this before, and the compromise we came up with was to shutter sync the witness cameras together at 23.976 fps, and accept that they were not synced to the A camera, which in fact was an ALEXA in this case.

Nick Shaw
Workflow Consultant
Antler Post
Suite 87
30 Red Lion Street
Richmond
Surrey TW9 1RB
UK

+44 (0)7778 217 555

Jake Pollock
 

Nick, 

Thanks for the input.  Would love to know if anyone has experience with this, but at least you give me hope that it’s possible!  Will talk with VFX to coordinate a test for them

cheers,
Jake Pollock


On 9 Mar 2018, at 7:18 PM, Nick Shaw <nick@...> wrote:

On 9 Mar 2018, at 10:01, jakehpollock via Cml.News <jakehpollock=yahoo.com@...> wrote:

I’m told by my rental guy, I can only do this if all cameras are the same model, but hoping to find out if it’s possible for VFX to use something other than Alexa. 

I've not done it with an ALEXA, but I have set up VFX witness cameras where the main camera was a RED EPIC, and the witness cameras were Sony prosumer camcorders. But in that case, both the EPIC and Sony camera could take a tri-level sync through the genlock input, so I just used an AJA Gen10 sync generator. I believe the ALEXA SXT uses a different sync method, and cannot take a standard tri-level sync, so you would need a master sync generator which can produce a sync signal in the appropriate format for all the cameras. I don't have a suggestion of a device which does this, I'm afraid. I believe the ALEXA MINI can take tri-level sync, but it is an option which has to be specially enabled on the camera by ARRI Service.

Another thing to watch out for is if you are shooting at true 24fps. Many cheaper cameras (most Sony prosumer ones, for example) can only do 23.976 fps, so cannot be shutter synced to a camera running at true 24. I have come up against this before, and the compromise we came up with was to shutter sync the witness cameras together at 23.976 fps, and accept that they were not synced to the A camera, which in fact was an ALEXA in this case.

Nick Shaw
Workflow Consultant
Antler Post
Suite 87
30 Red Lion Street
Richmond
Surrey TW9 1RB
UK

+44 (0)7778 217 555

Marty Brenneis
 

The way to approach this is to sync all cameras to a master sync generator. An Aja Gen 10 will supply a tri-level sync source and are findable in many areas.

The Ambient Lockit box will give both timecode and sync sources.

The VFX folks will have to use cameras that accept a sync source, I suspect they already know this and have a sync source since they have more than one witness camera.
A decent video rental house will have a sync generator and should have the knowledge of how to use it. It is easy to test in the shop to see if the camera reports "genlock" with the sync source.

Trying to use one camera as the master sync source is going to cause you pain and suffering as well as delays on set.
Your rental house is staffed by a technician who can deal with the usual daily hire type stuff. You need an engineer who Knows cine video technology and how to link various pieces of kit.

Getting farther than the tips in this thread will take a boffin who Knows what genlock, trilevel, timecode, syncgen and other words like that mean and how to attach the cables to make it work. A good boffin can set you up for success on your own way better than muddling around from various advice here. There should be some video engineers in HK or Taipei who can deal with this. They should also be familiar with the concept of RTFM.

I'm also sure there are several of us here on CML available for hire, but that might be a bit extreme.

If you want more detailed assistance you may post here or email me with exact model numbers of the cameras and any kit they have for syncing them as well as which Alexas you want to use.

Good Luck
Marty Brenneis
24frame.tv

Mark H. Weingartner
 

There is what they think they need and there is what will work (I speak from significant experience here)

There are several challenges
1. Can they run cables all over
2 Do the cameras they are using for witness cameras accept external genlock?
Timecode is not really an issue in a world where we use slates.  The issue is ideally syncing the camera frames so they are in phase - so that each frame of each camera is coincident with the same frame on another camera.

If you are allowed cables you can either get a master clock and feed the alexa and all the other cameras from the same clock or you might be able to use a DA and take from the alexa and feed the witness camera.  In the analog world you just took a monitor out from one camera into the sync port of the next camera etc (before tri-level sync)

If you are not allowed cables dont bother with wireless as the latency will throw you off anyway.
BUT 

it is often the case that the cameras chosen as witness cameras do not take an external genlock…

In which case the best you can do is provide a “sync event” so vfx editorial can sync the output from the witness cameras in post with the main camera.

We have resorted to this enough that for Green Lantern I built a system we cheerfully called the MegaBloop.

