Audio: Dual system sound

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Hal Robertson | August 2008




When you need to sync many mic sources into your video or want to boost the audio you're already collecting, you might want to look at dual-system sound.

0Have you ever wondered how they record audio for a motion picture? DVD bonus features clearly show the microphones, headphones, wireless packs and other audio goodies, but where does it all go? They don't have microphone inputs on film cameras, do they? While it is possible to record audio on film, virtually all film sound is recorded on separate devices. This technique adds a layer or two of complexity to the production, but it also has some powerful benefits. In the video production world, it's a technique you can use to improve the consistency and quality of your digital audio. Sounds like it's worth investigating (pun intended).


But Why?

1In the video world, this technique is called dual-system sound, because the audio is recorded on two devices: the camcorder and a separate audio recorder. I know what you're thinking: why go to the trouble of recording on another piece of equipment when my camera's got a mic input? Please be patient, because there is more than one answer.

First, not all camcorders have a microphone input. There are many perfectly adequate cameras that are locked into their tiny built-in microphones. As a content creator, you're probably leaving that camera on the shelf rather than risk the bad audio. With dual-system sound, that camera is now available - if not for your primary shot, at least as a B-roll camera.


Second, camcorder microphone preamps aren't always the highest quality. In fact, some of them stink. They often have limited headroom and increased noise, compared to a dedicated audio system. In addition, there are plenty of producers who don't like the compressed MPEG and AC3 audio of HDV and AVCHD cameras. Recording separately eliminates this concern.

Finally, dual-system sound forces you to pay close attention to your audio. It requires more setup, more operator involvement and some rec-ordkeeping. Extra work, yes, but the kind of extra effort that insures a quality product.

Gear Up!

2Let's assume you already record audio with an external microphone and/or mixer. That means you need only a dedicated audio recorder and some extra cabling for dual-system sound. Audio recorders come in several flavors. For some time now, the MiniDisc has been a favorite with video producers. It's small and runs forever on batteries, and the discs record about the same amount of time as a regular CD.



 The MiniDisc format is fading, but there are still plenty to be found on eBay, at swap meets and even in your local pawn shop. If you decide to go this route, make sure the recorder has a microphone or line input and some good basic controls.

Another popular option is the Flash-based recorder. Edirol, M-Audio, Tascam and Zoom are a few of the manufacturers offering this type of small, portable recorder. Most have professional XLR microphone inputs, phantom power and quality input sections. Since there are no moving parts, these recorders offer extended run times and can record as much audio as you want, using removable cards. Back in the edit suite, you just plug the card into a reader and transfer the audio files to your hard drive.

The most flexible recording option is a laptop computer. Using a USB or FireWire audio interface and some recording software, a laptop-based recording system gives you the ultimate in control. There are many affordable audio interfaces that include two to eight microphone or line inputs. Recording media can be a hard drive, thumb drive or Flash memory card. Software can be as simple as Audacity or as complex as ProTools. With a laptop, you can mix and match your options to create the perfect audio recording system.

We mentioned additional cabling earlier. With dual-system sound, you'll split the audio signal, sending one feed to the camcorder and another to the separate audio recording system. Why both? For redundancy and easier sync in the edit suite. The splitter could be something as simple as a Y-adapter from an electronics store. If you're using a mixer, there should be an extra output or two that will meet your requirements. In either case, it's rare to have the right connectors on everything, so adapt as needed, and test both systems for clean, clear sound.

Shoot and Edit

Shooting and editing with dual-system sound requires some extra effort, and you'll need a crew member to handle audio duties. 

The first audio task is to make sure everything is working and routed properly. 

Second, the audio person has to start and stop recording along with the video camera, so a clear system of signals or phrases is a must. 

Third, the audio person will keep track of the various takes and the audio file names or clips associated with them. 

3With a computer-based system and the right software, this can be almost automatic. With other recorders, it will take some practice. Be sure to verbally slate each take with scene or subject and take number. A clapboard provides another audible cue and will simplify editing later. If you don't have one, simply clapping your hands will produce the same results.v

You made it through the shoot, and now it's time to gather your audio and video elements for editing. This is where that list of takes and audio files comes into play. Drop the first good take on the timeline, and make sure you can see the waveform of the audio from the camera.

Next, find the associated audio clip, and put it on a blank audio track under the video sound. Locate the clapper spike in the waveform, and slide the audio recording over to match. Zoom in, if necessary, to get the best possible sync. Now, turn off the audio from the video recording and, if your video editing application allows it, link the video clip with the new audio file. Edit as needed. It takes some extra time, but if you have kept good records, you'll be syncing audio and video quickly in no time.

Options Galore

Using dual-system audio recording opens up some interesting possibilities. Depending on your recording system, you could easily record dialog and surround sound simultaneously. With enough recording channels, you could place each performer on a separate track. You can record two channels on the camera and two or more additional channels on the separate audio recorder. You can also add effects and processing to the audio clips before editing.

No matter which method you choose, you'll find that dual-system sound can offer you a level of options and control that most video producers have never experienced.

Contributing Editor Hal Robertson is a digital media producer and technology consultant.


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