The rapid growth in popularity of laptops and portable/handheld devices with HD multi-media playback capabilities means that connectivity to an HD display is more and more a supported feature on such devices. Especially on these gadgets cutting the video cord brings huge benefits to consumers, because of the ad hoc nature of their connectivity to an HD display.

Bluetooth enabled game pads are now also available for the PS3 game console. This creates a strong desire in consumers to have the display follow the game pad wherever convenient to play. Similarly, manufacturers of media hubs would benefit by allowing consumers to wirelessly control and view different content streams on any screen simultaneously in the home.

In enterprise applications cutting the video cord means no hassle with docking stations on the desk and no hassle with video cables to connect to a conference room projector.

Broadband radio technologies
At present there are a handful of broadband radio technologies competing for a wireless path to HD displays and projectors. These are summarized in Table 1.


Table 1: Broadband wireless technologies competing for wireless HDMI in the home

Table 1 is not intended to be exhaustive, but only lists the most notable ones. Furthermore, the bit rates in Table 1 are for the PHY and actual bit rates are usually lower due to MAC-layer overhead.

Next: Wireless HDMI key requirements and challenges
Wireless HDMI key requirements
Wireless HDMI must provide cable-free HDMI repeater functionality:

1. Transmission of the video data.
2. Transmission of the audio data.
3. Transmission of the EDID information.
4. Transmission of the CEC data.
5. Transmission of clock information.

Table 2: Key requirements for wireless HDMI

Although operation within a single room is a minimum requirement, it is very desirable for a wireless HDMI solution to extend this throughout the entire home, stretching the value proposition to consumers to the maximum.

Low cost does not necessarily mean same price point as an HDMI cable. The convenience factor gained from cutting the cord is well worth paying for in the minds of consumers. However, the magical retail price ceiling is somewhere around the $200 per Tx/Rx pair at the onset of the market and dropping to sub $80 as the market matures.

Wireless HDMI challenges
The bit rates for different HD and SD video formats transmitted over an HDMI cable are listed in Table 3.

Table 3: Bit rates for HD and SD formats

At these bit rates even the fastest of the broadband wireless radios in Table 1 is challenged to beam raw 1080p60 at the minimum of 24 bits/pixel. Note from Table 4 that although compressed multi-channel HD audio can consume significant amounts of bandwidth, it still is relatively insignificant compared to raw HD or SD video streams.

1) HRA = High Resolution Audio
2) MA = Master Audio

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Table 4: Compressed audio bit rates

Another challenge that faces wireless transmission links is the substantial fluctuations in bandwidth that can occur due to interference by other radios in the same frequency band and by obstacles in the transmission path, notwithstanding the significant advancements in array signal processing and beam forming.

Next: To compress or not to compress, the H.264 video compression standard, and Super Low Latency technolgy
To compress or not to compress
That's really not the question, given that HD video stretches any broadband radio to its maximum capabilities. But, more importantly, somehow applications always find a way to outpace available bandwidth, and from this fact it is evident that any future-proof wireless HDMI solution must deploy some method of compression of the video and audio signals. An easy to imagine scenario is that of a media hub, beaming HD AV streams to several displays in the home. Another one is the advent of 2K (24 and 48 frames/sec for 2048x1080 resolution) and 4K (24 frames/sec for 4096x2160 resolution) digital cinematic displays and projectors. At 48 bits/pixel a 4Kp24 raw pixel stream results in 12.8Gbps.

One may argue that it is enough to beam over the source material in native compressed format. I.e., such as in case of a DVD movie or cable TV program. However, there is unavoidably always a graphical overlay component for navigation or other interaction, such as on a Blue-Ray DVD. Game consoles are an extreme, where all data is either pure graphics or a combination of graphics and video material. Hence, it is impractical to send across the compressed A/V source and the uncompressed graphics as separate streams and then recombining video and graphics at the display side. Besides, the uncompressed graphics would require the bit rates in Table 3 anyway.

H.264 -- the video compression standard
Although there are several video compression standards in existence, which could do the job of reducing video bit rates to manageable data rates, the H.264 standard is the most evident choice. It has been defined with extensive combinations of profiles, levels and encoder tools, covering a wide range of application scenarios. For instance, quantization parameters can be chosen to produce video as pristine as the original. Compression can be performed using I-frames only, or including P and/or B-frames, exploiting temporal redundancy for better efficiency. The standard also allows for compression of video in YUV 4:2:2 or 4:4:4 format and in pixel formats greater than 24 bits/pixel, and even includes a lossless profile. H.264's efficiency produces pristine HD video at sub 50Mbps, roughly a 75:1 reduction in bit rate over a 3.6Gbps 1080p60 stream. But most importantly, the standard is widely implemented and H.264 HD encoders are finding their way into digital still cameras and camcorders at very affordable consumer price points.

Latency - the grand challenge of video compression
The key challenge of video compression in a wireless HDMI application is latency, caused by the sheer amount of calculations that are involved. This latency tends to be exacerbated as resolutions, frame rates and color depths increase, and plagues any video compression method, including H.264. In addition, bit rate control introduces additional latency. Smoothing out bit rate variations requires buffering, which in turn affects latency unfavorably. Strongly limiting bit rate variance is desirable for a wireless transmission link to prevent bandwidth and bit rate fluctuations, potentially with disastrous effects on the decoded video. Invariably, strong bit rate fluctuations happen with the occurrence of I-frames. And even between I-frames bit rate fluctuations can wildly vary depending on the complexity of the frame. Other factors may further impede latency negatively, such as pre-processing, etc.

