Canon 7D Sync Problems

H

hamiltonsavage

New member
Hello,

I was hoping someone could help me with an external microphone syncing problem I'm having. I shot a short film with my canon 7d. I recorded sound with the camera's onboard mic for reference, but had a boom operator mic the scene with a Sennheiser 416 and a Zoom H4n. I learned afterwards that the Sennheiser records at 44.1 kHz and the Canon 7D records at 48 kHz. I don't know if that makes any difference. I took the footage into final cut and tried to match the Zoom sound with the sound I recorded with the onboard sound and found that the sound was recorded at different rates so they wouldn't match. They were off by a long shot. This makes absolutely no sense to me. Is there any way for me to change the rate of the zoom audio that I recorded so that the video and sound will match? Is this normal? Does the make that one records at 44.1 kHz and the other at 48 kHz make a difference?

Thanks
 
Drop your 44.1 kHz audio files from the Zoom H4N into your Final Cut timeline. Then export those audio files from the timeline. Then import the new files into FCP and they will be a 48 kHz frequency.

Try to sync the 7D audio now with the new audio files. It should work.

In the future, you can take your H4N out of stamina mode - at least I think that's what it's called that yields 44.1 kHz sampling and maximized recording time - and set it to record at 48 kHz.


The only difference I'm aware of (between audio files of different sampling rates that you try to sync) is the popping sound you hear when the two tracks collide at the same phase in their vibration. I'm not aware that the different file rates would cause them to be way out of sync.
 
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jscottprice said:
...
The only difference I'm aware of (between audio files of different sampling rates that you try to sync) is the popping sound you hear when the two tracks collide at the same phase in their vibration. I'm not aware that the different file rates would cause them to be way out of sync.
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If the 44.1 kHz file gets its sample rate converted to 48kHz upon on import into the timeline they should sync but not all software necessarily does that. The user may need to explicitly set an option in order for it to happen. I'm not an FCP user so I don't know what it handles sample rate differences but in many NLEs, such as Nuendo for example, if the project is set for 48kHz and you import a 44.1 kHz file, it asks you if you want to convert the file to the project sample rate or leave it as-is. If you don't convert it, it will playback at a 10% faster rate than when recorded (for a 44.1kHz file into a 48kHz project). 44000 samples should take 1 second to play but if played at 48000 samples/sec it will take less than 1 sec to play all 44000. 10 minutes worth of original sound would play in about 9 minutes. But 10 minutes of picture will still take 10 minutes to play. If this was double system sync sound it would clearly go out of sync with picture in very short order.
 
That makes sense. I've never seen them go that far out of sync because most of the video I've worked with (when the audio file in question was 44.1 kHz) was 30 sec - 2 minutes long, and sometimes the original track from the video was removed anyway. I didn't realize that things could go out of sync that fast, though.

I know that in FCP (and I'm pretty sure in Premiere, although I've only dealt with this sampling issue in FCP) you have to export the 44.1 kHz sampled audio out of the timeline and reimport it, which will change it to 48 kHz. If the audio is the first thing you drop in the timeline, it will ask you if you want to change the sequence settings to fit the file; but after that, it tries to conform new media to the audio/video in the timeline.
 
FCP 6 and later "ought" to SR convert on the fly. You are supposed to be able to use mixed sample rates with out a problem. As a rule I would convert them because if you go to an audio post app like protools the mixed sample rates with mess up the OMF. But you should not have a problem.

If your going to convert I would probably do it in something else. Audio Hub is good though no longer available (though it is or should shortly be available as open source). There are a lot of cheap batch converters that should do a fine job.

But unless your on an old version of FCP that is probably not the problem. Did you by any chance shoot at a X.0 frame rate? 25, 24,30? FCP has to be set up just right for those (if you live in NTSC land) and if not you will get some serious drift.
 
hamiltonsavage said:
Hello,

I learned afterwards that the Sennheiser records at 44.1 kHz and the Canon 7D records at 48 kHz.

Thanks
Click to expand...

