Digital TV: The Need For Speed

With CES just finishing up, I wanted to take a closer look at the changes in the digital TV market, and what affect those changes have on high performance memory and serial links.

Just five years ago, the United States made the transition from analog to digital television. At the time, standard definition digital TV was common, with screens that contained 345 thousand pixels per frame. Recently, I was reading about new 8K TVs, boasting 7680 x 4320 pixels. By itself, this increase in pixel count would drive a bandwidth increase by a factor of almost 100.

But there is more to bandwidth than just pixel count. Both refresh rates and the number of bits per pixel also affect bandwidth requirements.

HDTVs typically support refresh rates of 60 Hz with some TVs supporting higher rates such as 120 Hz or 240 Hz. Regardless of whether you believe in the benefit of higher refresh rates, if the system includes an option for higher refresh rates, serial links and memory must support it. Using the above table, a single image on an HDTV requires 2.1 megapixels, and at 60 updates a second, a total of 124 megapixels must be transferred in one second.

Now that we know how many pixels are required per second, we just need to convert pixels to bits. A pixel represents a specific combination of red, green, and blue light, with many implementations allowing 256 levels per light component, corresponding to 8 bits. UHDTV commonly encodes each 8 bits of information into 10 bits, and therefore a total of 30 bits per pixel is utilized.

Ignoring signaling overhead, we can finally calculate the video signal bandwidth requirements. 4K UHDTVs demand approximately 16 Gbps, with 8K UHDTVs further extending the bandwidth to 60 Gbps.

From a serial link perspective, each of the RGB colors is split into a separate channel as part of the HDMI standard, and therefore signaling speeds of about 6 Gbps per channel is required to support 4K UHD TVs. Perhaps not a coincidence, many signaling technologies utilize speeds that are in a similar range (see table below), which means that issues regarding signal integrity are well understood. However, to support 8K UHD TVs, it appears that the industry will need to develop standards that support approximately 20-30 Gbps per channel, most likely leveraging signaling technologies developed for the communications and networking markets, which are already pushing to 28 Gbps and beyond.

At first glance, it would seem that current common memory technologies such as LPDDR3 and DDR3 would be capable of supporting even 8K UHDTV video speeds. However, when the application involves video encoding, data from Nokia Research Center (http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5090799&tag=1) indicates that about 20 times the video signal bandwidth is needed. To accommodate the bandwidths for 4K UHD video encoding at 60 frames per second, DDR4 or LPDDR4 running at top speed would be required. Note that these top speeds are at least a year away from being available.

As these new product categories continue to drive the need for more bandwidth, we at Rambus, are working closely with customers to enable the fastest serial links and advanced memory interfaces that meet or exceed these requirements for the consumer video markets.