01.16.2008 08:14 PM
How to Choose an Audio Processor - by Tim Carroll
Today, broadcasters face many new challenges, including federal broadcast regulations for digital television, loudness control issues and the ability to process stereo and surround sound signals appropriately.

There now exist on the market well-designed audio processors that address these challenges and make the broadcaster's life much easier by handling complex signal management within one system. While these processors make short work of the complexities of today's broadcasting, they also have features, including metadata support that will take audio processing into the future.

MAKING THE DATE
Congress mandated that after Feb. 17, 2009, terrestrial television stations across the United States must transmit only digital signals and may no longer transmit analog signals. While the digital transition will allow broadcasters to deliver DVD-quality pictures and sound (along with enhancements for multilingual broadcasting, better accessibility for the visually or hearing impaired and improved emergency alerting capabilities), it also presents new challenges for handling audio and video.

Legacy audio processing only had to account for stereo, but today's DTV signals can deliver surround sound of up to 5.1 channels in multiple languages. Furthermore, as broadcasters seek to increase the number of markets they serve, they are finding it increasingly important that alternate-language broadcasts are equal in quality to the main program.

Consequently, station engineers are dealing with a much more complex broadcast model and many more audio channels. That's where a well-designed audio processor comes in-by simplifying audio handling and delivering quality sound consistently to viewers.

Key factors in choosing an audio processor relate directly to the system's ability, now and in the future, to meet the needs of the broadcast station as well as federal broadcast regulations for digital television. When evaluating possible purchases, broadcasters should consider loudness control and surround sound image control, as well as the ability to handle both two-channel and 5.1 programs, and whether a processor contains built-in audio encoding.

QUALITY ISSUES
Loudness issues are among the chief concerns of broadcasters shifting to DTV. When the February 2009 deadline hits and analog broadcasting goes away, everyone will be watching and listening to digital feeds. At this point, broadcasters' loudness issues can turn into regulatory issues as well.

Loudness issues tend to be rooted in the simple fact that, for obvious reasons, not all programs are produced the same way. A long-form drama will necessarily be produced far differently than a soft drink commercial. Further, programs with a large dynamic range (i.e., a large variation between the softest and loudest sounds) also are more difficult to match to other programs, causing loudness to jump at program and commercial transitions.

Rather than simply applying a permanent "fix" at the transmitter that affects all programs, it is important to look for a processor with the ability to separate loudness from dynamic range and to provide the broadcaster and the consumer with control over the each.

Metadata support enables the audio processor to take advantage of an adjustable mix of traditional and metadata-based dynamic range control, so regardless of the type and quality of the home viewing environment, the viewer's decoder delivers audio within prescribed soft and loud limits.

The end result is that all viewers receive a satisfactory broadcast that falls within acceptable loudness ranges by default, and those viewers with high-end home theater systems can elect to hear the programming with the audio reproduced as originally intended.

5.1 IMAGE CONTROL
Though 5.1 sound is growing in popularity, a great deal of stereo sound content, both new and legacy, remains on the airwaves. Proven upmixing capability is important, as is operating in a mode that allows 5.1 audio signals to pass and stereo audio signals to be converted into 5.1 without glitches. With this capability integrated into the audio processor, broadcast facilities can output consistent 5.1 audio and keep the blue "5.1 light," indicating top-quality audio to consumers.

The audio processor's upmixing capabilities in integrating local two-channel audio with 5.1 audio from the network are also key to the overall quality of the broadcast. Internal detection of two-channel audio, which sometimes is mistakenly signaled as 5.1 channel audio, also helps to preserve accurate presentation of dialog.

While the digital standard specifies delivery of up to 5.1 discrete audio channels to all viewers, regardless of the number of speakers they use, a processor should provide a two-channel downmixed output suited to legacy stereo distribution paths or for local audio monitoring at the station.

METADATA SUPPORT
DTV audio also incorporates metadata that describes the audio, and that information is as important to get right as the audio itself. Managing both audio and associated metadata is a big job, particularly in a multichannel environment. Add to this the challenge of inserting local programming prior to transmission, and there is plenty of room for error. Look for an audio processor with metadata support that can streamline many of the processes necessary for delivery of audio.

Building metadata support into an audio processor is a difficult task, but the resulting functionality is valuable, particularly in multiple-channel DTV service. Can a broadcaster get to air without metadata support' In the short term, absolutely. But as each broadcaster's DTV services become more complicated and sophisticated, metadata support is a must. Rather than make a second-round investment in a more robust audio processor, the broadcaster has the opportunity to make this choice up front.

Metadata includes parameters such as dialogue loudness (dialnorm), dynamic range control (DRC) and downmixing (reproduction of a multichannel program for delivery by less than a 5.1-channel home theater system).

Through metadata, program loudness can be tagged via the dialnorm parameter and then normalized in decoders. The audio processor can read these existing tags, make sure that they are valid for the audio program, and adjust if necessary.

Correct metadata can dramatically reduce-or even eliminate-changes to the audio program that a processor would need to make to control loudness. Basically, if program producers get metadata right, less has to be done downstream, but a safety net is obviously required for cases where this is not true.

SUPPORT AND FUNCTIONALITY
Support for insertion of local, network and emergency alert audio is a necessity. For broadcasters who just need to plug in and get on the air, a processor with multiple inputs and easy methods for switching among them can dramatically simplify this process and save stations from having to purchase, install, and configure additional switching and mixing equipment.

In choosing to work with a particular product and vendor, the broadcaster should consider the support that comes along with the purchase. Above and beyond issues of audio processor functionality, the broadcaster should find a knowledgeable, well-connected partner that can help the station ensure the quality of audio, no matter the cause of any glitch or problem.

A company well-versed in broadcast standards, digital transport stream protocols and audio coding systems can be a valuable ally in working back through the transmission chain to track down and solve a problem affecting the quality of DTV broadcasts.

Another consideration in purchasing an audio processor is the maturity of the product's technology. Leading vendors have moved on to second-generation processing, having refined the performance and functionality of earlier-generation systems. With broadcasters increasingly under the spotlight, the reliability and prior track record of the audio processor technology is important.

The box also should include all the options the broadcaster needs for managing audio. For example, some products include Dolby encoding and can deliver space, setup and cost savings. While Dolby Digital (AC-3) is the mandated ATSC audio standard for the United States, broadcasters also should evaluate their needs to be sure they don't also need support for other standards, such as aacPlus or DTS encoding, used across the industry worldwide.

One question to ask of the vendor is whether or not it's a licensee of all four of these top encoding technologies. The longer a vendor has worked with these different technologies, the more likely it is that there will be transparent output of signals from the audio processor in the appropriate format.

While international standards are still in flux, U.S. federal standards for digital broadcasting aren't going to change significantly in the near future, and the investment in an audio processor of quality will stand a broadcaster in good stead as the February 2009 deadline approaches.

There are no curve balls coming that a well-designed processor can't handle. Today's technology can support 6.1 or 7.1 digital broadcasts, but what's more important now is getting DTV to air in a way that satisfies not only the FCC, but ultimately the viewer.

Whatever the broadcaster's needs going into 2009, there is an audio processing solution and vendor that can help ensure the digital transition is a success. The best bet for those looking to buy an audio processing system is to be willing to invest in a complete system up front and to find a vendor who can serve as a partner in building and maintaining high-quality DTV services.

Tim Carroll is president and founder of Linear Acoustic in Lancaster, Pa.


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