Surround-sound broadcasting has been a hot topic for both terrestrial and satellite radio. As with any enhancement to an existing broadcast format, backward compatibility is a must. The systems proposed or in use for surround radio adhere to this requirement, delivering surround audio to properly equipped new receivers, without compromising existing mono or stereo reception on existing devices.
Yet radio faces a unique problem in delivering such compatibility: The content itself may not allow it.
The bulk of surround-sound material that radio will broadcast is likely to come from the growing number of music releases produced in 5.1-channel formats. But unlike cinematic and television productions that include surround sound, these music releases – distributed on DVD or SACD formats today – generally include separate surround (5.1) and stereo (2.0) mixes of all material, and the two mixes may differ considerably.
Most importantly, because the surround mixes are intended exclusively for surround listening, they are typically not created with consideration of how they will sum – or “downmix” – to stereo or mono. These downmixes often can present suboptimal (if not downright unacceptable) results, pleasing neither the creative community nor listeners.
Naturally, this creates a problem for broadcasters, who do not have the luxury of bandwidth adequate to simultaneously provide two separate versions of their programs, nor the facilities to manage such dual audio inventories. They require instead a hierarchical approach – a single audio source format that addresses surround, stereo and mono listening environments with equally satisfactory results – just as film and TV soundtracks do.
The music industry’s different approach confounds broadcasters with a dilemma on how to best present surround content in a singular form. All the great engineering applied to compatible surround delivery formats will be for naught if the audio content does not allow itself to be transmitted properly through such systems.
Consider how FM stereo would have fared if much of the stereo music of the day had summed poorly to mono. Despite the backward-compatible design of the FM stereo multiplex system, the format likely would have languished and eventually failed if most broadcast content had been created in mono-incompatible form.
We believe surround sound broadcasting is an important part of radio’s future, but fear its prospects are threatened by the lack of downmix compatibility in its primary content source. We suggest that the music and broadcast industries work toward a compromise solution on this point, for their mutual benefit.
While we appreciate the artistic and technical reasons behind the music industry’s preference for creating surround mixes that are unfettered by compatibility concerns, we remain confident that satisfactory, downmix-compatible surround music mixing can become standard practice, as in the movie and television industries. Without such an effort, surround-sound radio broadcasting may never reach its potential.