DTV and Audio

There's no gentle way to say it: audio has traditionally been TV's "bastard child". By contrast, video has always been the family favorite: receiving the lion's share of attention and funding from engineers worldwide.
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There's no gentle way to say it: audio has traditionally been TV's "bastard child". By contrast, video has always been the family favorite: receiving the lion's share of attention and funding from engineers worldwide.

"It's true... audio has always come up for consideration only after all the other stuff has been done in television," says Bill Russell, vice president of Crystal Partners; makers of Big Ears parabolic microphones. "Audio is a poor relation to the video signal, and unfortunately few people in television pay much attention to it," adds Nigel Spratling, a partner in the PA-based audio processing firm Linear Acoustic.

Thankfully, engineers are starting to pay attention to TV's neglected child. Small wonder--all the pretty 1080i HD images won't impress anybody if the sound is as out of sync as Ashlee Simpson on Saturday Night Live.

Why DTV Audio Demands Respect

In the analog television universe, broadcasters could afford to ignore audio. After all, adding an analog audio signal to an NTSC video transmission was no big deal, even when broadcasters made the move to stereo. "When it comes to marrying analog audio to video, there are no real problems besides loudness and levels," says Spratling. "It is not as if there are any timing or synchronization issues to be dealt with: when sync issues occur, people know how to easily deal with them."

In contrast, digital audio is a much more demanding beast, especially when married to ATSC/8-VSB video transmission. After all, DTV audio is ideally meant to be heard in a 5.1 surround sound atmosphere. Add in a stereo SAP feed plus a mono visual description channel for the visually impaired, and "you are potentially contending with nine or more separate audio channels for transmission," Spratling notes. "Even when compressed with metadata attached [metadata being the instructions that tell the receiving set-top box how to decode and reassemble the various audio channels], this is a big package that uses up a lot of bandwidth."

Remedying Sync Issues

The existence of separate DTV video and audio streams can cause problems not found in the analog world. The big concern is synchronization: it is possible for DTV video and audio feeds to become noticeably out of sync as they travel from the network via satellite to their affiliates, and then by air to the viewer's DTV set-top box.

The sync problem was diagnosed by the ATSC Implementation Subcommittee (IS) in 2003, in the IS/191 report called Relative Timing of Sound and Vision for Broadcast Operation. "Each component in the [DTV] system imposes a latency on the audio and/or video signals flowing through it," explained IS/191. "Operationally, unequal delays can be imposed on the audio and video signals respectively, and these delays compromise audio-video synchronization."

To deal with this problem, "the video encoder and Dolby Digital can be easily configured to keep the audio and video in sync from the transmitter to the set-top box," says Tom Daily, marketing director for Dolby's professional division. "However, as broadcasters add more HDTV compression processes to their distribution systems, there will be more opportunities for picture and sound to go out of sync."

Correcting Audio Imbalances

When it comes to DTV audio, one might say that the devil is in the metadata. The problem is that the metadata which defines how digital audio should be reassembled--as defined by the broadcaster or even the program's producer--can end up being altered on its way to the viewer's set-top box.

Sometimes the problem can be caused by an affiliate engineer boosting a level on their board; something that was an accepted practice in the analog days. "Unfortunately, if someone boosts the audio gain on a show locally, they can unintentionally mess up the metadata that the viewer is receiving," Spratling says. "This can result in the show's levels being thrown out of balance on their digital television, degrading or even ruining the audio listening experience."

In other instances, the local broadcaster may apply generic metadata values to the outgoing Dolby Digital AC-3 audio stream, in a bid to provide a consistent on-air sound. While understandable, this approach can rob certain shows of their unique audio qualities (or instance, using 5.1 surround sound to creatively to render a striking audio environment).

To remedy this problem, broadcasters need to protect their programs' original metadata as carefully as they control audio-video program synchronization. Otherwise, the amazing DTV audio benefits of 5.1 surround sound will be lost on their viewers. In this case, the solution is discipline--get your fingers off that slider!--and close cooperation with the program providers.

Eliminating Bangs and Clicks

One problem that has cropped up in certain DTV audio transmissions is the transition between 5.1 program feeds and stereo commercial tracks. Sometimes the combination of program metadata and the specific set-top box being used can result momentary collisions between the two differing signal architectures. When this happens, viewers can hear an annoying "bang" or "click" emanating from their speakers.

To remedy this problem, Linear Acoustic offers an audio processor that synthesizes stereo signals into 5.1 surround sound. By doing so, there are no audio collisions during commercial breaks, nor are the rear and center speakers left to hiss quietly. "We also sell a processor called the Octimax that smoothes out loudness differences between commercials and programs without mangling the metadata," says Spratling.

Making Digital Audio Easier to Manipulate

One might not consider a digital audio console's interface to be a source of DTV audio distress. However, if the operator can't make sense of the system, or work with it as nimbly as he once did using an analog board, then the interface is indeed a problem.

Studer's Vista 8 Live Production Console is one answer to confusing digital audio boards. It features TFT touch screen panels that permit the operator to quickly access all adjustments in easily understood, highly graphical views. "We have tried to merge the familiarity and speed of an analog console's human interface with the powerful features of digital console technology," says Clayton Blick, national sales manager for Studer in the U.S.

Conclusions

Clearly, DTV audio is proving to be more complex than most people thought. The crux of the problem is the complexity of the ATSC DTV distribution chain: the use of digital technology plus the extra demands of 5.1 surround sound are generating synchronization issues that just don't exist in analog television.

Obviously, it's only a matter of time before these issues are resolved. However, audio's unexpected ability to compromise the DTV transmission process illustrates what can happen when you neglect a child for too long, and assume that they will act as required when needed to.

The problem is that the revolution now sweeping America is not digital television really, but digital audio/video. DTV engineers and network executives would be wise to mind this fact: successful DTV is more than just pretty pictures.