It uses three LED truck tail light modules  arrayed at the end of a painter’s pole held into shot for a sec or thrown around on set separately so that each of the witness cameras can see one.
A simple pushbutton momentary switch and a twelve volt battery complete the setup.  When slating the scene, you hit the button (don’;t press, hit… you want the shortest duration)
that puts a bloop on each witness camera that editorial can sync up.
We hae been doing this for years for witness cameras and it turns out well enough…   you can never be more than 1/2 frame out of sync with the taking camera or the other witness cameras - that is you can never be more than 1/100th of a second out     and animators seem to be able to soak this up… at least on the movies I work on.

Best,

Mark
Mark Weingartner, ASC
DP, VFX Supervisor, and Stereographer
Los Angeles based 





On 9Mar, 2018, at 04:07 08, Jake Pollock via Cml.News <jakehpollock=yahoo.com@...> wrote:

Nick, 

Thanks for the input.  Would love to know if anyone has experience with this, but at least you give me hope that it’s possible!  Will talk with VFX to coordinate a test for them

cheers,
Jake Pollock


On 9 Mar 2018, at 7:18 PM, Nick Shaw <nick@...> wrote:

On 9 Mar 2018, at 10:01, jakehpollock via Cml.News <jakehpollock=yahoo.com@...> wrote:

I’m told by my rental guy, I can only do this if all cameras are the same model, but hoping to find out if it’s possible for VFX to use something other than Alexa. 

I've not done it with an ALEXA, but I have set up VFX witness cameras where the main camera was a RED EPIC, and the witness cameras were Sony prosumer camcorders. But in that case, both the EPIC and Sony camera could take a tri-level sync through the genlock input, so I just used an AJA Gen10 sync generator. I believe the ALEXA SXT uses a different sync method, and cannot take a standard tri-level sync, so you would need a master sync generator which can produce a sync signal in the appropriate format for all the cameras. I don't have a suggestion of a device which does this, I'm afraid. I believe the ALEXA MINI can take tri-level sync, but it is an option which has to be specially enabled on the camera by ARRI Service.

Another thing to watch out for is if you are shooting at true 24fps. Many cheaper cameras (most Sony prosumer ones, for example) can only do 23.976 fps, so cannot be shutter synced to a camera running at true 24. I have come up against this before, and the compromise we came up with was to shutter sync the witness cameras together at 23.976 fps, and accept that they were not synced to the A camera, which in fact was an ALEXA in this case.

Nick Shaw
Workflow Consultant
Antler Post
Suite 87
30 Red Lion Street
Richmond
Surrey TW9 1RB
UK

+44 (0)7778 217 555

Art Adams
 

>My equipment vendor tells me it is only doable with all cameras of the same model.

Depends how sync'd they need to be. The EXT port on an Alexa can be fed into a box that Arri makes (I'm still learning the model numbers so I can't say which one, but it exists and is known) to sync other Alexas down to the scan line. I don't think you'd get that with another camera due to different read out speeds, but I don't know that for sure.

The R/S ports put out a basic sync pulse, but a simple camera may not care about that, or genlock. There's a wiring diagram somewhere online (I'm told) that will tell you what all the pin outs on the EXT port are, and that might give you something you can work with.

As Mark says, the bloop light could be a decent solution for syncing dailies, but not necessarily syncing frames. What are the witness cameras?

I bet a Skypanel would make for a good sync light, depending on the subject matter. Fast on and off, no need to hunt down special lights. Stick a convex mirror on a stick and hold it in the shot, pop the light with a DMX controller.

--
Art Adams
Director of Photography
San Francisco Bay Area

Justin Johnson
 

REDs will shutter sync to scanline accuracy as well.

Here is a link to their support doc describing the setup and procedure.


http://docs.red.com.s3.amazonaws.com/955-0160/WEAPON%20MONSTRO%208K%20VV%20Operation%20Guide/en-us/Content/8_TC_Gen_Multi/Camera_Array.htm



On Fri, Mar 9, 2018 at 10:43 AM Art Adams <art.cml.only@...> wrote:
>My equipment vendor tells me it is only doable with all cameras of the same model.

Depends how sync'd they need to be. The EXT port on an Alexa can be fed into a box that Arri makes (I'm still learning the model numbers so I can't say which one, but it exists and is known) to sync other Alexas down to the scan line. I don't think you'd get that with another camera due to different read out speeds, but I don't know that for sure.