BTW the Sennheiser doesn't record at all, it's a microphone. The Zoom was set to 44.1, it can record at a number of sample rates.
 
The sampling frequency has next to nothing to do with syncing the audio from your Zoom recorder with the video you shot with your 7D.

The sync problems stem from the fact that the clock frequency ( not the sampling frequency ) of your Zoom recorder and 7D camera do not match, so you have to adjust the audio timing to match the video.

I do this all the time with the long format medical lectures that I film using a Panasonic HS700 consumer cam and audio recorded with a Olympus LS-11 recorder. The clocks are close ( fine up to about 10 minutes of shooting ) but will go out of sync when filming longer events.

To correct the audio problem, I do the following:

1- Bring the video and audio into my editor ( Vegas Pro 9e ), match up the video audio at the beginning of the take. ( I cut both video and audio to match at the start )

2- Then I match up the audio with the end of the video, so the end of the video is now in sync with the audio, and then cut the audio to match the end of the video.

...At this point you now have the start/end of the video track, and the start/end of the audio track, but the duration of these two tracks do not match.

3- Export the cut audio track to a new audio file. ( the length of this audio file will be adjusted to match the exact length of the video )

4- Measure the length of the video file. Say you have something like 73 min 23 sec 17 frames, which equates to 73 min 23 sec + 0.5672 sec, or 73 min 23.5672 sec ( assuming the video frame rate was 29.97 fps )

5- Load up the exported audio file in to a reliable audio editor, and adjust the length of the audio file to match 73min 23.5672 sec, then save out the adjusted audio to another audio file.

This new file is now the final audio to be used with the video, and if everything was done correctly, you should have a perfect sync match to the video track.

NOTE: I have found that many audio editors are not accurate enough when changing the duration of a long audio file by only a fraction of a second. Fortunately there is one very affordable audio editor for Windows that works perfectly for this task:

GoldWave Audio Editor : $19

In the GoldWave main menu, click Effect, then select TimeWarp, then click on the Length radio-button and enter in the correct time you want your audio to be. Once adjusted save your audio to a new file, and then sync this new file with the video in your video editor.

I have used this technique for video / audio tracks up to 6 hours long, and was able to get frame-accurate synch on my first try.
 
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TheDingo said:
The sampling frequency has next to nothing to do with syncing the audio from your Zoom recorder with the video you shot with your 7D.

The sync problems stem from the fact that the clock frequency ( not the sampling frequency ) of your Zoom recorder and 7D camera do not match, so you have to adjust the audio timing to match the video.

I...
Click to expand...

The reason the clock frequencies have to match is that they control the sampling rate. For example, if the camera clock results in a true 48,000Hz file as it should but the recorder clock actually runs slow so only 47,900 samples are recorded in 1 second of real time, the real sample rates won't match even if the nominal rate is set to 48kHz in both devices. In the case of the original poster, he even had the sample rates in the recorder and the camera set so there would be an even greater discepency than mere clock variability would cause. If the NLE converts the imported audio file from 44.1 to 48, any residual discrepancy would be attributable to clock disparity. But if the 44.1 file DOES NOT get properly converted, playback speed, hence sync, will be wildly off. And if the "44.1" file was itself off a hair from a true 44.1 due to clock variability, the conversion process might very well not fully correct it.
 
Steve House said:
And if the "44.1" file was itself off a hair from a true 44.1 due to clock variability, the conversion process might very well not fully correct it.
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Yes, clock variability will kill the usability of any type of video gear.

I owned the original Zoom H4 for about a year, and found that the clock in this recorder was not stable, so in order to sync it with any video I shot, I would have to manually sync the audio every for every few minutes of video. ( worst recorder I have ever owned )

I also own a tiny little iRiver T20 MP3 player ( about the size of your thumb ) that has a line-in audio record function that is amazingly accurate. The clock would stay frame accurate with most video cameras I own for at least 30 minutes. It is a pain in the *ss to use because of a horrible menu system, but the dang little thing syncs really well.
 
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