SDI is like flies. A necessary part of the food chain that generates a great deal of revenue, but in a modern IP-based world, you kind of feel that there should be a way to do without it. In fact, are there any real technical or commercial benefits left in SDI? I wonder…
I like to look back on history when trying to predict the future. Often it’s not the warm fuzzy glow of a success story that teaches us lessons, but the hard won truth that is painfully extracted from failure and continued development. SDI was created in the late 1980s as a direct replacement for the analogue PAL and NTSC standards. Looking back on that era, it wasn’t certain that our current SDI standard would be the winner. There were many advocates for digital composite transmission rather than separation of the Y, Cb and Cr. There were many heated disagreements between the format and packing of the data in addition to the big issue of components versus composite.
Back in those days, networks existed that did high speed packet switching. But they were too expensive for broadcasting applications, so a uni-directional, circuit switching system was adopted for SDI that was a direct replacement for its analogue parent. In some respects, with hindsight, it might have been considered an opportunity missed for the broadcast industry to be dominant in packet switched networking. However, for that to be true, you would expect that it is obvious that we should now be performing packet switched IP transport for our signals…
If you have been browsing the recently published SMPTE standards, then you will find that the following documents have appeared:
- Image Format and Ancillary Data Mapping for the Quad Link 3 Gb/s Serial Interface ST 425-5:2015.
- Image Format and Ancillary Data Mapping for the Dual Link 3 Gb/s Serial Interface — Amendment 1 ST 425-3:2014 Am1.
- Image Format and Ancillary Data Mapping for the Quad Link 3 Gb/s Serial Interface — Amendment 1 ST 425-5:2014 Am1.
- Image Format and Ancillary Data Mapping for the Dual Link 3 Gb/s Serial Interface ST 425-3:2015.
- Ultra High Definition Television — Multi-link 10 Gb/s Signal/Data Interface Using 12-Bit Width Container ST 2036-4:2015.
- Ultra High Definition Television — Overview for the SMPTE ST 2036 Document Suite OV 2036-0:2015.
- Generation and Alignment of Interface Signals to the SMPTE Epoch ST 2059-1:2015.
- SMPTE Profile for Use of IEEE-1588 Precision Time Protocol in Professional Broadcast Applications ST 2059-2:2015.
- Y’D’ZD’X Color-Difference Computations for High Dynamic Range X’Y’Z’ Signals ST 2085:2015.
- Synchronization of 59.94- or 50-Hz Related Video and Audio Systems in Analog and Digital Areas — Reference Signals ST 318:2015.
- 12 Gb/s Signal/Data Serial Interface — Electrical ST 2082-1:2015.
- 2160-line Source Image and Ancillary Data Mapping for 12G-SDI ST 2082-10:2015.
- 6 Gb/s Signal/Data Serial Interface — Electrical ST 2081-1:2015.
- 2160-Line and 1080-Line Source Image and Ancillary Data Mapping for Single-Link 6G-SDI ST 2081-10:2015.
- SMPTE Bit-Serial Interfaces at 6 Gb/s —Overview for the SMPTE ST 2081 Document Suite OV 2081-0:2015.
- Serial Digital Fiber Transmission System for SMPTE ST 259, SMPTE ST 344, SMPTE ST 292-1/2, SMPTE ST 424, SMPTE ST 2081-1 and SMPTE ST 2082-1 Signals
The first thing to notice is that there are a lot of documents, and while many of them are UHDTV and timing related, a big number of them relate to high speed SDI for mapping the varieties of UHDTV that are appearing in research labs and on screens around the world.
SDI has several advantages over IP:
- It is simple and straight forward to set up
- There are no known cases of malware being injected into a facility over SDI
- If you connect a camera to a switcher with SDI during a high value sports game, then it will almost certainly still be working the same way at the end of the game as the beginning
- An SDI link’s capacity is fixed and deterministic, and comparatively simple to understand
Some of these advantages are also weaknesses. There is a much greater investment in the technology behind IP links than in SDI technology, and that trend is more than likely to continue. IP links are reconfigurable and can dynamically change both payloads and their physical links to adapt to live demands of a system.
So the commercial / technical balance often comes down to risk and reward. Replacing SDI with IP can be very lucrative financially (e.g. long-haul links), providing the risks are well managed.
How does UHDTV change the equation? Fundamentally today, there are not many implementations of the links in those standards listed above, so the only real option is to use an IP link and manage the issues as they arrive. It’s not quite as easy as it sounds though, because some of the UHDTV data rates are very high and multiple physical channels are required.
“Just use 100Gbps,” I was told by a colleague, but even that approach has issues. Most of the cheap, short-haul 100Gbps links today are actually 10 multiplexes of 10Gbps, with the new 4x 25Gbps standard making headway. The bottom line is that the sort of technologies required to go much faster than 10Gbps are still exotic in 2015, and we won’t be seeing a 100Gbps network adapter costing $10 in the back of a server any time soon.
That should be good news for SDI, except that we won’t be seeing a UHD RGB HDR HFR SDI link costing $10 any time soon either.
My belief is that UHDTV will increasingly push the world into a packet switched IP future because the investment in high speed Ethernet for data centres is similar to many of the requirements for UHDTV links in a media centre. The fact that the IP link can be reconfigured for multiple HDTV and other uses means that re-use of the hardware makes good commercial sense.