The R/S ports put out a basic sync pulse, but a simple camera may not care about that, or genlock. There's a wiring diagram somewhere online (I'm told) that will tell you what all the pin outs on the EXT port are, and that might give you something you can work with.

As Mark says, the bloop light could be a decent solution for syncing dailies, but not necessarily syncing frames. What are the witness cameras?

I bet a Skypanel would make for a good sync light, depending on the subject matter. Fast on and off, no need to hunt down special lights. Stick a convex mirror on a stick and hold it in the shot, pop the light with a DMX controller.

--
Art Adams
Director of Photography
San Francisco Bay Area

Gavin Greenwalt
 

REDs will shutter sync to scanline accuracy as well.
-Justin Johnson

Scanline accuracy would still only be between cameras of the same make/model which would all inherently have a different scan rate due to the different read-reset rate of say an Alexa vs a RED. I feel confident that whatever the Animation Supervisor is wanting, doesn’t need to be nearly that precise.  Standard Genlock should be sufficient.  If it’s more than just a witness camera and they’re trying to do on-set motion capture, then genlock would be the tip of the ice berg in considerations.   Any solution that requires perfect scanline sync would have a mountain of other error sources that would overwhelm the triangulation software long before a 1ms mismatch caused issues.  You might achieve perfect temporal cohesion between the witness cameras and the hero camera but 1-2ms of temporal delay is nothing compared to the geometric distortions that you’ll never perfectly analyze between lenses at differing focus depths.     Even if you had 3 of the exact same camera if the hand is in 3 different places-in-frame (from 3 vantage points) scanline locking is completely worthless since an individual point in space will be in 3 different scanlines and therefore measured at 3 different times for each camera, even though they are perfectly scanlines locked.   Scanline locking is only useful for stereo rigs where you’re filming from nearly the exact same perspective.  

Ultimately the witness cameras are just that, witness cameras.  They’re helpful but you have to fudge whatever they say to play nice with the hero camera.  And even then you’ll have fudge it again in comp to be pixel perfect.

That being said, if someone really really wanted to mix different camera models and maintain that degree of sync for some reason, the only way you could do that would be an external electro-optical shutter sync’d in addition to sensor-Genlocking all of the cameras. But then you would also end up with a global shutter which may or may not be desirable.  There is a filter which you can rent to do this, but, I can’t possibly imagine this is worth the cost/on-set inconvenience to get that precise.  http://tessive.com/time-filter-product-info/  

Gavin Greenwalt
VFX Supervisor
Seattle, WA

Mike Sippel
 

It’s my understanding that while conventional Alexas can’t accept genlock they DO sync quite precisely to timecode frame cadence.  If you need to synchronize a non-Alexa camera with an Alexa a workaround (provided the non-Alexa camera takes external genlock) is to use a timing device like an Ambient Lockit that generates both Timecode and Genlock in synchronicity.  The Lockit becomes the master clock – the Alexa(s) get locked to the Lockit via Timecode, the other cameras get genlock.  Because both signals are synced the cameras sync transitively.

 

Mike Sippel

Senior Field Support Technician

Arri Rental

Atlanta

 

Mike Sippel

Senior Field Support Technician

 

ARRI Rental
3980 Dekalb Technology Pkwy #800
Atlanta, GA 30340
Phone: (678) 248-5432
Website | Facebook | Twitter

ARRIRENTAL


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Bob Kertesz
 

This has been a very entertaining topic. My two cents, in no particular
order:

-Weigartner's solution of the bloop light. Occam's Razor manifested on
set. You can't go wrong, and I strongly suggest you consider some form
of it. A cell phone camera flash that can be seen by all cameras at once
would work.

-If you insist on using hardware, Marty Brenneis' good advice to hire a
proper engineer to get it done as good as it can be. Won't happen, of
course, but good advice anyway.

-Read Gavin Greenwalt's post again, the one based on experience, saying
why the post VFX guys' request is bullshit because once you think it
through it becomes obvious the proposed setup will not get accurate
results no matter what. I'm paraphrasing here - I'm sure Gavin is too
much of a gentleman to use such language, whereas I've repeatedly
demonstrated that I am no such thing.

-Bob

Bob Kertesz
BlueScreen LLC
Hollywood, California

DIT, Video Controller, and live compositor extraordinaire.

