Why am I experiencing unsynchronized audio during recording?

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I've run into an issue while recording audio with MXL-990 using Behringer U-Phoria UMC204HD.

We're recording audio with the above devices and video+audio with a video camera. The audio does not sync up with the video. It starts OK, but 10minutes in the audio comes in too fast - the video seems to lag behind. In other words: the sounds happen sooner than they should.

We've tried multiple cameras and multiple PCs for recording (linux, MacOSX), as well as multiple pieces of software (Audacity, Ardour, GarageBand, Oceanaudio), all yielding similar results. Changing the sample size/bitrate didn't help.

Here's an example of that. In the top track is the audio from the Behringer, and on the bottom track there's audio extracted from one of the cameras we've tested.

beginning of the recording

10 minutes in

There are two claps in the recording, 10 minutes apart. The first one is in sync, the other one, not so much. You can see the discrepancy in the screenshot above

Could it be that our interface is faulty? How can we confirm that?

Kuba Orlik

Posted 2019-11-24T10:15:17.140

Reputation: 121

Does the camera have a setting for variable frame rate and constant frame rate? (VFR/CFR) other users have found that to be an issue. – Timinycricket – 2019-11-24T22:05:52.123

@Timinycricket we've tested with 3 total different cameras on the same time, please look at this timeline: https://community.musictribe.com/vdgmh27479/attachments/vdgmh27479/bepc1002/163/1/audacity.png

– Arkadiusz Wieczorek – 2019-11-25T08:34:59.050

Also be aware that the drift is locked in during the recording. Once you have made the recording, everything after that point is viewed independent of the clock in the audio interface. – Mark – 2019-11-25T08:50:03.330

Answers

1

The most likely culprit is that you are using a non-integer frame-rate with your camera, but when you are bringing the footage into the computer it is finding it's way into the timeline at a different frame-rate. Check the frame-rate of the camera. Sometimes the frame-rate is not what it seems.

Mark

Posted 2019-11-24T10:15:17.140

Reputation: 7 535

Thank you Mark! with Kuba Orlik we tested the recorded sound from the interface of 3 different devices: Canon EOS 550D → 30fps, Samsung S6 Edge → 60fps, Huawei Mate 10 Lite → 30 fps at once and all 3 tracks from the cameras were synchronized, and the recording from the microphone wasn't – Arkadiusz Wieczorek – 2019-11-24T11:58:19.353

1A lot of these cameras will tell you it's 30fps but in reality it's 29.97 non-integer. Feel free to dropbox me some of your footage and audio. mark.p.edwards@gmail.com – Mark – 2019-11-24T12:05:30.273

1if you drop a 29.97fps footage into a 30fps timeline, your audio will slip for sure. – Mark – 2019-11-24T12:06:17.297

@Mark we've extracted the audio from the video file and extrapolated it over the audio from Behringer, and the discrepancies are still there. I've updated the question to reflect that – Kuba Orlik – 2019-11-24T12:21:56.667

1if it was an integer/non-integer related issue, the slip would be greater than what you are seeing. Unfortunately I think the problems is most likely a clocking issue with your recording device. The sampling clock isn't the same as the sampling clock running in the camera. Most professional equipment will permit 'genlock' to be used which allows you to clock both the recording device and the video device from the same source. Unfortunately with the devices you are using this is not possible. You will have to stretch the audio in post to match. – Mark – 2019-11-24T13:15:39.470

That's a very interesting point!

We've compared audio recorded by many different cameras, and they were all in perfect sync, even though they didn't use a 'genlock'. Why would that one device (our interface) be the only one device with a different clock?

Could the interface's clock be faulty? – Kuba Orlik – 2019-11-24T14:46:18.137

1very unlikely that it is a faulty interface. the clock drift is less than 0.5% so this would not be considered a fault. – Mark – 2019-11-24T23:48:21.570

suggest you try the interface with a different computer – Mark – 2019-11-24T23:49:46.123

We've tried on mac and still the same problem. – Arkadiusz Wieczorek – 2019-11-25T06:56:56.323

1I think your only option is to apply time-stretch to the footage in order to align it with the footage from your cameras. You should use a slate at the start and end if you are going to be working with long takes. – Mark – 2019-11-25T07:41:06.680

I've changed tempo without changing pitch of my record to -0.018% in audacity then it seems to be synchronized, but is so strange that I have to make workaround on my record to get synchronized audio. – Arkadiusz Wieczorek – 2019-11-25T08:36:08.597

1None of these cameras permit Variable frame rate, however it is likely they are not entirely truthful about their frame rate, but as we have established the issue is unlikely to be related to this. I still suspect an off-colour clock however 0.018% isn't enough to claim this is a faulty interface. Often you see these discrepancies between device clocks - and they become more significant with longer takes. – Mark – 2019-11-25T08:47:49.730

Does changing the tempo affect the sound quality? I've checked my audio from cameras through ffmpeg and:

  1. 29.97 fps [audio 48KHz, 1536 kb/s]
  2. 30.22 fps [audio 48KHz, 192 kb/s]
  3. 30.01 fps [audio 48KHz, 256 kb/s]

all audio which is coming from cameras is perfectly synchronized.

"I still suspect an off-colour clock" can I influence it by setting something, e.g the number of samples or not? – Arkadiusz Wieczorek – 2019-11-25T09:03:12.093

No, changing the sampling rate won't assist you at all. The master clock in the interface drives the sampling clock and I am pretty sure it's the master clock that is drifting. Drift is a common problem when working with Separate Sound - it's less common with more professional equipment - i.e. Sound Devices/Zaxcom field recorders - and you also have the opportunity to lock clocks with the camera. unfortunately with your setup you are going to have to wing it somewhat and make sure you have slates at both ends of each take in order to re-align the audio with camera sound. – Mark – 2019-11-25T09:22:24.410

You might want to consider investing in something like a Zoom F4 which will give you much better recorded quality and a more stable audio clock. You also get to use it as an audio interface. – Mark – 2019-11-25T09:23:40.583

I borrowed from my friend the same interface will test today if there is the same problem. – Arkadiusz Wieczorek – 2019-11-25T10:06:07.457

Good idea. Let us know how you go. – Mark – 2019-11-26T07:40:55.143

0

This is a troubleshooting suggestion rather than an answer. StackExchange says that I don't have enough reputation points to post a comment.

Play a video on your TV and film it with one of your cameras while also recording it with your Behringer. Then, play back the video along with the two recordings, and see which recording aligns with the video.

If the problem is the Behringer, then there might be a way to calibrate its clock. If the problem is the camera, then you just need a more honest camera.

Arnold Cross

Posted 2019-11-24T10:15:17.140

Reputation: 29

Thank you for your suggestion. We will try that! If it turns out to be an issue with the behringer's clock, do you know how it can be adjusted? – Kuba Orlik – 2019-11-26T10:10:20.010

1It can't be adjusted. – Mark – 2019-11-26T12:32:40.583

Talk to their tech support. For the accuracy that they advertise, I assume they use a crystal oscillator (as opposed to an LC circuit), and the crystal might be defective. It is possible to use an adjustable cycle counter for the clock, but that would have to be designed in. If they did implement that in their design, then their repair techs are probably the only ones who know the procedure. If the clock cannot be calibrated, then the whole circuit board would need to be replaced, and it's probably the main circuit board. – Arnold Cross – 2019-11-26T17:15:01.280