Addressing timing challenges in 6G-SDI, UHD video applications

March 03, 2014 // By Ayan Basu and James Wilson, Silicon Labs
Continued innovation in 4K digital cinema and ultra-high definition (UHD) television increases the need for low-jitter, high-performance timing solutions. Frequency-flexible, low-jitter clocks and oscillators play a critical role in enabling the market transition to 6 Gbit/sec and faster data rates.

Digital video transmission rates have steadily increased since the introduction of high-definition video. The latest trend in the industry for high-resolution video is the market adoption of 6G-SDI to support 4K digital cinema and ultra-high definition (UHD) television. Digital video data delivery at higher speeds required by 6G-SDI poses new challenges in designing broadcast video production and transmission equipment.

In particular, high frequency, low-jitter clocking solutions are a critical element to maintain proper signal integrity through the various components and interconnecting cables that constitute the high-definition video network. In addition, these timing solutions must be flexible enough to accommodate the multiple frequencies required by legacy video standards.

Higher-speed video standards on the horizon

The Society of Motion Picture and Television Engineers (SMPTE) was founded in 1916 to standardise video content distribution. Video equipment manufacturers have since adhered to these standards. In 1997, the SMPTE established the SD-SDI 259M standard, which was the first ratified definition of a serial digital interface (SDI) to send and receive uncompressed digital video over 75-Ohm coaxial cable within a studio environment. SDI supports transmission rates ranging from 270 Mbps to 360 Mbps.

Due to the fact that digitised video signals accumulate jitter across video components and interconnecting cables, SMPTE established limits on the allowable jitter content of SDI signals. As high-definition digital video advanced to 720p and 1080i, SMPTE defined the HD-SDI 292M standard to support higher bandwidth video transmission at 1.485 Gbit/sec.

In 2005, the SMPTE introduced 3G-SDI to enable the transmission of 1080p video at 2.97 Gbps over existing 75-Ohm coaxial cable. To support these higher video transmission speeds, the SMPTE has set increasingly stringent jitter requirements. Table 1 summarises the timing requirements for the SMPTE-ratified SDI standards.

Table 1. SMPTE SDI Timing Requirements

In recent years, continued technological innovation in digital video has pushed the boundaries of video resolution from 1080p (2K resolution) to 4K. Transmitting a larger number of pixels on the same infrastructure implies having to deliver the video payload at 5.97 Gbit/sec. The goal of the standards published by the SMPTE has been to guide the increasing data transmission rates to ensure that existing video production facility infrastructure can support and broadcast higher resolution video. Although the SMPTE body has yet to ratify a standard for 6G-SDI, video equipment suppliers are already meeting the demand for 4K by introducing solutions to support the faster data rates.

Early 6G/12G-SDI market adoption

In response to surging interest and demand for 4K video components, broadcast video equipment suppliers are starting to release 6G-SDI compatible products such as 4K production switchers, video routers, encoders/decoders, video monitors, video servers and video converters. In addition to enabling transmission of UHD video over standard BNC cable, this equipment supports simplified switching between UHD, HD and SD formats.

This enables broadcast video engineers to easily switch between different formats depending on their content production requirements. This flexibility eases the migration to UHD by enabling studios to leverage their existing investment in HD and SD equipment and continue to produce content in a variety of formats.

Semiconductor manufacturers have followed suit and have started to release ICs that are purpose-built for 6G-SDI. These devices include cable equalisers, cable drivers, reclockers and FPGAs with integrated SDI transceivers. One limitation with 6G-SDI is that 4K UHD can be transmitted at no more than 30 frames per second. Higher data rates are required to transmit video at higher frame rates. Anticipating this demand, manufacturers are starting to release 12G-SDI components that support data rates of 11.8 Gbit/sec.

Taking these “proprietary” solutions to available-product status [in the absence of, or in anticipation of, relevant standards] increases the risk of future interoperability concerns. For this reason, the SMPTE has assembled a new Working Group to define UHD single-link, dual-link and quad-link electrical and optical SDI interfaces with nominal link rates of 6 Gbit/sec, 12 Gbit/sec/s and 24 Gbit/sec to support next-generation multi-media, high-frame-rate data transmission.