Super low latency technolgy (SLL technology)
WW Communications has solved the low latency challenge in its WW602 and WW108 super low latency H.264 HD video codecs, through architectural and algorithmic innovations. Its patent-pending Super Low Latency Technology or SLL Technology alleviates the processing and bit rate control bottlenecks. With SLL Technology video processing latency is only bounded by the video pixel clock. This means that a 1080p60 stream is processed with less latency than a 480p60 stream, as the pixel clock for the HD stream is much faster. In fact, a single 1080p60 stream can be processed at sub 1ms encode-decode latency.

SLL Technology solves the latency issues associated with I-frame buffering by using a special technique, called macro-block intra-refresh. Rather than processing I-frames as a single frame, they are processed distributed across a group of pictures. This allows the I-frame bits to be streamed within the constraints of imposed bit rate and latency. An added advantage of this technique is the inherent error resiliency associated with the distributed method of processing.

The remarkable low latency achieved through SLL Technology has another significant advantage in a wireless HDMI application. Due to the extremely low latency, SLL Technology provides inherent lip sync. Provided that the audio signal is not substantially delayed, lip sync issues are not detectable when using the WW108 and WW602 without any time stamping and further means of AV sync.

Next: Wireless HDMI over IEEE802.11n
Wireless HDMI over IEEE802.11n
The selection of IEEE802.11n in the 5GHz frequency band as the wireless path to the display is a natural choice given the ubiquity, low cost and high interoperability of WiFi products and technologies. However, any of the other radios in Table 1 can be used as they mature and attain sufficient market presence. Indeed, UWB for instance is a low power broadband wireless technology that has very good prospects as the preferred choice of radio in external "dongle" accessories for proximity line of sight (PLOS) applications versus non-PLOS (N-PLOS) applications.

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Figure 1: WW602 test setup for wireless HDMI over IEE802.11n

IEEE802.11n in the 5GHz band is the preferred radio for non-PLOS applications, such as beaming the AV stream to a display in any room in the home. Our test results confirm very robust N-PLOS applicability with off-the-shelf IEEE802.11n solutions at 100 feet through 6 walls or concrete flooring. Figure 1 illustrates the test situation. A Sony PS3 game console was used as the source for testing with movie and gaming content.

Table 5: WW602 and IEEE802.11n wireless HDMI test results. Note: Maximum encoder and decoder bit rates were set to 65Mbps.

Table 5 shows the test results obtained with a 2x3 MIMO (2 Tx and 3 Rx antennas) solution on the encoder TX and the decoder Rx side, as well as the test results obtained using a 2x2 MIMO on the decoder Rx side. At 20Mbps the video quality of the WW602 still exceeded HD cable broadcast quality in the range of 6-15Mbps.

Next: Designing single and multi-stream wireless HDMI solutions
Designing wireless HDMI solutions with the WW602 and WW108
The latest WW602 and WW108 super low latency H.264 HD video codecs from W&W Communications support channel-adaptive bit rate, transrating and transsizing control. This allows a wireless HDMI solution to adjust in real-time the video data rate to match available channel bandwidth under any condition as shown in Table 5 (previous page), as well as streaming HD content to devices that only handle lower resolutions and frame rates than the source material.

Strong real-time error resiliency encoder and error concealment decoder tools on the WW602 and WW108 allow developers to provide consumers with a robust wireless HDMI video experience. Error resiliency tools include adaptive GoP, slice and intra-refresh sizes, as well as I-frame forcing and random versus linear intra-refresh. Error concealment tools include SKIP modes at macro-block and frame levels.


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Figure 2: WW602-based wireless HDMI concept block diagram

The WW602 is targeted at low cost, single stream wireless HDMI applications for laptops, game consoles, DVD players, settop boxes, DVRs, portable media players, etc. The WW108 with its multi-stream capabilities targets media hubs, adding to these the ability to wirelessly beam over different content to different displays in the home simultaneously.

Figure 2 shows a simplified block diagram of a single-stream wireless HDMI solution using the WW602. Figure 3 shows the same for a multi-stream, media hub application using the WW108. In both diagrams only the video is compressed and decompressed, whereas the audio and all control signals, such as HDMI CEC and EDID are passed-through across the wireless link.

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Figure 3: WW108-based wireless HDMI media hub concept block diagram

Conclusion
After the introduction of high-definition video in every manner of video content distribution and delivery, cutting the video cord is the next frontier in HD entertainment. While various broadband wireless technologies have come to compete for the over-the-air path to the display, it is not difficult to see that the need for bandwidth will keep outpacing availability. The only way out is the use of compression and at present the best candidate is the H.264 standard with a wide range of tools and profiles defined for maintaining pristine quality. The challenge is to "eliminate" latency introduced by the encode-decode path. W&W Communications, Inc. demonstrates that this is possible through careful encoder and decoder design, which forms the foundation of the company's Super Low Latency Technology.

About the author
Kishan Jainandunsing is currently VP Marketing at W&W Communications. He has over 20 years of high-tech management, marketing, business development and R&D experience. He holds a PhD and MS in Electrical Engineering from Delft University of Technology, The Netherlands, and has extensive experience in digital speech, audio and video processing. He can be reached through info@wwcoms.com.