As display resolutions grow larger and clock rates increase, bandwidth is becoming more of a problem for the switching and distribution of HDMI signals. Our industry is starting the transition to 4K and higher resolutions, but our signal extenders are (more often than not) hard-pressed to keep up.
There are several things that affect the clock rate of an HDMI signal. “The first and most obvious is the total number of pixels (including blanking) in each frame of video,” said Tom Kopin, Global Engineering Specialist at Kramer Electronics. The second is how quickly these pixels are refreshed from one frame to the next, and the third is the color bit depth of the signal. (For purposes of this discussion, we’ll limit our focus to signals in an RGB (4:4:4) format.)
It seems like forever that the ceiling for HDMI clock rates was capped at about 150MHz, a result of refreshing 2,073,000 pixels 60 times per second (1920x1080p) using 8-bit color. Some signal extenders that could handle this clock rate could keep up with 10-bit and even 12-bit signals. Others could not, but that didn’t become a problem until support for deep color became standard on Blu-ray players and later, laptop and desktop computers.
Think of an existing AV system where the displays (often TVs) are upgraded, as are the computers and media players connected to them. The EDID information from a display shows it can support 12-bit color, so the video source generates a 12-bit 1080p/60 signal. But the signal extender that runs between them is an older type and is incapable of passing anything faster than 8-bit color.
Aside from swapping out the extender for a new one, the easiest solution is to configure the video source to run in 8-bit color mode (or 10-bit mode). But on some computers, that setting is not easy to change. In this case, there are a few options to overcome the speed limit issue.
The first is to set the refresh rate of the media layer or video card to a lower speed— say, 30Hz. That will cut the clock rate in half. Another solution is to insert an EDID “spoofer” just before the connection to the display to generate a more limited EDID table that supports only 8-bit or 10-bit color.
There are also instances where signal extenders are capable of the higher clock rates needed for greater bit depths, but the signal still has trouble passing from the transmitter to the receiver. In this case, the limiting factor is the category wire used between the two.
For 1080p or 1200p signals with 10-bit and 12-bit color, basic Category 5 wire may be insufficient for the task, and either shielded twisted pair (STP) Cat-5 or Cat-6 STP wire (or optical fiber) should be substituted. This is the best approach if you are planning a system upgrade to support higher resolutions like Ultra HD (3840 x 2160).
SPEAKING OF 4K
It’s better not to focus on the resolution so much as the pixel clock. You will see many manufacturers claiming their products are “4K certified” or “4K compatible.” What that could mean is that their interfacing products are capable of passing lower frame rate Ultra HD signals with limited color bit depth.
As an example, the six-year-old HDMI 1.4 interface—still used in the vast majority of HDMI switching, distribution, and extension products— has a maximum clock rate of 10.2 gigabits per second (Gb/s). That’s plenty fast enough for a 1080p/60 signal with 16-bit color.
Version 1.4 is also just fast enough to pass Ultra HD (3840x2160p) at 30Hz, albeit only with 8-bit color. Such a signal in the RGB format has a clock rate of 8.91GHz, or 2.97 GHz per color channel. But if we increase the color bit depth to 10 bits per pixel, the clock rate soars to 10.7Gb/s and we’ve hit a roadblock.
And here comes a bigger challenge for our industry: A consensus is building among the movie and TV production standards organizations that 4K requires at least 10-bit color, especially to support next-generation TV enhancements like high dynamic range (HDR), wide color gamut (WCG), and high frame rate (HFR).
That means we’ll be facing a serious bandwidth crunch in the next few years as all of these enhancements come to market. Ultra HD Blu-ray will start rolling out in December of this year, and computer displays with 5K (5120 x 2880) image resolution are already available. Sooner or later, you’re going to dealing with much higher clock rates no matter which interfacing standard (HDMI 2.0, DisplayPort 1.2/1.3, superMHL) you migrate to.
It’s all about bandwidth…
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