By Yasmin Hashmi, 3Droundabout
3D-only broadcasting may be some way off, so in the meantime, as with mono AM and stereo FM radio, co-existence with 2D will remain a practical necessity. The challenge for service providers is in making 3D transmissions backwardly-compatible so that the same infrastructure can be used for 2D and 3D content.
The key technology in first-generation of 3DTV broadcasting services is frame packing, where the left and right image are packed into one video frame with twice the normal bandwidth. This squeezing of the left and right images into a single high-definition frame has the advantage of allowing the service provider to reuse part of the existing production infrastructure and the whole of the existing distribution infrastructure, including most HD set top boxes already deployed at the users’ premises.
The drawback however, is that if frame packing is to be used in a transmission system without an increase in bandwidth, it will require subsampling/decimation of the left and right source images to make each squeezable into half the size of an HD frame. Depending on the chosen technique, this may have undesirable consequences.
In the case of vertical or horizontal subsampling, the image resolution of one direction will be different to the other, causing a degradation of the perceived image quality. If the applied subsampling technique is staggered subsampling or quincunx, the balance between horizontal and vertical resolution is preserved, but at the cost of a loss of diagonal resolution.”
In order to produce the composite image, the staggered pixels must be aligned, giving rise to a loss of correlation where the edges look somewhat jagged, and consequently, the following encoding stage may generate compression artefacts, and/or a higher bitrate is required in order to obtain a satisfactory quality. Moreover, due to the geometrical layout of the composite frame which contains both the left and the right views, the resulting video will not be backwardly compatible with 2D TV sets – legacy HD decoders will be able to decompress the video, but 2D TV sets will display the composite picture and not, as would be preferable, one of the two (L or R) images that form the composite.”
A new frame packing system
To overcome these problems, Sisvel Technology has devised a new solution, the 3D Tile Format, that allows the storage of two 720p frames in a single 1080p frame. In this case, if the L and R pictures are originated in the 720p format, no decimation is needed and the reconstructed L and R pictures will preserve their original resolution and will not suffer from an imbalance in the vertical and horizontal resolution.
In addition, since one of the two images remains unaltered, the resulting format can also be made service compatible.
If H.264 encoding parameters are managed properly, this enables the deployment of a service-compatible frame packing format.
Part of the metadata encoded in the H.264 bitstream is the ‘cropping rectangle’, which used to signal to the decoder the part of the decoded frame that has to be output to the display. This parameter was introduced because the encoder is able to process images having a vertical and horizontal size multiple of 16 pixels (the size of a macroblock), while 1080 cannot be divided by 16: thus the cropping rectangle has been defined to cut out a 1920 x 1080 image from the 1920 x 1088 used by the encoder.
A legacy decoder, while receiving an H.264-encoded 3D signal would display only the area of the frame enclosed by the cropping rectangle. In other words, existing H.264 decoders, without any change in firmware, can provide a 2D-compatible picture, while requiring a simple firmware upgrade (through downloading) in order to be able to feed a 3D display.
In summary, the 3D Tile Format, along with in-band 3D-switch signalling or H.264 SEI messages offers the following advantages:
• If the original L and R pictures are in 720p format, there is no loss of resolution both for the 3D picture and for the compatible 2D picture.
• The imbalance between V and H resolution caused by common frame packing formats is avoided.
• Service compatibility is assured, namely an H.264 HD legacy decoder, without any software change, can feed a 2D display, and all that is required to make the decoder able to feed 3D displays is a simple software patch download.
The 3D Tile Format is a new form of 3D broadcasting that is fully compatible with 2D television displays, allowing 3D and 2D viewing of a single broadcast stream. It uses an innovative technique for formatting stereoscopic images that integrates two 720p frames within a single 1080p frame. The reconstructed right and left images maintain full 720p spatial and temporal resolution, giving viewers of both versions the full benefit of the original picture.
The 3D Tile Format also promises to provide better transmission quality of 3D content than current solutions, and, because it is backwardly compatible, allows broadcasters to transmit to both 2D and 3D users without the need for increased bandwidth.”
The 3D/2D-compatible system is already in use at QuartaRete TV in the Piedmont region of Italy as part of its DVB-T broadcast service and is being tested for implementation by several broadcasters elsewhere in Europe.