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Arri Alexa TC drift by up to 7 frames


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Hey guys,

We're using a SD 788T SSD as our master clock, jamming Ambient lockit boxes to it and then jamming both Alexas. We have not been putting the lockits on the cameras as camera department were initially not keen to have them onboard.

We started with the standard morning and after lunch jam, but we're told by post that both cameras are drifting by up to 7 frames by the end of the day.

We jammed each camera 4 times yesterday - morning, mid-morning, lunch and mid afternoon - to the same result. Even though each camera is accepting the timecode jam and says "system good" after each jam, the drift is incrementally increasing.

Today, we put the lockits on the cameras and are awaiting the results. Has anybody else had similar issues?

It seems as though the cameras timecode is drifting even though it's jamming.

Both cameras are showing an identical drift.

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Reboot and double check that they're all at the correct speed. Is this 25.00fps or 23.976 or 24.00? Which software version?

 

Alexa manual here:

 

http://www.arri.com/camera/alexa/downloads/

 

I have generally had flawless luck with Alexa timecode -- never, ever a problem. But we've only shot 23.976 and I've only used Denecke timecode boxes. In varispeed, the timecode reverts to Hour 0 and you do sometimes have to give it a wack to rejam after a varispeed shot.

 

If the slate is not drifting, then this is a possible camera issue, unless sound timecode is drifting. I would have the DIT verify the timecode in the camera files and in the sound files using a 3rd-party program.

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Most definitely something adrift there mate........... Alexa normally rock solid for hours, also highly unlikely that 2 Alexa's are out.

 

Are you using a Timecode Slate, if so is it consistent with the SD788T or the Alexa's or neither ?

 

Are you re-selecting 'Int TC' as the 'Source' on the Alexa's after jamming ?

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This is why even with Alexa I leave a TC box on the camera.  I HAVE had Alexas lose TC jam across a battery or frame rate change,  so why risk it even if the problem only occurs seldomly?  I tell camera dept people objecting to the box that I think it's just wonderful that the Alexa is so stable TC-wise, but how about we put the box on anyhow so that on the remote chance it does screw up the TC we're still good to go without a scramble, right?  Since Alexa doesn't take external sync, it could be a dinky box like TIG: tiny, no weight….

 

philp

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Since Alexa doesn't take external sync, it could be a dinky box like TIG: tiny, no weight….

 

Or a Tentacle? ;) that's one of the reasons I ordered so many of them: to make it ever harder for the camera crew to reject my TC box.

I second the SOP of leaving the TC box on the Alexa all the time. In fact, I'll do that with every camera if at all possible.

The Alexa is well known to lose TC after a fps change or after reboot.

 

Re 788T being the culprit: even with a dead TC battery the 788 will hold TC fine as long as it's turned on. If the battery is really empty it'll lose TC, but it will be really obvious as it will start from zero again.

Another possible reason could that you have set the TC mode on your 788 to 24h (i.e. Time of day). Upon every turn on the recorder will re-sync TC to it's TOD clock, which a lot less acurate than the TC circuit

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Another possible reason could that you have set the TC mode on your 788 to 24h (i.e. Time of day). Upon every turn on the recorder will re-sync TC to it's TOD clock, which a lot less acurate than the TC circuit

 

No, in 24Hr Run, the 788T will only re-jam to its TOD clock if the generator has reset after a 4 hour shutdown.

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EX Alexa manual:

 

Note: It is recommended to sync after every shooting break to avoid the risk of having
to resync during the shoot.
Note: The camera will keep its TC for about 10 minutes after disconnection from the
power source. If a power source is connected, the camera will remember its TC for
as long as the power source is connected, even if it is switched off.

 

mike

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Note: The camera will keep its TC for about 10 minutes after disconnection from the

power source. If a power source is connected, the camera will remember its TC for

as long as the power source is connected, even if it is switched off.

In practice however, this does not seem to be true. Not in my experience anyway
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Are the cameras drifting away from the timecode slate (assuming you're using one) or drifting away just from the audio recorder (or both)?

 

If it were a timecode framerate discrepancy, (24-frames vs 23.976 frames for example), then the drift would be much more than 7 frames.

 

If both cameras are in sync with each other and are also in sync with the slate, then it is probably that the timecode generator in the recorder is drifting.  

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  • 1 month later...

 

No, in 24Hr Run, the 788T will only re-jam to its TOD clock if the generator has reset after a 4 hour shutdown.

Is this feature common to all Sound Devices TC capable recorders ?

Thanks Matt for this info ;-)

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Have the Lockit boxes and 788 been tuned?

Are you letting the 788 run free or does it have a Lockit box?

Maybe run the Lockit box back into the 788 once you have jammed the cameras for a day?

Run a TC slate as a 4th 'source' (after tuning all devices) then have post compare TC stamps and visual TC to establish which devices are staying in time with which others.

Good luck,

 

sb

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6 Series recorders have a 2 hour shut down timeout, IIRC all 7 Series recorders are 4 hours.

I would take the 2 hours with a grain of salt. It's all dependent on the life of your internal timecode battery. The flashing blue light will let you know the 664 is still holding timecode. No more blue light means it's reverting to the TOD clock. 

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