How to Sync Audio and HDV video
 

I use a Sony FX1 HDV camcorder, and I am looking to use a separate digital recorder to record audio for a project. The audio will include talking heads, which will need to be synced to the video in post-production.

I am a newbie at audio issues, and I am wondering, what do I need to do to reduce or eliminate the possibility of the audio going way out of sync in post?

Also, does the sampling frequency affect syncing?

The audio frequency would be best if it was the same as the audio recorded by the camera. Simply because when you put the camera audio and the digital recorder's audio up on the editing software time line. The software may prefer them to be the same.
Once you have done this then the method I use for sync is to find a sharp soundwave peak (or a clap board) that appears on the camera's reference audio track as well as on the main recorded audio. Put a marker on the clips or on the timeline and move the peaks into position together, zoom right in to view a single frame, move them until they are perfect... dependent on your software's capabilities (I use Speed Edit I can get within a frame)
Or you could find a recorder that can record a timecode and feed that timecode from the camera to the audio recorder. This involves capturing the timecodes and matching them.
My above method is easier with less gear needed. However, every time you turn the audio recorder or camera off then you have to go through the whole procedure again. It's best to leave the recorders running, capture everything and just have to match the clips up one time only. They will stay in sync throughout the program.
I usually use this method for multi-camera and multi-track video shoots of concerts. So leaving the recorders running is no problem.

Thanks for the info, I'm going to try recording at 48hz with a clapper board, and see how it goes.

The only reliable way to insure sync doesn't drift out over the duration of a shot is for both the camera and the audio recorder to share a common timebase. Since the FX1 doesn't accept genlock, the most practical way is to use an audio recorder such as the Tascam HD-P2 that accepts composite video or blackburst sent from the camera and slaves its clock to it. A slate will establish sync, the common timebase will keep it. Otherwise, a scene may start in sync but after 10 minutes or so some drift may become apparent. (How soon and how bad are extemely variable.)

Steve, thanks for the info. I have a few questions:

1. If a camera has genlock, how do you connect the camera and audio equipment (and with what cables)?

2. With a device such as the Tascam HD-P2, do you connect the camera to the device, send a "blackburst," and then it saves it and you can access the timecode for future recording, or do you have to do it each time?

Genlock normally is via a coax cable with BNC connectors. It does not carry TC.

Rick is right - genlock and blackburst have nothing at all to do with timecode. They deal with the sample clocks in the camera and the recorder.

Genlock is an input on the camera to allow it to accept "house sync" (your FX1 doesn't have one). Broadcast studios where multiple cameras are switched live through a video switcher must have all of the camera synchronized so they all scan the video frame exactly in lock-step, they all start scanning line 1 of each video frame at exactly the same instant. If they don't, there's a momentary jump in the picture and loss of sync when one camera is switched to the other. To insure this, there is a master sync generator that feeds all of the cameras, switchers, and other equipment in the studio with a common clock pulse instead of letting each camera rely on its own internal sync generator.

The same sync issues also apply to recording double system sound. Your camera, and DV in general, uses an audio sample rate of 48,000 samples per second. The sound recorder does too. But 48kHz is a nominal rate - due to tolerances, temperature differences, etc the real rate can be very slightly off that. When you record and playback on the same device it doesn't matter - sound recorded in 1 minute plays back in 1 minute. But when the recording is moved to a different device with a different clock, things change. If, for example, the audio recorder is running at 48,500 samples per second instead of exactly 48,000, when played back on a device that is running exactly on-spec, a recording that was made in exactly one minute will play slightly slow and take a little longer than 1 minute to play back.

Now imagine your camera is really running at 47,500 and the audio recorder at 48,500. You record exactly 1 minute on each and load it into the editor which is running at exactly 48,000. The video will play slightly fast and finish a few seconds less than a minute while the audio will play slightly slow and still have several seconds to go when the video finishes. If they're in sync at the start of the minute, they'll be out of sync at the end. The solution is to once again use a master clock so that BOTH devices use the same timebase and they're running at exactly the same speed. With a camera that has genlock, the house clock controls the camera. The same house clock also has an output signal called "wordclock" which is the standard for controlling audio equipment. So genlock feeds the camera while wordclock from the same clock feeds the audio recorder. That house clock can be either a single physical clock generator hardwired to all the devices on the set or a set of highly accurate battery-powered clocks called "lockit boxes" that can be tuned precisely to each other and then attached to each camera and recorder. The Sound Devices timecode capable recorders actually have the equivalent of a lockit box built in them.

Another way to achieve this lockup is to use blackburst. Blackburst is a continuing signal, the sync pulse embedded in the video that the camera sends to tell a display when a scan line starts. Devices are available that take video blackburst and convert it into wordclock so that when you have one camera and a recorder, the camera can become the "master" replacing the house clock in my above discussion of genlock, wordclock, and audio recorders. The Tascam recorder has such a converter actually built-in so it has an input connector for video sync. (It's not the only recorder with this feature but all the others that have it start at several times the price of the Tascam.) You connect the camera's composite video out to the recorder's video sync input. The camera feeds sync to the recorder continuously as long as you're rolling.

