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Using Dual System Sound in Video Production

For whatever reason, the sound capability of most hybrid video/still cameras is clearly inferior to their video. Some cameras don’t have any external audio jacks, while others have a microphone input but no headphone output. Very few cameras sound as good as their pictures.

The audio preamplifiers in these hybrid cameras tend to be weak — often an afterthought in design. Most camera preamps are noisy, have very limited headroom and poor metering display. They also produce highly compressed audio files.

The workaround for poor internal camera audio systems is dual system sound. This means the sound is recorded not only on the camera, but separately on a high-quality digital audio recorder. Then the audio is synced to the video image in post. This technique dramatically improves the sound quality of any production.

Like everything else in video production, getting good sound requires attention to detail. Levels must be set correctly and audio should be monitored throughout the recording process. Professionals never use a recorder’s automatic gain control. This feature is for audio notetaking and can generate artifacts that negate the value of dual system recording altogether.

Virtually every major audio manufacturer now makes mixers designed for video sound recording. Most record at 24-bit/48 kHz WAVE or AIFF uncompressed audio to an internal flash card while feeding a separate output to the camera.

Zoom F8 – 32-bit Float

A newer generation of portable audio recorders use the 32-bit float method, which reduces the need for riding gain. This technology works differently from the traditional 16 and 24-bit standards. With 32-bit float, a wider range of audio values are recorded. Recordings can have a dynamic range of up to 1,528 dB — much wider than human hearing.

With 32-bit float recording, there’s not even a level knob of most recorders. These recordings can be adjusted to the correct level after the fact in post-production. This is a major breakthrough for field audio recording.

However, it should be remembered that 32-bit float will not fix all audio problems encountered on a video shoot. Managing noise from outside sources on the set still matters. Making sure microphones are capturing signals properly and having no defects in cables or other sound gear are still important. It’s no magic bullet fix using 32-bit float.

Yet, for small crews, having to carefully ride audio gain on location can be eliminated once the equipment is set-up and determined to be working correctly. Users just need to make sure their editing software can handle 32-bit float files and that the new technology fits into their standard 24-bit workflow.

New 32-bit float files do not eliminate 24-bit altogether. At some point in the post-production workflow, an audio engineer will need to make adjustments to ensure that the audio doesn’t get clipped when being downsampled to 24-bit. In effect, the gain riding task is being shifted from the field to the edit room. Yet, it is definitely easier to do in post.

Using 32-bit float is big advantage for one-man-band video crews, lone journalists and documentary filmmakers who are capturing unpredictable, unrepeatable sound sources. On the downside, users can expect 32-bit float audio file sizes to be about 33 percent larger than standard 24-bit files.

When the outboard audio system is set-up, the audio recorder and camera should be started and stopped simultaneously. Some kind of signal — a clapper stick, time code or even a hand slap — should be used to mark the sound on both the camera and audio recorder.

There also should be some kind of system — either a human assistant or automation — to separate the good and bad takes during the recording day. Having accurate records of takes can save a huge amount of time later in post.

Before editing begins, both the video and audio elements need to be assembled and organized. This is where the record keeping becomes important. Most edit systems have features for dual system sound, but having a good grasp of the different elements can save a lot of time.

The best practice is to drop the first good video take on the timeline with the camera’s internal audio. Then locate the sound spike from the clapper or use the time code and then slide the outboard audio recording to match the video.

This process may cause the editor to have to zoom in to assure the closest possible sync. Once lined up, turn off the audio from the original video and link the new audio file. With well catalogued picture and sound elements, this process can be relatively easy.

An even faster way is with software such as PluralEyes 4, which can automatically sync the video and outboard sound. PluralEyes automates audio/video synchronization without having to use a clapper or timecode. It employs algorithms so the user doesn’t have to organize clips before syncing. The user just drags and drops all the clips at once to the software. It’s as simple as that and avoidsw a lot of busy work.

Needless to say, dual system sound opens many creative opportunities, including separate dialog, stereo or immersive sound tracks — all recorded simultaneously on different channels for later mixing. With enough recording channels, each performer can be recorded on a separate track. Good low-cost 32-bit float systems for these applications are Zoom’s eight-channel F8n Pro and Sound Devices 12-channel MixPre 10 II.

In an increasingly competitive video production environment, average audio is no longer good enough. To make audio shine with even a low-cost camera system, dual system recording is the best approach. It can take more time, but the payoff is big and the needed gear is getting cheaper by the day.

Frank Beacham
Broadcast Beat - Production Industry Resource