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MixPre-6 II Audio Drift

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    MixPre-6 II Audio Drift

    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!

    #2
    I'll just throw this out there. The most common cause of audio drift is recording in a different sample rate. 48khz is the standard check your MixPre setting to insure its 48khz.

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      #3
      Originally posted by Peter C. View Post
      I'll just throw this out there. The most common cause of audio drift is recording in a different sample rate. 48khz is the standard check your MixPre setting to insure its 48khz.
      Definitely make sure your MixPre was set to record 48KHz. A difference in frame rate can also cause issue in the NLE, but you’ve already crossed that one off the list.

      There are cases where two devices running 48KHz actually have slightly different versions (off by just a few Hz), which can also cause issues in post. The solution there is to have everything locked to a common word clock, but the MixPre-6 doesn’t have that capability. That said, this isn’t an issue I run into very much anymore when it comes to production sound and cameras. I’ve run MixPres for extended interviews without an issue, but I use Tentacles on everything. I know the record trigger can be handy, but I never use the HDMI and keep the MixPre running completely independently.
      Knoxville-based location sound mixer.

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