Frame-compatible, in this sense, generally means stereoscopic 3D content that can be read and displayed by current-generation digital cable set-top boxes. Although most cable 3DTV content combines two half-resolution high-definition images, the CableLabs specs (PDF) offer technical parameters for both high- and standard-definition 3DTV streams. The specs, which are publicly available for any industry, not just cable, also address coding and signaling for 3D's top/bottom and side-by-side video formats.
The specs will likely evolve, but this iteration aims to create a uniform, national technical footprint for 3D programming and forge interoperability between cable boxes and 3D-capable TVs. The Society of Cable Telecommunications Engineers (SCTE) , meanwhile, is also chewing on cable's 3DTV requirements and developing work that may morph into more formal standards. (See Cable Engineers Push Standard for 3DTV Signals and SCTE Looks at 3DTV .)
The new spec, dubbed Content Encoding Profiles 3.0, replaces VOD-Content Encoding Profile 2.0, which has been widely adopted by the cable industry. Although the new specs define requirements for VoD, they "may also be used for linear services as applicable," CableLabs spokesman Mike Schwartz confirmed via email.
Another key difference is the addition of profiles for content encoded in MPEG-2 and MPEG-4/AVC formats. The addition is important because MSOs and programmers, save for some early examples, are expected to use the more efficient MPEG-4 codec for on-demand and linear 3D programming.
Comcast Corp. (Nasdaq: CMCSA, CMCSK), for example, is delivering ESPN's new part-time 3D channel in MPEG-4 and requiring customers who have 3DTVs to get a hybrid MPEG-2/MPEG-4 box. Avail-TVN has yet to decide if it will rely strictly on one codec or the other for its upcoming 3D service slate or if it will use transcoding to deliver the format that specific affiliates desire. (See Avail-TVN: All 3D, All the Time.)
'Safe harbor' bit rates
The specs still supply suggested "safe harbor" bit rates that can be applied to 2D (for standard- and high-definition) content and 3D content, which is delivered only in high-definition. The CableLabs specs still advise the use of a 3.75-Mbit/s MPEG-2 transport bit-rate for SD, and 15 Mbit/s for HD. But that's just a baseline number. Fast-moving sports video, CableLabs points out, may require higher bit rates, on the order of 4.125 Mbit/s for MPEG-2 standard-def streams.
Those suggestions are less cut and dry when it comes to using the more bandwidth-efficient MPEG-4/AVC encoding. In that category, CableLabs suggests a minimum of 1.875 Mbit/s and a max of 3.75 Mbit/s for standard-def programming. For HD programming, which would also factor in 3D, CableLabs lists six suggested peak transport stream bit rate profiles, starting with 5.625 Mbit/s and topping out at 18.750 Mbit/s.
For 3D in particular, CableLabs has defined three video formats, and all are in HD:
- 1280x720p60 TaB (Top-and-Bottom)
- 1920x1080p24 TaB
- 1920x1080i30 SbS (Side-by-Side)
Encoding for any type of video is part science and art, so it's no surprise that CableLabs is offering plenty of wiggle room. For example, Comcast's on-demand coverage of The Masters earlier this year was encoded for MPEG-2 at 18.75 Mbit/s after tests showed that quality suffered when it was encoded at the original 15-Mbit/s safe harbor threshold. (See 3DTV Warning: Poor Quality Could Poison the Well.)
Based on current safe harbor figures, most frame-compatible 3DTV services should fit inside the space of a regular HDTV channel. Future, full-resolution 3DTV cable services that can deliver 1080p signals to both eyes could end up costing much more bandwidth. However, there are patented processes coming about that aim to keep those capacity requirements in check – perhaps a 35 percent bandwidth penalty over the half-resolution, frame-compatible technique. (See TDVision Wants to Fill Up Cable's 3DTV Glass .)
— Jeff Baumgartner, Site Editor, Light Reading Cable