In these early days of 3-D broadcasting, side-by-side technology appears to be the dominant choice and has been used successfully on a number of sports projects (including the current World Cup tournament on ESPN) to date. Although no formal standard has been set for transmission, side-by-side transmission works for broadcasters because it saves bandwidth and operates within the MPEG-2 compressed HD infrastructure already deployed.
The basic patent for side-by-side 3-D display, which expires in 2011, is held by RealD. Essentially, two HD image streams compatible with existing broadcasting standards — “frame compatible” — are compressed as left and right images. They are positioned next to each other horizontally, transmitted, decompressed at the TV set and displayed as a time-multiplexed image. The image display is synched to the LCD shutter speed of the glasses worn by the viewer.
Side-by-side methods are not perfect, however. Mainly, there are limitations in picture resolution, because the side-by-side technique displays two 1080p 3-D images electronically stitched together. And there are a number of varieties of side-by-side transmission, developed by other companies, making it impossible to guarantee interoperability between various broadcast signals and TV sets. Today, 3-D glasses that work with one manufacturer’s TV set won’t work with another brand.
Many consider side-by-side schemes a temporary solution for 3-D transmission. If there is no need to retain frame compatibility with existing MPEG-2-based broadcasting, there are several other ways to transmit 3-D images with no resolution degradation at full 1080p/60 resolution. A popular method is the dual-stream approach.
With dual-stream technology, the left and right images are divided over multiple channels for transmission, reassembled and then displayed by the 3-D TV receiver. Korea’s KCC plans to begin trial broadcasting in selected regions this October using MPEG-2-coded left imagery and H.264-coded right imagery in the 6MHz terrestrial broadcasting waveband.
This design is compatible with 2-D broadcasting, which is essential for terrestrial broadcasting, while enabling simultaneous 3-D transmission. The use of highly efficient H.264 coding for the right image will mean an image with resolution far superior to the halved resolution provided by side-by-side technology compression.
Sending multiple image streams simultaneously allows the TV set to switch between them. The second stream is sent as differentiated data from the first stream instead of the whole right image, making it possible to cut the bandwidth by about 25 percent.
Dolby Laboratories has also announced its own specification for broadcast 3-D delivery, whereby 3-D images are encoded and carried using frame-compatible techniques through a conventional 2-D broadcast infrastructure. The company said that it is fully compatible with currently used side-by-side and over/under approaches, although it favors side-by-side transmissions to maintain the highest-quality HD resolution, especially with interlaced signals. The spec also includes accommodations for enabling the transmission of full-resolution 3-D (1080p/60 HD) in the future.
Another company, Sensio Technologies, has developed a different dual-stream scheme for the high-quality decoding of quincunx (checkerboard) compressed stereoscopic formats. Company president and CEO Nicholas Routhier said the technique retains more resolution than side-by-side methods because it’s deleting (compressing) less picture information. A patent application has been filed that protects the checkerboard decoding technique in multiple markets and applications, from digital cinema to mobile phones.
While the broadcast industry is still debating which technique to establish as an official standard for 3-D broadcasting, some international standards bodies like SMPTE and Europe’s ITU have quietly dropped support for side-by-side broadcasting and are working on new standards based on H.264. The H.264 encoding technology is being adopted for 2-D broadcasting across the world as a replacement for MPEG-2. It also offers a technical advantage that neither MPEG-2 nor side-by-side provide: the ability to broadcast 3-D imagery at the highest full-frame resolution. This is the primary reason why so many people are interested in making H.264 the basis for worldwide 3-D broadcasting standards.