How We See CE

For wired connectivity, HDMI has emerged as the enabling digital interface for HD in consumer electronics. The key to its success is the ability to deliver uncompressed HD 1080p video and audio at a rate of over 3 Gbps.

Wireless high-definition video transfer is a more challenging problem. Obviously, the key requirement is to establish a communication channel with enough bandwidth and SNR (signal-to-noise ratio), i.e., enough capacity, to handle the high video rate. What adds to this challenge is that any wireless channel will at times be variable and unpredictable. Its characteristics can change rapidly; due to fading and interference, its SNR and capacity can vary considerably. In data transfer, buffers and re-transmissions often are used to compensate for these problems. This is impossible in video connectivity, where the transfer must be done in real-time with no delay and with no degradation in quality.

Traditional wireless video approaches have failed to provide an adequate solution to the problem of wireless HDTV connectivity because they treat the delivery of video essentially the same as data delivery. In a wireless data modem (e.g. 802.11n, UWB) all bits are treated equally; they all receive the same level of protection from channel degradation. However, in video, different bits have different levels of importance and the effect of an error greatly depends on which bit is corrupted.

The traditional data modem approach to video delivery is to separate the source coding from the channel coding based on Shannon’s separation theorem. The system first compresses the source signal, and secondly maps these bits to the wireless channel without looking at their visual significance. This allows for the independent optimization of source and channel coding. This is a nice approach in theory, which assumes that the channel is known and stable, and that delay may be unlimited, but in practice such systems cannot efficiently optimize over the varying wireless channel capacity. In order to avoid interruption in the video delivery, traditional data-modem systems operate at a data rate that is below the worst-case channel capacity and consequently underutilize the channel capacity. Some systems attempt to solve this problem by attempting to average out capacity with buffers, or by implementing a return feedback channel, but these solutions are complex, expensive and can add unacceptable latency.

An alternative approach is JSCC (joint source-channel coding), where there is no separation between source coding and channel coding. This is a key aspect of a video-modem architecture. JSCC enables a video-modem to dynamically map the different elements of the video stream onto the wireless channel by virtue of their visual significance. There are two key advantages of this. First, by using all available channel capacity to send out less sensitive video data, JSCC enables a better utilization of the available channel capacity, even when it is varying.

Second, JSCC maps the video elements onto the wireless channel in a way that gives elements with more visual importance a greater share of the channel resources, i.e. they are transmitted in a more robust manner. Elements that have less visual importance are allocated fewer channel resources, and therefore are transmitted in a less robust way. Allocation of channel resources occurs dynamically with no feedback, as the channel capacity changes, and can include, for example, setting power levels, spectrum allocation and coding parameters. This allows a video-modem to enable robust and cost-effective video delivery systems that can intelligently adapt to the varying and unpredictable nature of the wireless channel. While beyond the scope of this blog, it has been shown that a video-modem, by using JSCC and four MIMO channels with 40MHz bandwidth, can deliver full uncompressed 1080p video.

For example, a typical uncompressed video signal is represented by a stream of 8 or 10 bit numbers, each representing the primary color value of a given pixel. The MSB (most significant bit) of each of these numbers has greater visual significance than the LSB (least significant bit). Errors that occur to the MSB can result in a drastic change to the value of the pixel, while LSB changes result in much less significant changes. Wireless data-modems ignore this characteristic of video. The compressed signal is mapped to the wireless channel in a way where the same level of protection is provided to all bits. This results in both an inefficient use of channel resources (LSBs do not require this amount of protection/channel resources), and a low quality video link (because MSBs do not receive enough protection/channel resources).  

The result of the video-modem approach is that any errors in the wireless channel are not noticed as they only affect the less important bits. Very high rates of video information can be transmitted because the human eye can tolerate the errors that fall on the less important bits – especially as such errors are typically of very limited duration. The WHDI (Wireless Home Digital Interface) is based on this video-modem approach, using intelligence to deliver HD video wirelessly.

What also needs to be kept in mind is that any wireless video system (even those at 60 Ghz) must take into consideration and deal with varying channel capacity in an intelligent manner. Failure to do so can lead to a poor user experience.  For example, one of the 60 Ghz Wireless HD products recently on the market is subject to visible distortion when a person walks in between the Tx and Rx (at a space of several feet away from the Tx). It appears that when the system encounters reduced channel capacity it simply drops a significant portion of the bit stream (perhaps up to 50-75%) in a blind, data-modem manner. This is especially noticeable when viewing PC graphics or Electronic Program Guide data. This is in stark contrast to the high quality user experience provided by WHDI’s video modem solution.

Leslie Chard
WHDI LLC

About the author

Leslie Chard is President of WHDI LLC, the Secretary for the WHDI (Wireless Home Digital Interface) consortium, whose members include Amimon, Hitachi, LG, Motorola, Samsung, Sharp and Sony. Mr. Chard previously helped found the HDMI standard and was President of HDMI Licensing, LLC.