SDI remains the technology of choice for UHD workflows, but it still has issues, says David Ackroyd
While the IP transport infrastructure for video and audio takes time to mature and overcome broadcast timing requirements and latency expectations, SDI continues to be the known quantity for 4K systems design and is growing in use mainly in the production environment.
This is supported with the recent introduction of high-speed 6G and 12G-SDI standards. It is clear that IP transport has many advantages for transmission, contribution, offline transfers, post-production and archive, but in live production, sports and events, where broadcasters focus on their customers’ demands, SDI will be required to maintain the expected service level.
At the emergence of transformational technologies such as UHD, equipment manufacturers and early broadcast adopters have need of new equipment that measures and generates signals to test designs and systems.
Key requirements for operational acceptance in studio and OB environments are timing, latency, routing density, mixing, monitoring, legacy HD/3G compatibility, signal tracing and fault-finding, but also ruggedness, simplicity and cost.
Furthermore, the technology for including 12G-SDI in designs is now available and most SDI products will include this capability.
We have identified three main requirements for a successful UHD-1 workflow: connectivity, data division and data integrity.
The first - how to transfer uncompressed signals from camera, to router, to switcher, to editor, to storage and then to transmission - is possible at present over SDI using quadlink 3G, 6G, dual link 6G and single-link 12GSDI standards.
Display connections are also possible via DisplayPort 1.2 and HDMI 1.4 and 2.0. With no clear preferences yet being adopted, there is a need for equipment to manage signals from all of these standards.
To maintain compatibility with legacy SDI infrastructure, 4K material can be transported using either ‘2-Sample Interleave’ (2SI) or ‘Square Division’ (SQ) formats (the former is now an SMPTE standard), which need to be automatically handled by products and systems.
Each has its merits and the combination of these data-packing formats in systems, alongside the single and multi-link transport standards make it necessary to check and test the formats throughout the signal chain.
The SMPTE 2SI standard is designed to allow use of legacy 3G infrastructure but minimises the processing delay that the SQ has.
So we can see how important it is for equipment to be able to handle both SQ and 2SI formats.
Finally, when designers are dealing with high-speed SDI standards such as 12Gb/s, the issue of signal integrity presents itself. On a given system, SDI waveform timebase jitter needs to be four times lower at 12Gb/s than 3Gb/s when measured in picoseconds.
This is a considerable challenge for receiver chips to be able to lock to a signal and for transmitter chips to generate low jitter output.
The challenge in overcoming this issue is to understand where output jitter sources lie and to be vigilant in removing them.
- David Ackroyd is vice-president of business development at test and measurement firm Omnitek
This article is taken from the September/October issue of Broadcast TECH. Click here for the digital edition