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Tuesday, December 15, 2009

The Importance Of Sound

In discussion with DPP, sound master Mark Adams describes the ins and outs of working with sound for multimedia and HD video productions

This Article Features Photo Zoom

First, let’s briefly talk about what’s meant by “24p” and “30 fps.” A motion-picture film camera feeds 24 frames, still images, of film through the gate every second. Video, unlike film, is recorded electronically, so we don’t really speak about video in terms of physical frames; it’s more like electronic fields comprised of odd and even lines that make up electronic “frames.” In standard NTSC video, there are about 30 “frames” per second. Instead of capturing a single “moment in time” using one image to represent one frame, as with in film, NTSC video cameras capture two “moments in time” for each “frame.” The video camera separates the two “moments in time” into odd and even lines. Then it creates the “frame” by combining the odd lines from one “frame” with the even lines from the next “frame.” This is called interlacing.

An HD camera recording in 24p captures 24 “moments in time” every second, similar to motion-picture film. Since the frames are made up of complete images as opposed to interlaced fields, we use the term “progressive.” 24p cameras also take each of the frames and separates them into odd and even lines. However, 24p cameras combine the fields differently.

In many cases, an HD camera recording in 24p is actually recording in 23.976 fps in order to comply with NTSC standards. NTSC is the National Television Systems Committee, and it’s also the current analog television system broadcast standard for North America, Japan, South Korea, Taiwan, Burma and some Pacific Island nations. If you’re shooting in HD 24p and plan on distributing the footage as NTSC video, make sure your camera is really recording at 23.976 fps. In this case, in order for your sound to sync up properly, you want to make sure you’re recording your sound at 23.976 fps, as well. If your digital audio recorder doesn’t have a 23.976 fps option, you may be able to record your sound at 29.97 non-drop frame, and it should sync properly with the video during the down-conversion process.

To the best of my knowledge, NTSC standards require that we must use 59.94 fields to make up 29.97 fps. So, if you think you’re recording HD or SD at 30 fps, the camera you’re using may in fact actually be recording 29.97 fps to comply with the NTSC standard. So, in this case, you want to make sure that you’re recording your sound at 29.97 fps. The keys are to know how the project is going to be delivered—television broadcast, straight to DVD, the web, etc.—and then make sure that your production frame rates meet those delivery requirements, and finally that the post facility that you’re using is capable of working with those frame rates so that everyone involved with frame rates for your project is on the same page.

DPP: What does one need to know in terms of high-fidelity sound capture when shooting with a DSLR and using an external recording device?

Adams: The bottom line when it comes to syncing sound using a digital audio recording device that’s separate from your DSLR is to, one, make sure you have a digital audio device that has multiple time-code frame-rate options; two, make sure you know what frame rate the camera you’re using is recording on and make sure your sound is being recorded at the proper frame rate; and three, make sure you use some type of slate, either a digital time-code slate or a “hand-clap” slate in front of your camera and microphone at the start of each take.

To optimize the benefits from these tools, hire a professional sound technician who’s familiar with digital audio recording in the field. They can provide professional sound quality to your project and make sure your dialogue synchronizes properly with your video.


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