radiomagonline.com

New HD Radios are capable of delivering stereo compatible surround sound formats. There are a few separate and very different surround sound technologies that are competing for adoption by radio stations converting to digital broadcasting. Although the surround solutions aren’t likely to be compatible with one another, they all will be compatible with two-channel HD Radio tuners, which will play the surround broadcasts in two-channel form. We’ll soon dwell into how they do it. iBiquity already has endorsed the installation of Circle Surround II encoders at AM and FM stations. Stations use Circle Surround encoders to transmit matrix 5.1-channel music from such sources as multi-channel DVD-Audio discs and Super Audio CDs. Many home and car audio systems already are equipped with Circle Surround II decoders, and they would reproduce the multi-channel version of a song the way it was mixed. Home and car systems using other surround decoders would also reproduce the music in surround sound, but not necessarily in the way that the music was originally mixed. iBiquity has also endorsed Pro Logic II Surround Sound and Neural Audio’s Surround Sound. One of the technologies, developed by Neural, has already been adopted by XM Satellite Radio. Neural claims that the sound broadcasted can be decoded by Circle Surround and Dolby decoders, however stating that Neural’s own decoder will deliver better channel separation and stability. The location of a voice, for example, won’t be pulled toward the channel with the dominant music passage. Now – this is all very nice but they don’t necessarily work as well as the companies say. Made it this far? We’ll go into a bit more detail below.

So how is Surround Sound possible in HD Radio?

So far there are three main techniques that can be used to broadcast surround sound over the HD Radio system: discrete, matrix/watermark and MPEG Surround. Quality chart shows pretty much what we would prefer but lets see the up- and down-sides of all systems a bit closer.

radiomagonline.com

Discrete 5.1 broadcasting

A discrete system discretely encodes 5.1-channel programming on the IBOC signal, using a 5.1 channel version of the already deployed HDC codec. For radio broadcasters – the technique is far from ideal however because of the bandwidth limitations in HD Radio. Even with the minimum quality, discrete surround sound encoding would use up all the available bandwidth, eliminating the possibility of multiple programs or data services (Doesn’t bother me but I don’t own a radio station and don’t pay the bills). At a minimum, discrete coding would require downmixing inside your radio for stereo-only environments, complicating implementation. Pretty much making it impossible for anyone considering broadcasting in discreet surround to have anyone listening. Otherwise thumbs up and audiophile wise – 10 out of 10. Can’t get better than that! Please will someone see the light…?

Matrix Systems

Matrix systems, such as SRS Circle Surround, Dolby Pro Logic and Neural Surround use matrix encoding to encapsulate a surround signal within a stereo signal. The L-R energy of the stereo signal is used to carry the surround sound information over standard stereo radio wave using phase difference. This allows matrix-encoded surround sound programming to be transmitted on the analog and digital portions of an HD Radio signal. When such an encoded signal arrives at a radio with a same brand matrix decoder, the stereo signal is upmixed for 5.1. Also, because the stereo signal is already the basis of transmission, the program can play back in a stereo-only environment with no modifications to the radio. The downside of matrix techniques is accuracy. Due to the limit on the amount of information that can be transmitted using matrix techniques, the quality of the surround mix cannot match the original 5.1 channel mix nor can it exactly match the artist stereo mix (one can’t fit more than 2 channels of audio to 2 channel broadcast without losing information). Also, by encoding the surround sound as part of the stereo waveform, it does not take full advantage of the digital information path offered by HD Radio. Anyone who has ever worked with matrix systems will know this and that’s the main reason this is near always overlooked.

MPEG Surround

MPEG Surround is a compromise between the discrete and the matrix methods. It provides the accuracy of discrete encoding while preserving the stereo compatibility and bandwidth efficiency of the matrix encoding. Born out of collaboration between LSI Corporation(previously Agere), Dolby Coding Technologies, Fraunhofer and Philips, MPEG Surround is a compression technique in the process of being standardized by the Moving Picture Experts Group. It is a way for digital system to transmit high-quality surround sound in a way that is backward compatible to stereo. MPEG Surround adds a side-information stream to the (mono or stereo) core bit stream, containing spatial image data. Stereo playback systems will ignore this side-information while players supporting MPEG decoding will output the reconstructed 5.1 surround mix. Since incorporating high efficiency AAC codec’s it is possible to broadcast in 64kbps or less. The nature of the MPEG Surround technology allows it to scale in quality proportional to the amount of bandwidth allocated to the surround signal. Quality in this case is measured by the ability of the encoded signal to accurately reproduce the original 5.1 channel audio signal sent into the encoder. MPEG Surround would provide surround sound to a stereo broadcast with room for extra audio programs or other digital broadcast services.

Most of the surround sound broadcasts used nowadays will be encoded and then decoded at the receiver. Meaning – if you don’t have the surround decoder – you’ll get a very different mix indeed. The thinking that lies in our stereo broadcasting stations is “We don’t do matrix and we don’t want to “cram up” airwaves with quality”. And that is the reason why it’s near impossible to implement a surround only station (at the moment). Understandable, as not everyone has enough speakers connected to their radio…except in their car. And well to be honest – who listens to radio in anywhere else than in his/her car. And if most cars come with 6 or more speakers – then why is there only us common folks that see the light? In the long run it is inevitable that the transition will come. And it might be closer than we think as Neural Audio has teamed up with DTS (Digital Theatre System – started by Steven Spielberg), THX (Tomlinson Holman’s eXperiment –THE standard for surround sound quality), car radio manufacturers (Denon, Onkyo, Pioneer Electronics (USA) Inc., and Yamaha) and satellite radio Sirius XM (Sirius XM already offers surround sound programs). Neural Surround car entertainment systems are available to all new cars. And compared to Satellite Radio – HD radio is the same terrestrial AM/FM that’s been and, touch wood, always will be FREE. The only bad thing is for all of us in Europe – HD Radio is only available in US – we’ll have to do with DAB. The users of HD radios continues to grow (below is a graph by Bridge Ratings according to their survey regarding growth projections). To add to that people that will buy a radio if it does surround – add another 2 people on the list. And if you are reading this article – add 3.

www.bridgeratings.com