UHDTV production interfacing challenges: is it all about SDI?

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Since High Definition Television (HDTV) became state-of-the-art in productions and the ‘norm’, Ultra High Definition Television (UHDTV) has gained ground and arguably has become the new favourite future production and distribution format. Although UHDTV is known over the last two decades, it failed to become mainstream. However, when the first so-called ‘4K UHDTV’ or ‘QuadHD’ screens entered the consumer market in 2013 the perception changed – it was a big success!

A study from Futuresource predicts that between 7 to 8 per cent of TV sets will be 4K ready by next year, in 2015 . Thus the immediate demand for native UHDTV content will increase and productions will be under pressure to decide what infrastructure they will use.

Currently, several proposals – as multi-link or single-link – are being discussed and attempts to standardise these approaches are being undertaken in SMPTE and ITU. The harmonisation process of those proposals is essential in order to build cost effective and interoperable equipment and workflows. However, some solutions will be useful for the short-term and for the early UHDTV productions.

While this article describes new developments of studio interfaces focused on the SDI technology, more long-term alternatives are underway. Felix Poulin, EBU, tells you more about a flexible solution of an IP-based production infrastructure that can also be adapted to UHDTV productions in a separate IBC Daily article, ‘The Network Studio: Here to Stay’.

And, what about High Frame Rate (HFR) or High Dynamic Range (HDR)? UHDTV is mostly seen as an increase in resolution, i.e. four times HDTV resolution (or 4 x 1920x1080 pixel). Early tests from the EBU have shown that this is not enough to achieve a satisfying increase in image quality. At IBC2012, the EBU already presented the conclusion that UHDTV must also include the following essential characteristics: HFR, HDR, a wider colour gamut and an immersive audio system.

Since this demonstration, more details on the influence of those parameters are known. For instance, a study from the BBC has shown that in Europe 120 Hz will create flicker issues and 100 Hz is therefore needed. The higher frame rate can be also a valuable feature for an enhanced HDTV system, 1080p100.

In order to transport those higher frame rates for HDTV and UHDTV, the signal needs to support a 1080p100/120 sub-image. To do so, an appropriate method to map those sub-images into Serial Digital Interface (SDI) will need to be defined. Further studies on HFR have been undertaken by the BTF-HFR group (Broadcast Technologies Future group).

The EBU´s Strategic Program Beyond HD has started studies on a new HDR evaluation method together with Dolby and the EPFL in January 2014. Those studies showed a need to re-consider the evaluation method, based on ITU BT.500, in regard to the environment. Even though no final conclusion can be yet drawn on the level of HDR from the studies they indicate a preference towards brighter images with more dynamics.

Another important aspect to consider is that more experts agree that only a reasonable combination of all parameters can result in a long term UHDTV format. This was reflected in the joint DVB-EBU UHDTV and HDR workshop in 2013 and 2014, respectively, and is also recommended by SMPTE´s TC24TB UHDTV Ecosystem Report. In addition, the ITU BT.2020 standard combines all of these parameters.

Tribute in a higher data rate

All UHDTV parameters together demand their tribute in a high data rate. Table 1 shows typical data rates in the Payload of upcoming SDI interfaces under discussion/standardisation. This table clearly shows that only 1080p images, with up to 60 frames/second and 10 bit, fits in the current 3G infrastructure. Everything beyond this would require a multiple of the data rate.

If you are more interested in more traditional interfaces, the SMPTE is working on standardising a UHDTV interface, described in SMPTE 2036-3 ‘Ultra High Definition Television – Mapping into Single-link or Multi-link 10 Gbps Serial Signal/Data Interface’. Even though 10 Gbps already sounds a lot for most of the formats on the table you will need to use a multi-link option.

Another interesting group to consider is SMPTE’s 32NF70 group on UHD-SDI. This group is in the process of developing SDI variants as 6Gbps (SMPTE 2081-1), 12Gbps (SMPTE 2082-1) and 24Gbps (SMPTE 2083-1).

These can either avoid the usage or at least reduce the number of multiple links. For example, if you like to produce in 2160p100 or p120 at 12 bit 4:4:4 (~48 Gbps) you can use two 24 Gbps links instead of having the need for 16 3Gbps links.

Those who will want to work with formats at lower data rates can alternatively use an interim solution of standardised multiple 3G SDI interfaces under SMPTE ST 425-3:2014 and ST 425-4:2012 for dual-link, and SMPTE ST 425-5:2014 and ST 425-6:2014 for quad-link.

In the end, several short- to long-term solutions for studio interfaces are underway. As seen from above, every broadcaster and production company will need to define their long-term strategy in order to make careful and cost efficient decisions about which interfaces for UHDTV to use. The requirements for any interface, may it be SDI or fibre, are the same, such as low latency, fast re-synchronisation low jitter, cost efficient, ancillary data and embedded audio.

In the long-term the trend can be seen in single-link interfaces, although multi-link solutions guarantee backwards compatibility to current 3G infrastructures.