Planning for Surround

January 9, 2012


In today's marketplace, radio station programmers and managers are looking for new and creative ways to attract and keep listeners. One way is through new technologies such as HD Radio and the enhancements it brings. Multicasting is rapidly gaining momentum as a way to provide more diverse programming choices to the listener. Another idea, which is not new but has once again surfaced, is the concept of implementing surround sound on FM radio.

Multi-channel broadcast history

Multi-channel audio is not a new concept for radio. The first FM stereo broadcasts began in 1961. In the early '70s we saw the era of Quad come and go, but it never really caught on. The '80s brought FMX, which was thought to be an answer to improved separation and reduced noise, but arguments about multipath distortion effectively put this idea to rest. The '80s and '90s introduced us to new technologies in recording methods designed to create an effect of 3D audio. Some of these effects were amazing, especially when listening through headphones. Some, however, had mono compatibility problems. It seemed like surround was another great idea that wouldn't pan out because of technical issues.

So with a history like this, why should we try it again? Because the time may finally be right. Today we have come to expect surround sound from our experience with movies and home theaters. The format most compatible for broadcast radio is 5.1 surround. Figure 1 shows the layout of this system.

Figure 1. The proper orientation of a surround system.

Surround sound might just be one of the killer applications to attract and keep the radio audience not only for HD Radio, but also for FM analog signals. But what will it take to actually get it on the air? Many stations have already spent huge sums during the past decade rebuilding and consolidating their studios and may not be too excited about spending even more money to add surround. Fortunately, there are several options to consider when implementing surround that range from easy and inexpensive to literally changing the entire infrastructure of the studio design. Let's start with the basics.

If the primary goal is to get surround sound on the air and the budget is almost non-existent, consider a music-only implementation, such as that shown in Figure 2. All that is needed is a source with the ability to play back the original discrete five-channel surround material and an encoder. The surround-sound encoder downmixes the multiple channels into a two-channel audio format and encodes the surround information within it. The two-channel signal can then be transmitted via traditional FM stereo. From this point on, the audio infrastructure remains unchanged as a normal stereo signal is fed through the station's air chain to the transmitter. It remains unchanged until the listener's receiver splits it back into 5.1 channels. Listeners who do not have a surround decoder will hear the audio just fine in stereo or in mono because the surround information is ignored.

Don't stop at just the music

This basic method of implementing surround allows a station to claim bragging rights to surround, but stopping there misses out on what could truly be the biggest part of the wow factor. Music in surround is important, but it's the locally produced elements that give a radio station its personality. When a listener hears a great song in surround followed by a sweeper that's not recorded in surround, what happens? The station's sound suddenly falls flat and all of the energy is lost. The local elements such as sweepers, promos and even commercials can provide a show place where the station's production creativity can really shine.

Figure 2. A music-only implementation of surround.

HDTV owners know what it's like when the new set is brought home. They usually look for shows broadcast in HD first and watch programs just because they're in HD. The same holds true with surround sound on radio. Listeners will be so excited over their new toy that if they hear something exciting, they may stay with the station and listen right through the commercials. Think of how happy your clients will be knowing that people are actually listening to their messages rather than changing the station.

To produce local content in surround, a greater investment than that for the basic surround studio is required. Figure 3 illustrates a full surround production room. The heart of the studio, the control console, must be capable of carrying the five audio channels. An older control console may have program, audition and utility buses that can support the five channels. Many newer control surfaces may already have surround capability. The DAW must support the 5.1 channels as well. In the Figure 3 configuration the console output is routed through the surround encoder to create the two-channel downmix. Again, from this point on, the infrastructure remains unchanged.

These two examples are economical ways to convert an existing studio to a surround studio. However, if you are designing a new facility and have the luxury to start from the ground up, consider keeping the entire audio flow in the discrete five-channel format. In this case, the design of the production room is similar to that of Figure 3, except the audio is stored on the automation system in a five-channel discrete format. This requires considerably more hard drive space as well as audio routing switchers with more channel capacity and surround output audio cards in all of the automation workstations. The discrete output of the control room console now feeds the surround encoder, which converts the source to the two-channel stereo mix compatible with the air chain.

Why keep the audio in the full discrete mode? Future flexibility. While our current transmitting facilities can only handle two channels, you never know what the future may bring. In the future we may not be limited to two channels. There are also new methods of transmitting the surround sound information on the way taking advantage of data channels on the HD Radio carrier. With a data-imbedded discrete system, such as the MPEG Spatial Surround system, the artistic stereo channels are transmitted along with a spatial data bitstream to give the receiver the surround information. If you're building from scratch, don't cut corners or you might be sorry later.



Proper monitoring

No matter how far you decide to go with your studio conversion, it's extremely important to include proper surround monitoring in the design. Surround monitoring doesn't have to be complicated, and can simply be a consumer-style receiver with a built-in surround encoder. Install surround monitoring in the production room and the control room. The station is distributing a special product; let the announcers enjoy the surround experience. The more ears listening to the final broadcast, the more likely that problems will be prevented.

Figure 3. A full-surround production studio implementation.

The monitor installed for the production rooms should also have the ability to decode all of the major surround systems: Dolby Pro Logic, SRS Circle Surround and Neural Surround. In the near future you will also begin to see the MPEG Spatial Surround on the market for use with HD Radio. When producing a spot, listen not only to the final product through the chosen surround decoder, but also through the other decoders. The radio station may be transmitting in Neural Surround, but the listener in the car has his radio set for Dolby. You need to know what the listener will hear.

Cross-compatibility listening tests have shown that for the most part the listener will have a pleasant experience whether listening to the proper decoder or someone else's, but don't let your guard down.

Proper training for the production staff is also a must. While many production managers may have been in the business for many years, surround production will still most likely be new to them. Train them to produce elements that will capture the audience.

What about multipath?

Multipath distortion has been responsible for squashing previous attempts to improve audio separation. The varying multipath signals can cause noise and distortion in the received audio signal and can even cancel the carrier altogether. And excessive stereo separation can increase the effects of multipath. Because surround systems encode their surround information in the stereo channels, the L-R channel of the composite stereo signal may increase in level and cause issues with multipath. Tests have shown that, in most cases, the encoded audio has little, if any, affect on multipath. If you suspect an increase in multipath, view the L-R channel on a scope with and without the surround encoding to see if there is a major change in the L-R signal. You may actually find high L-R energy in the non-encoded signal to start with. Don't assume the surround is the cause of the problem.

Don't forget multicasting

Multicasting tops the list of the benefits that HD Radio has to offer. If it were to come down to a choice of broadcasting an HD2 channel or broadcasting in surround, the HD2 choice would win. For this reason the surround proponents have been challenged with making their systems work well even at reduced data bit rates. Early testing at rates of 64kb/s and below are showing promising and impressive results. When building new multicast stations don't forget to allow for surround, even if it's not used at first.

The future of radio is not grim, but can be rather exciting if we continue to move forward with new HD Radio channels, new programming formats, new digital features and new improvements to audio such as surround sound. When remodeling a facility, keep all of the new technologies in mind during the design stages so that you will be prepared when asked to turn on a new feature. This will help you win the game and become the hero.


Fluker is the director of engineering for Cox Radio, Orlando.



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