Practicalities of surround
Aug 1, 2005 12:00 PM, by Chriss Scherer, editor
Now that the FCC is soliciting comments on NRSC-5, there has been renewed interest in the IBOC rollout. While the standard defines the basic operation of the transmission system, the enhanced functions of data, surround sound and multicast operation have yet to be fully defined. Several stations now transmit multicast programming, and multicast receivers are becoming available to consumers.
Surround sound for IBOC is not yet mature. While some of the surround technologies are established, there's no easy way for consumers to decode the signals. This is advantageous for broadcasters, because few stations have the capability to provide a significant amount of programming anyway. This could change soon.
Surround sound has potential for success with IBOC. While most radio listening is casual, which lends itself to the additional program offerings of multicast, surround sound will likely find its initial niche as a special feature medium. Concerts and special programs are an obvious possibility. Some classical stations may consider surround for more regular use.
Searching for standards
There are currently five surround systems that have been demonstrated for use with HD Radio, so there is no single standard. Three of them, SRS Circle Surround, Neural Audio's surround system and Dolby Pro Logic II have been approved by Ibiquity as compatible with HD Radio. The other two systems � from Fraunhofer and Coding Technologies � operate differently and have not yet received the Ibiquity stamp, although they have been shown to work with the HD Radio system.
As it is, there are few receivers available to decode a surround signal, so there is not much incentive for stations to begin regular broadcasts. SRS Circle Surround or Dolby Pro Logic II can be found in several models of home media systems, and Kenwood announced that it will release a mobile receiver for many formats, including HD Radio and Circle Surround.
The five systems can be divided into two distinct types. Circle Surround, Pro Logic and the Neural system all encode a surround source into a stereo audio path, while the Fraunhofer and Coding Technologies systems create a stereo channel and an associated data channel that is used to create the surround image.
Because the first three encode the surround information into a stereo signal, it is easy to understand how these systems can work with HD Radio. In reality, they could work with any stereo audio path. As long as the appropriate decoder is installed in the receiver, any or all of these systems could be used. Because the surround audio is encoded into a stereo path, compatibility with non-surround systems is maintained by not decoding the imbedded signals. The SRS and Dolby systems are matrix processes, and the Neural system is a watermarked process.
The last two are different because they split the data path of the FM signal into two portions. Instead of using 96kb/s for the entire stereo audio path, Fraunhofer and Coding Technologies use a large portion of the path � about 80kb/s or more � for a stereo signal, and up to 16kb/s for the surround data. The surround data is created in the encoding process by comparing the surround mix to the stereo mix. The stereo portion is left intact for systems without a surround decoder. For simplicity, I'll refer to these as discrete systems.
The HD Radio signal is a data path, and with the development of multicast, this path can be allocated with different bit-rates for different uses. Again, more than one surround system could be implemented and the receiver could detect which process was used for the encoding.
The surround sound formats have different needs for transmission.
Fraunhofer's system is touted as being compatible with MP3, AAC, AAC Plus and other codecs. Coding Technolgies uses the HDC coder, which it built for Ibiquity.
Surround around the station
Before a surround signal can be transmitted, stations must be able to work with surround content. There are several points along the signal where surround sound can be implemented, but we'll start at the beginning of the audio air chain and move forward.
Creating surround content will likely be the most challenging for any station. While music may be provided to the station in a surround format, live concerts or other station productions will need surround capabilities. In addition, if any program elements are delivered in surround, station imaging would sound better in surround too, or it will sound flat compared to the program.
The audio production needs for a surround sound studio can be handled by many audio systems today, although some are not broadcast specific. Still, radio console manufacturers are answering this call, as was demonstrated at NAB2005. The router-based broadcast console manufacturers have shown that they are ready to meet this need.
If surround content is delivered to a station, it will need to be encoded into the transmission format being used. All the systems have encoders and decoders to do this. Each audio entry point will need an encoder, as will each audio monitoring point need a decoder.
To encode and decode audio with the matrix and watermark systems, Neural offers the 5225 Upmix decoder and 5225 Downmix encoder; SRS has the CSE-07D and CSE-07 hardware encoders, and CSD-07D and CSD-07 hardware decoders, as well as VST and TDM software plug-ins; and Dolby provides the DP563 encoder and DP564 decoder. Fraunhofer and Coding Technologies do not have commercially available encoders and decoders at this time.
Creating the stereo mixdown from the surround source can be a source of heated debate. The matrix and watermark systems perform this function automatically. The discrete systems can take advantage of using a manual mixdown for the stereo version. While a manual mixdown could maintain some artistic control, the amount of material being encoded may make this step impractical. A compromise will have to be made between the expended effort and the desired result.
Once audio is encoded into the surround format, it must be stored for later playback. The matrix and watermark systems encode to stereo files, so nothing needs to be done to handle the files. The discrete systems create two elements for the audio file. Computerized audio storage systems can easily store and deliver these files. Enco and Broadcast Electronics demonstrated their systems' abilities to deliver discrete surround-encoded audio at NAB2005.
The final step is routing and delivery of the surround content. Again, the matrix/watermark systems are ready to go because of the existing stereo infrastructure. The ability to transmit one of the discrete systems will depend on the facility itself. An analog infrastructure will need to be upgraded.
The discrete-encoded audio is transmitted as a bitstream, but this is not a traditional digital audio stream. Some of the router-based audio systems have demonstrated their ability to carry these signals.
So if your station decides to carry surround-sound audio, which system is the one to choose? This is a decision that the station will have to make on its own, but that decision can be tempered by weighing the various aspects of the systems.
The matrix and watermark systems work with existing stereo facilities. Audio encoded in this way can be mixed and edited with little or no effect on the encoding. However, surround aficionados may notice that the surround audio may not be exactly like the original. In addition, there is concern that the encoding process may introduce unwanted artifacts, such as excessive stereo difference information.
Meanwhile, the discrete systems maintain the stereo quality and accurately replicate the surround field. The drawback is that existing stereo facilities are probably not yet able to route and mix the encoded audio. The matrix/watermark systems have the advantage that they are easier to implement, and can be done so gradually.
The ability to transmit surround-sound programming adds a sparkle to IBOC's consumer acceptance, and it provides another element to keep terrestrial radio in step with other forms of digital entertainment. While the specific format used widely has yet to be determined, stations should plan now for a possible surround-sound future.