Live Sound University Article Thu, December 04, 2008

Have you decided to take the big plunge into pictures by turning your once humble audio room into a full-fledged video facility? Or perhaps you merely wish to have some basic audio-for-video capabilities so you can do audio for TV commercials as well as radio spots. Either way, welcome to the world of synchronization. Despite what you my think about sync to picture, it isn’t as easy as it seems. “Hey, I deal with timecode every day,” you say. Sure, in our own private world. The real challenge, however, is making your studio work with coming in from, and going out to, other facilities.

First, some questions to ask yourself before taking the leap. Above all, does the added income of work justify the expense of the new gear? Is there a video post demand in your city/locale? Are you going to add video editing capabilities as well? What will you say when the client wants you to edit out two seconds of the video--and they WILL ask that—what then? If you really plan on doing video in house, be prepared to plop down at least the cost of a new SSL for a “decent” video-editing suite such as an Avid workstation? An alternative is to find a local video studio, one that doesn’t do much audio, and buddy up with them. You throw them video work, they throw you audio-- kind of and I’ll scratch your back and you scratch mine situation.

Here’s where you need to be prepared when dealing with video. What format is the client (or video house) going to give you: 24fps, 25fps, 29.97fps, or 29.97df ? What format is this intended for? What is the delivery medium? From the point the material comes into your studio until point that it goes back out, there are a distressing number of things that can go wrong.

If I haven’t scared you away, I’ll get into the good stuff. FIRST LAW OF AUDIO TO PICTURE: When dealing with film/video, ALWAYS use 48k as your sample rate. Why should you use 48k? Because it’s the standard. If you try to take your project anywhere else, or get a project from anywhere else, you’ll be quickly grabbing for the aspirin if you’re working at the wrong sample rate.

When dealing with video, you must think of two sync signals: “speed” and “location”. Where are we (location)? How fast are we going (speed)? When dealing with SMPTE timecode in a purely audio world, SMPTE is both speed AND location. However, in video, SMPTE is “location” and video sync/house sync defines “speed”.

SMPTE contains the location, or address. Address answers the question, “Where are we?” How fast we get to where we are is determined by the frame rate. Common frame rates are 30fps (frames per second), 29.97fps, 29.97df (drop frame), 24fps, and 25fps. For audio work, 30fps is the standard, and you (hopefully) will stripe your 2-inch and ADATs with 30fps when dealing with a purely audio session. Note that 30 fps is simply that: in the period of one second thirty video frames will pass. However, the film standard is 24fps: in one second of film 24 frames will pass. Adding to the confusion, 25fps is the standard for most of Europe, Asia, and South America. (This 25fps standard is commonly referred to as PAL or SECAM.)

Here’s where it gets tricky. NTSC 29.97fps is the video standard for the US. You’re probably asking, how can you have .97 of a frame? Because 29.97fps is actually 30 “frames” of video running slightly slow. It just takes a little longer to run its full 30 frames, 0.1% slow to be exact, which makes for 3.6 seconds over the course of an hour. Think how funny it would look at the end of your favorite hour-long TV program if the dialogue was 3.6 seconds late compared to the picture! Ready for more confusion?

Introducing 29.97 drop frame. Remember good old 29.97fps that ran 0.1% slow? A common problem with standard 29.97fps is that, over time, the counter (hours:minutes:seconds:frames) will run slow compared to the clock on the wall. The display on your video machine says it’s only been an hour, yet the clock on the wall says it’s been one hour and 3.6 seconds. And with broadcast TV stations (usually) running on time, they wouldn’t be too happy about cutting off 3.6 seconds of a show or commercial every hour. Enter the solution: 29.97 drop frame. This format “drops” pre-determined frames so that that counter display on your video machine runs with the clock on the wall. Drop frame skips frames 0 and 1 at the beginning of every minute that is not a multiple of 10. By doing this “math,” drop frame effectively removes 108 frames per hour, which gives us exactly 3.6 seconds. Neat.

Now that were done with the “address or location” part, what keeps video gear running at a constant speed? The answer is video sync.

Video sync (or house sync) is the timing reference for video which answers the question, “How fast are we going?” House sync is most easily thought as word clock for video gear. House sync lines up all the frames together on video machines, much like word clock lines up all the samples in an audio stream. House Sync runs between all gear that is locked to picture, including video decks, DA-88’s, DAW’s, etc. There are two standard “speed” rates in the world of picture: 60 and 59.94. The 60 speed is used in film applications, grabbing the 60Hz out of the wall and turning it into a “pilot tone” recorded to a Nagra or some other type of remote recording device. As you likely guessed, 59.94 is, is 0.1% slow from 60 and thus, becomes the “speed” standard for video work. Unless you’re doing remote recording for films, your studio will use the 59.94 house sync reference. But then you ask, “What if I get a film project and my studio is setup for 59.94?”

More on that in Part Two of this discussion, where we will be” dealing with sample rates that are “pulled up” or “pulled down”.

Ideally your new video room would have a House Sync generator and the output signal would be fed to every piece of digital audio and video gear in your studio. Not many audio manufacturers include House Sync input, so there are a few house sync to word clock converters available keeping everything on one master clock generator.

A few rules of thumb to take away from this. First, 95% of your work will be at 29.97fps, house sync of 59.94, and a sample rate of 48k. Stick to those rules and you’ll be set most of the time. Dealing with the other 5% is what makes you buy stock in companies that make heartburn medications, and also makes you a true audio for picture engineer. In Part Two, we’ll dive into how to actually set all this stuff up, along with common delivery formats, and dealing with that pesky 5%. Also included in Part Two, dealing with audio that is edited to video, but has a final delivery format of film (“pull up” or “pull down” sample rates).