Sony AVC mp4 audio sync

[John Rofrano]

[Nathan Halder] “Is the .MOV throwing Vegas for a loop? Should I render the raw .MOV to something else, then edit it?”

It couldn’t hurt. I assume the MOV file is some highly compressed interframe long GOP codec that isn’t frame accurate. Rendering to a intraframe codec like CineForm will probably make it easier to sync up if for no other reason that it will play back smoother.

Don’t forget the audio may need to be stretched to match the timing of the video. The audio will always drift over time when recorded separately. Try stretching the audio first and see if that helps. Sometimes you need to split it to keep it stretched in several sections.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Nathan Halder]

Didn’t think of that – thanks John.

I have used this borrowed Zoom audio recorder before with no drift while syncing to video that is 24fps. Must have been lucky this hasn’t happened until now. I’ll try rendering it and/or stretching the audio and see what happens.

[John Rofrano]

[Nathan Halder] “I have used this borrowed Zoom audio recorder before with no drift while syncing to video that is 24fps”

I have to say that it is extremely rare that it would drift in only 5 minutes. Usually I get around 20 minutes before I see drift with my Marantz PMD660 recorder, but didn’t you say the video was from an iPhone? How stable does that record?

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Nathan Halder]

Apparently not very. 🙂

[Dave Haynie]

Which Zoom recorder did you use? The H4 and some other other older ones have a reputation for a pretty dicey clock. The cheap everyday crystals used to drive microprocessors come in at about 100ppm (parts per million), which translates to a loss of sync of one frame at 24fps every 6.9 minutes. I’ve read a fair bit of anecdotal evidence that suggests that’s about what you’re going to get with the Zoom…. it’ll certainly vary from unit to unit and video device to video device… your camera may be super accurate, but it’s it’s off, there’s a 50/50 chance it errs in the same direction as the audio recorder. So you can’t know the whole story just from a few specific-case measurements.

On the other hand, the clock module I use in my digital radio designs (the day job) is 1.5ppm, which would mean one 24fps frame every 7.7 hours. The typical cheap watch crystal, usually 32.768kHz, used for the timebase in some consumer gear, is usually around 20ppm, which gets you one frame de-sync every 35 minutes or so. And again, that 20ppm is a worst-case number…. at room temperature, you’re probably sticking with a frame every hour or so.

I haven’t found any hard numbers on the Zoom H4n (I have one, and no problems to date, but I figured I’d look it up), and I’ve read claims of it being off one frame in 100 minutes, so that could well be running on a 10ppm rate clock. But again, that’s only so useful if your camcorder walks all over the place.

And of course, do check that your project sample rate and video sample rate are correct. Most of expect “24p” video to be at 23.9fps, but the iPhone isn’t really a video device.. does it maybe give you a hard 24fps or something?

-Dave

[Jim Greene]

I haven’t heard of people with audio drift problems with the Zoom H4, but I have an old H2 that drifts terribly. I use it for recording live musicians at ceremonies and for the length of a single song or two its fine, but not much after. It does a great job of recording live audio but not good at keeping sync with my cameras. I now use Edirol R-09, Tascam DR-07 and Zoom H1, which all keep sync for my needs.

-Jim.