Commentary: Where does Surround Sound Fit Into the Digital Jigsaw Puzzle?

There is no doubt that surround sound enhances listeners' experience. Movies have been using it for years. Television joined the fray, first with "movie night" presentations, then with sports, special events and most prime-time programming. Now that DTV has found its stride, many of those broadcasters want to provide everything in surround.
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There is no doubt that surround sound enhances listeners' experience. Movies have been using it for years. Television joined the fray, first with "movie night" presentations, then with sports, special events and most prime-time programming. Now that DTV has found its stride, many of those broadcasters want to provide everything in surround.

There's no reason digital radio should be any different. Or is there?

Digital radio is just beginning to define itself. Years of technical machinations seem to be (nearly) over, letting the programmers begin thinking of how they would like to use this new tool.

There are many different possibilities, some of which may conflict. An examination of the requirements of each major component of the system can help find out where surround sound can find its place.

Listener requirements

Beginning with the listener is the best way to keep a clear view of what we ultimately would like to accomplish with digital radio. Listeners can be found in the home, in cars and in portable situations.

Home listeners fall into two major categories: the kitchen radio/background noise folks, and those who listen in a more controlled environment, for specific programs and entertainment.

The kitchen radio listening experience will probably not require surround; in fact, a mono signal is probably most appropriate.

The "entertainment" listener probably already has a home theatre setup, with a well-defined (and comfortable) sweet spot, so this listener can benefit from something a lot more enveloping than the stereo delivered now. Just about any programming, except for the talking heads, will benefit from a surround presentation. Home listeners don't change stations very often, and will probably not listen to a station that does not offer consistently clear reception.

In-car listeners are different. The "drive hour" programs are also very different, with much more information than entertainment programming. These listeners change stations more often, and suffer from intermittently poor reception problems that are out of their control.

One of the main objectives in a car is to provide a much larger sweet spot than conventional stereo, and a sweet spot that includes listeners in the rear seat. A surround system can offer this, but has to avoid flipping back and forth between surround and a stereo or mono sound field as the reception conditions change to avoid annoying the listeners.

Absolute spatial fidelity is not required in a car, and may in fact be undesirable if it penalizes some of the listeners in the vehicle. A surround system that works with the FM stereo signals as well as the IBOC signal, and that might even distribute the mono FM signal (recovered under severe reception conditions) to all the speakers would minimize listener annoyance.

Portable applications have other limitations. Size, weight and power consumption limit the amount of technology that can be stuffed into receivers.

They suffer from the same (or worse) reception problems as car receivers, but their headphones do offer perfect, consistent speaker placement. This allows technologies like Dolby Headphone to offer a surround experience.

Broadcaster requirements

Broadcasters know that they have to offer listeners compelling programming and new services. Digital radio can offer multicasting and data services (as pioneered by DTV broadcasters) but has also to continue to provide features like reading services for the blind, without affecting the existing FM main and supplementary services.

Surround sound can help market digital radio, but in the opinion of several broadcasters I have interviewed, surround falls below multicasting and datacasting on their priority list.

The existing station infrastructure is a huge concern. Many stations wired for stereo are not scheduled for a rebuild for at least 5 to 8 years.

Existing libraries, playback equipment, servers, program delivery systems, STLs, etc. are all built for two channels. An all-digital system, connected via Ethernet (or other flavor-of-the-day network) promises a flexible route to a multichannel infrastructure, but is an overbuild and requires a huge change in operational practices.

Multichannel "islands," including local multichannel production facilities, could be built within the existing two-channel infrastructure, but would still have to pass multichannel signals through two channel paths.

A five-to-two-to-five channel encoding and decoding or matrixed system, such as Dolby Pro Logic II, could solve these problems, and would serve the roughly 34 million currently deployed Pro Logic decoders.

Program providers

Program providers are the folks who make the music, produce the jingles and create the ads. They like surround. To paraphrase one advertiser, "Mono tells listeners about (the product), stereo helps to create the illusion, but multichannel drops listeners right into the environment I'm trying to create."

If recent Audio Engineering Society workshops are an indication, the studio community is experimenting enthusiastically with multichannel production and recording techniques. Surround proponents have the advantage of already being "islands," so they can produce multichannel and two channel matrixed versions of their products for distribution within the existing infrastructure. The multichannel versions can be archived for the day when that becomes the common format.

IBOC requirements

The digital radio system simulcasts programs on the IBOC and FM carriers. This allows the IBOC signal to "blend" back to the FM analog signal, protecting the program when poor reception makes the digital information unrecoverable. It also uses the FM signal for tuning, avoiding a few seconds of delay while the IBOC buffers fill with data.

Multicasting shares the 96 kilobits per second of data delivered by IBOC between services, making the blending and tuning functions more complex.

The subjective quality delivered by digital radio depends mainly on audio codec design and data rate. Subjective tests of Ibiquity's HDC audio codec done with stereo program material last year showed that a significant portion of the listeners were not satisfied with the quality for data rates of less than 48 kbps.

It would be prudent to do more testing before committing to an aggressive multicasting format.

In conclusion, surround has many benefits, but has to fit within the many restrictions imposed by the digital radio system, existing broadcast plants and the new service offerings being planned.

RW welcomes other points of view.

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