High quality images for more than four decades - whether you've wanted
them or not.©

* * * * * * * * * *

Tim Sassoon
 

BTW, if Alexa’s aren’t actually required, just sync, and they’d rather have more cameras, Sony advertises that their new POV cameras sync up. And have “Zeiss” lenses and shoot uncompressed 4K to external HDMI Slog2 :-)


The sync box for up to 50 units:



Tim Sassoon
Venice, CA




On Mar 9, 2018, at 11:47 AM, Mike Sippel <msippel@...> wrote:


It’s my understanding that while conventional Alexas can’t accept genlock they DO sync quite precisely to timecode frame cadence.  If you need to synchronize a non-Alexa camera with an Alexa a workaround (provided the non-Alexa camera takes external genlock) is to use a timing device like an Ambient Lockit that generates both Timecode and Genlock in synchronicity.  The Lockit becomes the master clock – the Alexa(s) get locked to the Lockit via Timecode, the other cameras get genlock.  Because both signals are synced the cameras sync transitively.

 

Mike Sippel

Senior Field Support Technician

Arri Rental

Atlanta


 

Mike Sippel

Senior Field Support Technician

 

ARRI Rental
3980 Dekalb Technology Pkwy #800
Atlanta, GA 30340
Phone: (678) 248-5432
Website | Facebook | Twitter

ARRIRENTAL



This message is confidential. It may also be privileged or otherwise protected by work product immunity or other legal rules. If you have received it by mistake, please let us know by e-mail reply and delete it from your system; you may not copy this message or disclose its contents to anyone. Please send us by fax any message containing deadlines as incoming e-mails are not screened for response deadlines. The integrity and security of this message cannot be guaranteed on the Internet.




This email has been scanned for email related threats and delivered safely by Mimecast.
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Jake Pollock
 

Thanks to everyone for their valuable input, gotta say how thrilled I to have access to such a generous community of colleagues!

I’m taking Mark’s advice for the bloop light sync.  VFX and I are in communication about shooting 48fps, thus reducing any phase issue to less than 1/200th of a second.  Because this is their first time doing face replacement (flashback of 50 year old actors in their 20’s), they are doing internal tests at 48fps as proof of concept.  

If they decide we need to genlock sync, we will test the Alexa with Blackmagic Minis and the AJA Gen 10.  Will keep everyone posted if there are any new developments.

cheers to all!
Jake Pollock
Cinematographer
Hong Kong 

On 10 Mar 2018, at 8:43 AM, Bob Kertesz <bob@...> wrote:

This has been a very entertaining topic. My two cents, in no particular
order:

-Weigartner's solution of the bloop light. Occam's Razor manifested on
set. You can't go wrong, and I strongly suggest you consider some form
of it. A cell phone camera flash that can be seen by all cameras at once
would work.

-If you insist on using hardware, Marty Brenneis' good advice to hire a
proper engineer to get it done as good as it can be. Won't happen, of
course, but good advice anyway.

-Read Gavin Greenwalt's post again, the one based on experience, saying
why the post VFX guys' request is bullshit because once you think it
through it becomes obvious the proposed setup will not get accurate
results no matter what. I'm paraphrasing here - I'm sure Gavin is too
much of a gentleman to use such language, whereas I've repeatedly
demonstrated that I am no such thing.

-Bob

Bob Kertesz
BlueScreen LLC
Hollywood, California

DIT, Video Controller, and live compositor extraordinaire.

High quality images for more than four decades - whether you've wanted
them or not.©

* * * * * * * * * *

Scott Janush
 

VFX Editor here.

A visible bloop light, as in a MoCo pass sync reference, will save your ass everytime, for WHEN the sync goes to hell.

If you shoot at the same frame rate as your main cameras and have a solid visual reference, life will be good.

-S

Scott Janush
Visual Effects Editor: Teen Spirit

Cell: LA / 323.646.3416
Skype: sjanush

On Mar 9, 2018 1:25 AM, "jakehpollock via Cml.News" <jakehpollock=yahoo.com@...> wrote:
Currently prepping a feature that will involve a scene with face replacements. VFX is requesting their two witness cameras be able to sync up with my Alexa. 

My my question is if it’s possible to sync two non-Alexa cameras with my two Alexas. And if it’s possible, what camera models can you recommend?
Usually VFX use fairly low grade witness cameras, but they feel that it will be critical to have frame-exact sync.