Timecode does NOT control recording or playback speed. All it does is give you a convenient way of aligning the audio and video at a single point in the scene - it's really just a glorified version of a clapper slate and does nothing more than they do. It's the sample clock issue that needs attention if the scenes that are in-sync at their start are to STAY in sync as a shot longer than 5 or 10 minutes plays out.

Hey Steve thanks for clarifying a lot of things for me. I've been going about trying to keep things simple at shoots and genlock is just too much hassle for multi camera shoots of concerts with assorted cameras roaming and panning about. However, in post I have notice drift over our typically 25 minute orchestra compositions. It's usually not significant for a 6 minute pop song and syncing up for every songs edit is usually what happens.
I will definitely experiment with Genlock on some studio shoots where more cabling is convenient.
...but for a quick fix in editing, you could always cut the video tracks and re-sync them to the audio every few minutes of an edit.

I posted a technique here:My Video Problems :: View topic - Synchronise external and camera audio tracks. some time ago which might be of some use. No substitute for doing it properly of course, but quite a bit cheaper! :-)
The important thing to remember is that the camera 'clock' has to be the master - even if that is the one that is slightly wrong.
Can save having to sync a long take in small chunks, which I've found can be a bit time comsuming.

If you have cameras possessing genlock and external timecode inputs, the hard-core way to do it and still have roving, untethered cameras is to use the aforementioned Lockits. No cables required while shooting; they're battery powered and about the size of a cigarette pack, just velcro them to the camera. You have a Lockit box on each camera feeding it blackburst or tri-level sync if you're shooting HD plus one for the audio recorder feeding it wordclock. The Lockits also provide timecode to their hosts. Tuning them all together and jamming them with common TOD timecode before the shoot gives you accuracy across all the devices to within 1 frame per day.

Very nice. Maybe repost it here?

I do have a question: If you are working with seconds, do you really need to change the sample rate of the audio clip? I think Vegas lets you work with samples where it would definitely be necessary--and I would think the answer might be more accurate.


Sure its a subsitute for doing it properly! Its a necessary workaround for low budget people--you've just made the procedure easier to manage. I mean, the information that Mr. House gives is great but not meet the budget ability of the OP.

Well, we don't know what the OP's budget is so that may or may not be true. It's certainly true that Lockits aren't cheap - they cost just about as much as the Tascam HD-P2 does, each! Nor are the cameras that can use them cheap - the Canon XH-G1 is the lowest priced one that immediately comes to mind. But in any event, my discussion of Lockits is irrelevant to him since the FX1 lacks the necessary sync and TC I/O to use them anyway. That discussion was addressed more to Don Xaliman's post where he said he worked with multiple cameras in a studio environment - it sounded like his cameras did have genlock inputs. But the importance of syncing the sample rate clocks in the camera and audio recorder still applies to the OP if he wishes to avoid having his audio and video drift out of sync requiring him to fix it in post. When working with a single prosumer camera such as the FX1, the Tascam provides a way of "doing it properly" at a relatively economical budget point. (Very inexpensive, actually, for professional level recorders.) Since the OP apparently hasn't purchased his recorder yet, the ~$1000 Tascam's ability to sync to video versus the inability of such recorders such as the similarly priced Marantz 671, Fostex FR-2, and Edirol R-44, the $600 Fostex FR-2LE, and sub-$500 handhelds like the Zoom may make it a viable option for him that's worth the price difference.

If you will forgive a blatant commercial plug, our product PluralEyes was designed to be an easy, low-cost and automatic solution for exactly this problem. $149 USD.

Bruce

Steve, just to clarify this point, it is correct to say that, if I am using the FX1, and I want to be able to sync up audio and video without physically connecting the camera and the audio recorder, then sending a constant blackburst signal with a composite video cable is not the way to go?

This is a huge shot in the dark, but is there any way to send a blackburst signal from the camera to the audio recorder in a wireless fashion? I figured I'd at least ask.

This is a semi-unrelated question, but how was audio synced to film on motion pictures because NLEs?

It is POSSIBLE that this may work: Super Small Long Range Wireless Video & Audio Monitoring System Wireless Video Equipment at Markertek.com This is a case where cheaper may be better--a more expensive system may reclock the input signal which would defeat the porpoise. But if the transmiter spits out exactly what it took in, it could work. Lots of loose ends to sort out.

Do an google image search for 'film synchronizer'. But you really do it the same way now as before: mark the frame where the clapper hits on the film, mark the frame on the 35mm magnetic film (sound has been transfered from 1/4" to mag film) where the clapper hits and sync 'em up. They, as you edit in the original NLE, keep marking picture and sound at the same place and cut at the same place and you're footage will stay in sync--or not if you didn't lock the camera and audio recorder together (the more things change, the more they stay the same!).

Steve, the old adage Good, Cheap, Fast: pick any two certainly applies here. "Good" is pretty much necessary which only leaves a choice of cheap (manual resync) or fast (hard sync between devices). While I have used and appreciate good equipment, in my personal life, it is a luxury that I need to find subsitutes for.