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Digital Audio Demands Editors’ Ears

Mixing up to 16 audio channels of sound can result in real mix-ups

There was a time when video producers worked with easy-to-use analog audio. It was recorded in one or two separate channels along with analog video and editing was simple; editors selected the video clips needed and the audio went along with those clips.

Ward Beck AMS16

Then digital audio came along—accompanied by its cousin digital video—and everything changed; suddenly synchronization became a real problem, even for the major broadcast television networks (remember watching early HDTV broadcasts and the lead character’s voice lagged behind the visual actions of his mouth).

S uch synchronization issues have largely been cleared up in the broadcast domain, but they remain to dog government video producers. And that is not all: The “embedding” of multi-channel audio within compressed HDTV signals has created new and novel problems of proper channel allocation, volume, and balancing for video producers. Frankly, the old saying that, “to err is human, but to really screw up you need a computer” applies all too often to the handling of digital audio.

T ektronix produces digital audio monitoring products and Mike Waidson is the company’s senior video applications engineer. “Historically, it has been common for video producers to forget about audio when they’re editing programs together,” Waidson said. “You could get away with such benign neglect in the analog days, but not today. There are so many complexities involved with digital audio, especially embedded digital audio, that you have to pay attention to what you’re doing.”


First things first: Users need to forget everything they think they know about audio.

S econd: Users need to enter the “brave new world” of digital audio, where audio channels are typically contained in the same data stream as HD video, and is not only one or two channels, but as many as 16 different audio channels included in a single HDTV raw feed. Those channels are then mixed down to either stereo or 5.1 surround sound in the editing bay; depending on what type of program distribution (web, DVD/Blu-ray) the producer has planned.

E ugene Johnson, managing director of Ward-Beck Systems, another producer of digital audio monitoring products, says with so many channels to work with, it is not surprising that things can and do go wrong. “For instance, I have seen people mistakenly flip the center and 5.1 subwoofer FX channels when dubbing DVDs,” he says. When those are flipped and a play back of the mix is attempted, the voiceover will likely come through the low range-optimised subwoofer, while the audio effects come through the center speaker, he said. “The result is unintelligible.”

S ynchronization is also an issue. The only way to ensure that digital video and audio clips line up properly is to synchronize them to a common time signal, which ensures that moving mouths line up with voice tracks, and that pops and other audio hiccups do not occur when new tracks are edited in.

Top: Tektronix WFM-7000; Bottom: Wohler Technologies AMP2-16V

“This is why properly-equipped digital editing suites generate a digital audio reference signal, or DARS,” says Waidson. “The DARS is a common reference ‘clock’ that all of the audio sources are synchronized to, with the DARS then being linked to the editing suite’s common video reference signal,” he said. “Like an analog time code, this gives all of the audio and video sources a shared, agreed-upon time reference. This eliminates the synchronization issue.”


DARS may eliminate the synchronization issue, but there are many other challenges associated with digital audio. Fortunately, they can be resolved by using digital audio monitoring technology.

D igital audio monitoring removes the guesswork from the audio editing process. To do that, the incoming digital audio stream is “de-embedded” from the HDTV signal stream, and reconstituted back into its individual channels. Once that has been done, each channel can be displayed both visually (using bar graph volume unit [VU] meters) and audibly (over audio speakers).

With each channel now visible, their volume levels, balance in relation to other channels, and location in stereo and 5.1 mixes can be determined accurately. That is then fed into the video switcher, for ultimate embedding in the finished HD video stream.

F unctionally speaking, digital audio monitoring equipment restores audio production to the good old audio days. That is why such equipment is a smart addition to any digital video-editing suite.


Alert Government Video readers will have noticed that Tektronix and Ward-Beck Systems both make digital audio monitoring equipment. So does Wohler Technologies.

T ektronix offers a full range of waveform monitor (WFM) and waveform rasterizers (WVR) for measuring and monitoring the real-time performance of video and audio. Its HD level products are the WFM-7000 and the WVR- 7200 (the WFM are standalone products with built-in LCD screens, while the WVR products are rack-mounted units that feed a number of external monitors, as defined by the user).

U sers can monitor all of their digital audio channels to ensure that the balance and allocation is correct, Waidson says. Users who increase one level to the 8000 series WFM/WVR can include 3G/3D video as well, he said, adding that provides a level of future-proofing in your plant.”

Ward-Beck Systems makes two digital audio monitors; namely their rack-mounted AMS8-1 and AMS16. The AMS8-1 handles and displays (using LCD bar meters) four AES digital audio inputs (eight channels), while the AMS16 handles/displays eight AES digital audio inputs (16 channels). That means the AMS16 can be connected to two HD video sources.

“When the source selection is made, the AMS16 will provide two reclocked outputs on the rear panel and display the audio signals on its bargraph meter,” Johnson says. “In addition, the unit will also provide discrete audio outputs of the selected source on the rear panel.”

Wohler Technologies has a full range of audio monitoring systems, covering from two to 16 channels with its top-end AMP2-16 Series supporting 16 channels of embedded audio, multi-rate 3G/HD/SD-SDI, AES, or analog audio inputs and outputs audio monitoring with Dolby D/E/DD+ decoding capability; encased in a two rack unit chassis.

“The AMP2-16V allows for users to be prepared to monitor audio in any situation, and examine the corresponding video and associated metadata,” says Jeff McNall, Wohler Technologies’ director of product line management (audio and video). “We have just recently added the capability to re-embed the audio as an SDI output,” he said, adding, “Users can now mix-and-match, as well as trim audio from all different source formats and have it come out as an SDI stream.”


If the specs and numbers in the last section appear daunting, fear not. As a video producer, all that has to be done is to determine the actual digital audio production needs, then contact the vendors and see what they have that meets those needs.

When talking to the vendors, ensure they clearly explain how digital audio monitoring can be integrated into a production suite, and check how easy their equipment will be to use. After all, video producers are already up to their necks coping with the move to HDTV. Dealing with digital audio as well should not be any more complicated that it has to be.

O ne thing is certain, digital audio monitoring is a must. The reason? Users cannot afford to gamble that a production’s audio is synchronized; will be at the right levels; and on the right channels.