My equipment vendor tells me it is only doable with all cameras of the same model. Was hoping to use the Alexa LF for added resolution over the Minis we’ll use for the principal package. However, I can’t imagine using those for VFX. It’s not only a budget issue, as well as a minimal footprint on set. The two scenes will be shot on location and I feel that studio-grade witness cameras will box us in and seriously limit our angles.

Any input or suggestions would be greatly appreciated! 

Cheers,
Jake Pollock 
Based in Taipei, currently in Hong Kong 

Mark H. Weingartner
 

Hey Jake - Contact me off-list  if you want me to send out one of my kits…   or the instruction sheet (which will help your team build their own)
Weingartner
LA based VFX DP etc

On 9Mar, 2018, at 21:24 58, Jake Pollock via Cml.News <jakehpollock=yahoo.com@...> wrote:

I’m taking Mark’s advice for the bloop light sync.

Jake Pollock
 

Mark,

Thanks for the generous offer, but I have a pretty good idea of what you were talking about.  My gaffer is great at rigging up crazy stuff, so I’m sure he’ll be able to do something that will satisfy our needs!

Jake Pollock
Cinematographer
Hong Kong

On 10 Mar 2018, at 2:58 PM, Mark H. Weingartner <vfxmark@...> wrote:

Hey Jake - Contact me off-list  if you want me to send out one of my kits…   or the instruction sheet (which will help your team build their own)
Weingartner
LA based VFX DP etc
vfxmark@...
On 9Mar, 2018, at 21:24 58, Jake Pollock via Cml.News <jakehpollock=yahoo.com@...> wrote:

I’m taking Mark’s advice for the bloop light sync.

Marty Brenneis
 

Now you have given me enough clues to assist. If you use the BMD cameras for the witness cameras you can sync the whole mess up.

The discussion drifted a bit up there when people were talking about line sync, that is not important for this application.
What is important is frame sync. It really is better if the 1/48th of a second slice of time is the same across all the cameras, your VFX experience will be quicker, and better looking with less work on the post end.
This is a case where spending a day of shop time to setup the camera system right can save perhaps thousands of hours trying to sync up the frames and deal with half frame sync in the VFX process. It should also result in a better looking product at the end of the day. (And those should be shorter days)

The BMD mini cameras have a genlock input on the HD15 power connector. That will take a bi level or tri level sync signal and frame lock the camera to it. your life will be way better if you find a good local tech to make a set of power cables with just the genlock cable added rather than flailing with the breakout cable with all the connections on it. (Contact me off list if you wish to hire me to make some)

The Alexa will framelock to a timecode signal as well as an SDI genlock. (Perhaps someone at Arri will chime in if this takes Trilevel too since SDI is not really a sync source)

Your best bet is to get something like an Ambient Lockit box or some other video sync generator that also produces timecode.

Somewhere on set near the camera rig you will want a breakout box with DAs in it to send trilevel to the BMD cameras, TC to the Alexa and TC to the recorders that the BMD cameras are connected to. Timecode is your friend here, it will make syncing up the frames way easier.

To test the sync put something in a drill spinning at some low speed and shoot it with all the cameras. I would position them straight ahead looking at it rather than the final off axis setup. You might want to vary the speed a bit to make sure you get a good look. You want the motion of whatever is spinning to be less that one rev a frame. A disk like an old DVD with tape on it stuck on some sort of shaft is a good test subject. I made one like this with an LED on it.

It will then be easy to see that all the cameras are getting the same slice of time.

Pass the media to the VFX team to make sure their software is processing the timecode and not tossing it in the bit bin.

Spending the time in pre production will pay off in post. I will warn you that the producer may not appreciate this since they won't see the cost overrun in the VFX and post budgets.

Time sync has been one of my passions since my early days at ILM.
Let us know how it works out.

Marty Brenneis
droid for hire
24frame.tv

Eric Wenocur
 

Now there's a clear, cogent description of the problem and potential solutions! And let's remember
that "bi-level" sync is what we used to call blackburst (meaning analog H-sync, V-sync and
colorburst). Some would argue that you need tri-level to get sufficient resolution for syncing HD
and above; others on this list may know better than I how serious that is. And if you're using
double-system sound, don't forget to send some sync to the audio recorder!

In this age of many frame rates, resolutions, and file types, I wonder if the basic understanding of
what genlock is about is starting to disappear. Just like the hit of colorbars that used to be at
the head of the recording. Sorry, that's old guy talk--and I'm not even that old!


