In the pursuit of ever higher resolution video standards hardware engineers and the ASIC designers who support them are continually challenged to reach higher and higher data speeds. The moving target is now firmly placed on 12G-SDI (SMPTE ST-2082) which can support 4K resolutions at frame rates of 60Hz (4Kp60). Manufacturers of professional video equipment need to start supporting this technology or will be left behind. So what are the hardware challenges of sending data at these incredible rates and what is the most effective way for manufacturers of video equipment to develop this hardware?
In this blog post we’ll explore the hardware challenges, understand what the root causes are and provide the solutions to these difficult challenges.
The fundamental objective of SDI signalling is to efficiently move a video signal from one location to another. The infrastructure that has been built up in studios and broadcast facilities over the years is coaxial video transmission of the digital video signal. While optical transmission of these new higher bandwidth signals is starting to make good sense, coaxial cable interfaces will likely remain dominant at least in the short term. There is an upper bandwidth limit on any channel that carries information and coax cable is no exception. While coax cable for SDI transmission is designed to have higher bandwidth capabilities the pressure to always increase bandwidth would make the cable prohibitively expensive and difficult to use if the sole responsibility for bandwidth fell onto the cable manufacturers. So over the years numerous techniques have been developed at the hardware level to ensure the transmitted signal is higher quality and the receiver is more sensitive to ensure the SDI transmission maintains reliability over longer cable runs even with higher speed requirements. Let’s explore the problems and understand the solutions manufacturers need to integrate into their products.
Understanding Signal Integrity
Signal integrity will ultimately determine whether or not the video signal transmitted by the source device can be properly decoded at the sink device. At 12G speeds we can’t think of the signal as a digital signal but rather an extremely high frequency radio signal that is subject to all the analog effects as other RF signals. We’ll get into some of the effects but let’s first look at how a signal is evaluated for quality. The two images below are scope captures of a high frequency signal that are called eye diagrams.
It turns out the quality of a measured signal is closely related to how open the ‘eye’ is in the center of the waveform. Note how the first image has a cleaner look with wider eyes when compared with the second image. One of the contributing factors to poor signal integrity is jitter; a deviation from the true periodicity of the signal which makes it harder to decode signals especially with embedded clocks. So the question becomes, how do we reduce jitter in an SDI video signal?
The key to reducing jitter and meeting specifications in an SDI video system is to use a reclocker. A reclocker is a basically a clock and data recovery (CDR) device that can phase lock to the SDI signal using a voltage controlled oscillator reference. The process of reclocking or retiming essentially cleans up the serial data signal and makes a fresh version of it, which helps to eliminate jitter that may be present in the signal at the expense of a bit of added latency. In some systems the latency difference or phase change relative to a reference signal due to a reclocker may be unacceptable therefore in systems that include reclockers it is wise to allow the reclocker feature to be bypassed if required by the user. Reclockers are available simply as a standalone device or often included in both drivers (source side) and equalizers (sink side) as silicon manufacturers have acknowledged the need for higher integration in SDI solutions.
Channel related signal degradation
Every medium that a radio wave travels through provides some attenuation to the travelling signal and coax cable is no exception. The following graph shows the attenuation of different types of coax cables according to the frequency of the signal (note SDI cables are not shown but they follow a similar response).
While the exact attenuation of each type of cable is unimportant for the discussion at hand it should be understood that SDI signals are subject to attenuation within a length of coax and that the attenuation is frequency dependent. To overcome some of the disadvantages to running a high bandwidth signal through coax cable there are two major pieces of hardware we want to employ.
On the source side, an SDI coax driver accomplishes two objectives. First it matches the coax impedance to ensure maximum power transfer from the driver to the coax cable. Secondly the driver helps to combat the coax attenuation by ensuring the signal entering the cable is suitably strong ensuring enough signal is received by the sink device after passing through the coax.
Because the coax attenuation of the signal is frequency dependant, an equalizer is used on the sink side to normalize the frequency spectrum of the signal. Essentially the equalizer acts as a frequency dependant pre-amplifier that has an inverse transfer function to that of the coax cable. Without the equalizer the received signal would have high amplitude low frequency components and low amplitude high frequency components which makes decoding of the signal more difficult.
The use of drivers and equalizers greatly increases the signal to noise ratio of the 12G-SDI system which enables coax runs of over 100m – which is a requirement in many broadcast applications.
Given the high bandwidth of the new SMPTE ST-2082 that makes 4Kp60 possible over 12G-SDI there are certainly other design challenges. The silicon for these applications is now being rolled out by IC manufacturers which make the technology possible. Hardware and layout engineers implementing these designs require careful planning and signal integrity simulation and analysis to ensure it is implemented correctly at the board level. Manufacturers of broadcast quality FPGA based video devices can expedite their designs by using verified development hardware such as FMCs and Xilinx development boards to reduce time to market. This approach also benefits from using the currently available Xilinx 12G-SDI cores to speed development even further.
Working on a 12G-SDI video device? Contact Fidus Systems for a complete list of HD video development hardware available to expedite your design. You can also leverage our expert video experience in 12G-SDI and other HD formats by using our contract design services. Contact us today!
Trevor Smouter – Fidus Hardware Designer