MixPre-6 II Audio Drift

[Rustic River]

Hey friends, I just wrapped a shoot where I shot 4.5 hours worth of interview content on three cameras (Sony FX6 & two Sony A1's) plus a MixPre-6 II recorder. The 4.5 hour recordings were taken in 30-60 minute takes, give or take. I had three Tentacle Sync E mkII's for this shoot, one for each camera, and I had my MixPre-6 II connected to the FX6 via an HDMI cable to give the MixPre timecode. All devices were set to 23.98 recording with the FX6's Tentacle set as the master for timecode. On the MixPre-6 II I had it set to HDMI TC while on the FX6 I had it set to HDMI Timecode Out. I also had the FX6 set to HDMI Rec Control and the MixPre-6 II was set to Rec Trigger HMDI Flag to trigger record/stop from the FX6. Lastly, the MixPre-6 II was set to record at 32 (float) for the bit depth with a sample rate of 48kHz.

 

Virtually everything worked perfect during this shoot. The FX6 fed timecode accurately to the MixPre-6 II and also successfully triggered rec/stop on the MixPre-6 II while all three cameras received the same timecode from the Tentacle Sync E mkII's. Using Tentacle Sync Studio I was able to get everything synced quickly and easily in post and I was able to output an XML file and bring it into Premiere to have a timeline from the entire day synced up perfectly.

 

The issue I ran into was that while all cameras looked synced (and playing the audio back on all three from their scratch mics confirmed this) the audio from the MixPre-6 II was not. It started off great, but seemingly drifted over time to become off by more than a second toward the end of the day. Obviously this isn't going to work. Fortunately I recorded a backup track of the interview audio to my FX6 directly and it sounded great, so we just rolled with that in post to save us from the headache of fixing the audio drift from the MixPre-6 II's files. But, I want to fix this so we don't run into it again.

 

Now, I'm not a professional sound engineer and what I know about audio and timecode is from what I've taught myself through exhaustive reading along with YouTube demonstrations, so what I've learned over the years is all self-taught. As such I'm a little puzzled as to where to go from here. I've done some reading and think perhaps I need a genlock device of some sort to fix this problem. But, that wouldn't be ideal considering how much I've already spent on Tentacle Syncs (I have four) as well as the fact I simply like the Tentacles and their app/software. As such I wanted to see if there's a way to fix this problem using what I have.

 

Rather than connecting the FX6 to the MixPre-6 II via HDMI TC Out, should I instead run my fourth Tentacle into the the MixPre-6 II and set the MixPre-6 II as the master for all Tentacles? Would that help me solve this problem as I would be using the MixPre-6 II's own clock as the master source? Or would they still drift apart no matter which is the master? If that doesn't work, is there a single genlock device I could buy that I could use for the FX6 to the MixPre-6 II while still being able to somehow use the Tentacles?

 

I would love some pointers and direction on this as I called Sound Devices and they too were puzzled as to why I ran into this and didn't have an answer for me. Thanks in advance for any help you can give. I appreciate it!

[Constantin]

Diagnosing issues from afar is never easy, so we may or may not be able to solve this. 
However, 30-60 mimute long takes is well within the length where digital devices start drifting apart. It’s unusual that the cameras would stay in sync, unless they were recording shorter takes (?), but it’s not unheard of. 
Genlocking all devices would indeed solve this, if all devices are genlock capable. Cutting and rolling again more frequently would also help with this. 

Did you check the camera sync when the were all lined up in Premiere or where/when/how did you check sync?
 

[Rustic River]

Hey, I did check the camera sync in Premiere after using the Tentacle Software to create the synced timeline and all three cameras were synced perfectly. I also recorded scratch audio on all three cameras and was able to double check the sync that way and it was perfect. Also, just doing a visual look at the sync of all three images shows they synced perfectly. It's just the MixPre that was off. Lastly, during recording I often checked the timecode to make sure the FX6 and MixPre were still aligned and they looked to be every time I checked.

 

The issue with doing shorter takes is that this is for a medical client and we are doing long form Q&A's and they are more comfortable speaking in front of camera when they are all just rolling continuously. As such I let them roll for long continuous takes as making them feel comfortable is a top priority for me, especially when it comes to getting relaxed and natural responses.

 

I've done this type of filming for this client for 4 years now and the cameras have always stayed in sync no matter how long the record days are. It's only when I threw in the MixPre-6 II that I had a device not stay synced.

[Vincent R.]

Could it be a HDMI thing/delay/drift? 

[al mcguire]

I am confused as to what device is the time code master.

In my world it would always be the master recorder in this case the MixPre 6.

All Tentacles would be jammed to this standard.

I think the HDMI TC firing the MixPre is for the one man band who wants better audio.

 

[TVPostSound]

 

Time Code does NOT set speed (Velocity) its only an address.

Reference black burst/tri level sync set speed.

 

You WILL drift without both.

[Derek H]

I’m not sure you’ll get better answers here than you already did on Facebook but worth  a try. My money is still on the HDMI connection having some unintended consequence.