Eric Wenocur
Lab Tech Systems
301-438-8270
301-802-5885 cell


On 3/10/18 10:45 AM, Marty Brenneis wrote:
> Now you have given me enough clues to assist. If you use the BMD cameras for the witness cameras you
> can sync the whole mess up.
>
> The discussion drifted a bit up there when people were talking about line sync, that is not
> important for this application.
> What is important is frame sync. It really is better if the 1/48th of a second slice of time is the
> same across all the cameras, your VFX experience will be quicker, and better looking with less work
> on the post end.
> This is a case where spending a day of shop time to setup the camera system right can save perhaps
> thousands of hours trying to sync up the frames and deal with half frame sync in the VFX process. It
> should also result in a better looking product at the end of the day. (And those should be
shorter days)
>

Marty Brenneis
 

Warning: Geek talk ahead that may be TL;DR for many

Solving problems like this in the VFX world is something I have been at for many decades.
There are 2 major parts of my process that I'll share when it comes to working the problem.
Learn the ultimate goal and learn the processes involved at the ends of that goal.

This is a great example for this.
The goal is to have multiple angles of the face that are time synced so each of the 3 frames frame represents the same period of time.
Since the sync is not being used to switch video with, line sync perfection is not needed in the way it would be for sending to a broadcast switcher or linking 3D cameras.
A few lines of slippage of time sync between the frames is close enough. When it gets to 25 or 50 percent different as it would if it was not synced then the CG team will have to spend time fudging the images and the end result may not be as good.
I'm guessing that the CG folks might not care if the resolution of the witness cameras is different from the main camera. That opens up the field for what cameras are used.

If the cameras are run wild they will have random frame alignment. Not good for the folks at the end of the pipe. That means all cameras need to accept a sync input.
The witness cameras might be mounted on some sort of rig or are placed to camera for each setup so smaller is better.
If the set already had video wires running from the main camera (more reliable) then a few more cables for the witness cameras will usually not be an issue.
This is where the VFX team needs the assistance of a tech with on-set video experience. Hiring a second video assist operator or adding a tech to the video assist team will make life much smoother. From an on-set point of view, recording the signals from 2 witness cameras is the same as capturing the feeds from 2 main cameras. It is just video frames going onto media and getting logged.

The special place here is the sync source.
Question 1: What external sync signal will the main camera take. If the answer is none, bin it and get a real camera. Extra bonus points if the camera can jam to timecode and not drift.
Question 2: What witness cameras are you using. Again find ones with a genlock input, it might be a timecode input. Bonus points if all cameras can follow timecode.
Question 3: Does the sync have to be wireless? If there are already wires running around on the set then one more is not going to make trouble. This is the Most Reliable Way.
Sync should come from a master sync generator rather than one of the cameras. You should always be able to have signal from any camera at any time and not care of a camera is having the cable moved or battery changed or worse yet the battery crapping out.
For wireless sync there are sync and timecode devices that an be jammed and have low drift. I usually jam mine at the start of the day and after lunch. If it is convenient I also jam them at the beginning of any major setup. Put one at each camera and at the recorders and you have fewer wires but more chance for slippage.

Once the whole thing is worked out then test it. The wonder of this new digital age over the age of film is instant gratification. Shoot something that has lots of motion in the frame, send that to a quad monitor and see if all cameras are seeing the same slice of time. Be sure to try it with the sync turned off and cameras randomly reset to see what the error artifact looks like.
Then shoot a subject and send it down the pipe and see how the CG team feels about the material. I also send a sample of bad sync so all can see issues if errors happen along the way.

On set time is very expensive, an hour delay is worth way more than a couple of days in the shop. If you can get the act down to shave 5 minutes off each setup you will have more time to hang out at craft service.

I share this because this process has worked well for me on many weird shooting situations. I was at ILM and there was no such thing as a 'normal' shot. The real power and wonder of the early years at ILM was the fact that all departments worked together. I had access to the entire pipeline and could connect the dots between departments. I know it was annoying to some that when they asked for a piece of gear I would ask, "What is your goal?"
Sometimes I would guide them to a better piece of gear to solve their issue.

Before you get too deep in the tunnel vision of gear step back and use the camera in your head to get that wide master shot of the process and goal.

Wow, you read this far. Cool!
Marty Brenneis
droid for hire
24frame.tv

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