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Sound differences in video post.. Audio Interfaces
Old 10th September 2011
  #1
Gear Nut
 
luca1's Avatar
 

Sound differences in video post..

Hi, this regards filming live bands.

I have a problem that has been bugging me for a year or so and I should have just asked, it seems like the answer is right in front of me but I just can't figure it out :/

I film using 3 cameras and I record the audio separately by mic'n up the individual sources.

My gear:

CAMERAS
Canon HV30
Canon XHA1
Lumix TZ10

SOUND
Profire 2626 Interface
Various Mics
A1CS pro audio laptop

Once recorded, I dump the audio into Ableton or Protools and mix it all down to a stereo file giving it a light master, bearing in mind these are home projects.

Once I have the stereo mix down, I drop that into Sony Vegas with the video files and line them all up at a snare hit (or other sharp transient).

The video's all stay in sync but over 20 minutes or so the audio (mix down) runs out.

I record the audio in 48kHz 16bit, mix down in 48kHz 24bit and dump everything into vegas with the project settings audio tab set to 48kHz.

They are all digital files and I am careful to keep everything at 48kHz, the only change is from 16bit to 24bit.

Can anyone elaborate as to why the audio runs out? This identical problem has happened over all my projects, usually I ignore it just cutting the video to line up with the audio, but I would like to dump everything into Vegas, line it all up at the start and have it stay in sync as I'm sure this is the norm.

Regards,
:Luca
Old 10th September 2011
  #2
Lives for gear
Many people will probably give more elaborate answers, but the simple way to put it is that a second isn't always a second.

Cheap cameras and audio recorders such as the Zoom H4N don't have very reliable clocks, and so will drift in and out of sync with each other. The Zoom recorders are well known for this problem (especially amongst Canon 5D MkII shooters), there the internal clock runs slightly slower of faster (depending on the model) and therefore record 48000 samples in slightly less or slightly more than 1 second, instead of exactly 1 second. When this is played back at exactly (more or less) 48000 samples/second, the result is a longer or shorter playback time.

Obviously in your case, the cameras are all working to their internal clocks which are slightly different to the clock in the Profire.

The higher end recorders have much better clocks and therefore won't have a noticeable amount of drift, however to be sure of this, locking to a timecode generator is always the safest plan.
Old 10th September 2011
  #3
Lives for gear
 

Yes to the above. Here's what I'd suggest: lock down the time base of your audio by clocking it to something steady and reliable (ext word clock generator, higher end recorder like Sound Devices etc). You can rent this stuff for the gig. Then cut your picture to the mixed sound. You will never get those cameras to stay in sync with your audio for a long period of time--as was said their clocks are not accurate enough for that. So in cutting the picture to the mixed sound you will be correcting the sync at every edit, by eye. In truth this is how even big-time concert films are done. I've done MANY concert shoots this way and it worked out well. What we learned is that there is no way to have end to end sync in a single shot w/o an edit for 30 min or more with that kind of prosumer camera, and that sync is in the eye of the beholder--you can fudge it a good deal with music.

phil p
Old 12th September 2011
  #4
Gear Nut
 
luca1's Avatar
 

Wow great replies!! I never knew the H4N's were known for that.

When I originally dumped the audio into Ableton, the H4N was off from the Profire tracks, now I know why

I have done my past videos by lining them up edit by edit and had great results, but just thought that everything (in an ideal world) should be synced start to finish. I understand now why pro's Genlock, I wondered why they still did that if their gear was good enough and they were using a NLE.. now I know why

Great info, thats why I love this site. You guys make my life so much less stressfull

I'll look into a more professional audio recorder and locking my devices by timecode, will be exciting to put theory into practice.

Regards,
:Luca
Old 13th September 2011
  #5
Lives for gear
 

Quote:
Originally Posted by luca1 View Post
Wow great replies!! I never knew the H4N's were known for that.

When I originally dumped the audio into Ableton, the H4N was off from the Profire tracks, now I know why

I have done my past videos by lining them up edit by edit and had great results, but just thought that everything (in an ideal world) should be synced start to finish. I understand now why pro's Genlock, I wondered why they still did that if their gear was good enough and they were using a NLE.. now I know why

Great info, thats why I love this site. You guys make my life so much less stressfull

I'll look into a more professional audio recorder and locking my devices by timecode, will be exciting to put theory into practice.

Regards,
:Luca
In a more pro world we CAN have everything locked end to end, think "Live At The Met" or a high-end rock concert video. But that requires gen-lockable cameras and a lot of peripheral gear, and I'm not at all convinced that the final product is any better for all this (many producers feel the same way). Remember that in digital audio the real sync is done by clock and sample rate. You can name the frames anything you like (timecode) as long as they are being recorded at an exact sample rate (clock). Those prosumer cameras don't take external TC either, mostly, just make sure they have some kind of audio feed (best) or camera mic (will do), and try to lock down your audio sample rate with a really steady clock.

phil p
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