Those of us who create and manufacture digital cinema projection equipment face the challenge of knowing what standards to aim for as frame rates jump from 24 to 48, 60 and beyond. At 24 fps, the DCI-specified peak bit rate of 250 Mbps for the picture is satisfactory. It seems logical then that, by doubling the frame rate to 48, we also need to double the bit rate to 500. Indeed, the general industry direction for exhibiting 48 fps 3D material is leaning toward a bit rate of 450 Mbps, leaving some room for peaking to 500.
As we move toward HFR and its necessary twin, high bit rate (HBR), the whole production and exhibition chain must move in unison. Cameras, servers, IMBs and projectors all have to be modified and advanced to keep up with the necessary speeds. Nowhere is this more evident than in the giant screen venues, which require multiple synchronized projectors and servers that can handle high bit rates and high frame rates. As an industry facing change, we need to come to agreement on what is necessary and update the DCI and SMPTE specs for the D-cinema industry. Before we can, we need to address some technical issues, namely the need for HFR content to test, how to measure frame rate specs of equipment, and what to do with mixed content within the same show. An additional concern is the special needs of Giant Screen exhibition, namely servers capable of streaming 4K 3D data to dual synchronized projectors.
First, there is the problem that testing new equipment at high frame rates and high bit rates requires content. This won’t be an issue if filmmakers begin filming at 48 or 60 fps. We’re looking forward to The Hobbit presentations later this year to see the full potential of the media.
Another technical issue is the varied ways in which bit rate is measured. We saw at NAB and CinemaCon this year that most manufacturers of D-cinema projection equipment now stream 48 fps data at aggregate speeds of 500 Mbps. However, this does not necessarily mean that all the internal independent components within the JPEG 2000 codestream, each of which may have limitations, can run at bit rates of 500 Mbps. DCP providers need to be aware that these limitations exist when making decisions about mastering. We will need to ensure that manufacturers report both aggregate and component bit rates.
An interesting dilemma that has not yet been solved is what to do with content of different speeds played in the same show. Servers and projectors will behave differently when switching between content with different frame rates and this can lead to viewing problems. What if a 24 fps trailer is played before a 48 fps presentation of The Hobbit? We will need to hear from exhibitors and content owners about want they want to provide in terms of an acceptable user experience. The Inter-Society Digital Cinema Forum (ISDCF) is aware of this problem and has been conducting tests with various manufacturers and content owners involved.
One major limitation to implementing HFR stereo 3D that we have solved is that of moving data at sufficient speeds from server to projector. This has been accomplished by swapping the HD-SDI cable for an Ethernet connection, as well as embedding IMBs in projectors. IMBs have now become industry-standard equipment shipped with all servers, and a necessary part of any new spec. Coupled with improvements in Series 2 projectors, including image brightness, the IMB’s increased speeds will certainly enhance image quality and alleviate some of the viewer discomfort during stereo 3D projection.
How Qube handles these challenges
The Qube XP-I server is capable of a bit rate of 1 Gbps, while each Xi IMB can handle up to 500 Mbps, with no component bit rate limitation. This is in keeping with current storage throughput and image decoder specs. Qube servers have the same component and aggregate bit rates.
The Qube XP-I server and Xi IMB are capable of frame rates up to 120 fps per eye. This gives a frame rate of up to 240 fps for dual projection driven by a single server streaming a single DCP for stereo 3D.
Qube has also shown that exhibition of 4K 3D content on Giant Screens is possible from a single XP-I server, streaming data at 1 Gbps and 30 fps to dual synchronized ultra-bright projectors. This greatly enhances the 3D viewing experience at Giant Screen venues.
When updating digital cinema specifications, we should aim high with respect to HFR in anticipation of where filmmakers might go. In this way we will be future proofing the next set of standards.
Rajesh Ramachandran is the CTO of Qube Cinema.