Multimedia analysis: High definition video in networked environments

Multimedia analysis (March 2009)

At a glance

• high definition (HD) video content is becoming widely used and produces considerable quantities of data.
• The bandwidth required to transmit HD content depends on a number of factors, including the compression algorithms and HD format chosen.
• A range of alternative cabled and wireless network standards are available, or still in process of being ratified. No single standard has emerged as the clear leader.

The rise of HD video

A considerable quantity of HD content is already available to schools: many films are being released in HD and both online and digital media sources are looking to provide some form of HD output, while more recent camcorders and cameras, Blu-Ray disks and many games consoles produce HD images. YouTube, the popular video sharing site, recently allowed users to upload HD video and the BBC Trust is consulting on a new 'Canvas' platform that would (among other services offered) deliver HD content to Freeview set-top boxes and through Freesat to satellite receivers.

HD formats and video bandwidth

HD video comes in three main formats: 720p, 1080i and 1080p. The number represents the horizontal scan lines, while 'p' means progressive (where the picture is refreshed each time) and 'i' is for interlaced (where alternate lines are refreshed on each pass). The associated horizontal resolutions are 1,280 pixels for 720p and 1,920 pixels for the 1080 formats. A further 2160p Quad HDTV format may lead to devices in 2015.

The amount of data produced by an HD video stream depends on the HD format, frame rate (frames per second or fps), colour sampling and audio codec chosen. Analogue television is currently broadcast at 25fps in the UK (24fps in US), but HD does not give optimum visual quality at this frame rate, so equipment manufacturers are tending towards 60fps.

According to Microsoft, 1080i HD video at 60fps requires nearly a 1Gbps to deliver uncompressed content to devices, whereas compressed 720p video at 24 frames per second produces 25Mbps. (If recording HD video, these equate to 410GB and 11GB of data per hour, respectively.) Transmitting this data across any form of network will require additional data for error correction and other network overheads.

A number of techniques can compress HD data, but each will introduce loss (with consequent reduction in picture quality). Down sampling, which reduces picture quality, converts to lower resolution formats, while upscaling (which can also create video artefacts) is used to create a video stream for higher resolution displays. TechNews covered HDTV in June 2006.

HD video in school environments

Schools and colleges benefit from high speed, switched Ethernet networks. While 100Mbps is inadequate for reasonable quality HD video, gigabit Ethernet provides plenty of bandwidth, if HD is not widely used or in contention with other significant network traffic. Nevertheless, as HD video traffic increases, even with prioritisation in quality of service (QoS) schemas, networks will come under considerable pressure.

Inevitably, what happens in the consumer market will have a knock-on effect for educational institutions. It was not so long ago that wireless projectors seemed far off, yet they are now used by many schools, so it may not be long before the wireless HD video technologies from the home become available too.

The digital living room

A number of developments covered by TechNews in July 2005 are coming to fruition. Set-top boxes and online television streaming services are converging people's viewing habits, while Wi-Fi or 3G enabled mobile devices are delivering such content throughout the home. A BBC blog post gives a short case study of how the current 'connected home' might look, but implies a complex web of devices, wires and software hook-ups beyond either the capability or interest of most consumers.

Delivering HD video wirelessly to displays throughout the home is one of the industry's immediate goals, with a range of technologies competing in this arena:

• Wi-Fi . The latest 802.11n draft standard could give a theoretical maximum throughput of 300Mbps in the 2.5 to 5GHz spectrum. In the real world, this would be inadequate for either of the 1080 HD formats without significant compression and loss of quality.
• Ultra-wideband (UWB). This is not a standard but a range of technologies that produces a series of simultaneous, short, low-power pulses across a set of frequencies in a broad spectrum. The low power reduces interference with other devices, while the spectrum spread allows for signal propagation issues created by objects in the room.
• Wireless USB. This standard is based on the WiMedia Alliance's implementation of UWB, which can deliver a maximum of 480Mbps at 3m and 110Mbps at 10m, using selected frequencies between 3.1GHz and 10.6GHz.
• WHDI . This draft standard relies on compression algorithms that intelligently examine a video stream to identify the least and most significant portions for the viewer. By applying greater compression to the least significant information, the developers can broadcast HD video in the unlicensed 5GHz spectrum. Depending on local spectrum restrictions, Amimom claim that they can deliver compressed 720p or 1080i HD video in a 20MHz channel and full 1080p in 40MHz. While the coding schemes are different, WHDI could share Wi-Fi antennas, which operate on the same frequencies.
• WirelessHD . This is targeted at delivering 1080p HD video at 24 fps using 60GHz frequencies. This spectrum, which is also unlicensed, can deliver far higher data rates over greater distances than 5GHz, although walls and other objects are a more significant hindrance to signal propagation. An array of antennas is used to form a beam through constructive interference, which focuses the signal into a particular spatial region. Fast switching of these arrays can rapidly reroute the signal to avoid fixed objects, or even people moving within the room.

Cabled home networks

Given the challenges faced by wireless systems, which can realistically be expected to provide 'in-room' HD video for the time being, are there cabled alternatives? A 'high speed' HDMI cable can reliably transfer 1080p data about 6m (25 feet), above which the HDMI web site recommends use of active components to ensure signal quality. Cat 5 and Cat 6 cabling can be used with boosters to give cable runs of up to 50m, but these will be relatively expensive and only connect pairs of devices.

The CoAir chipset from Sigma Designs employs multicast technology and Ethernet or standard coaxial cable (installed to provide television outlets for the aerial in most houses) to network the home; UWB transmitters provide the final wireless link to attached hardware. Since it is based on UWB, CoAir can only provide a maximum data rate of 480Mbps at a range of 3m.

'Power line' networks have also been promoted for pervasive home networking, but have struggled to gain acceptance in the market place. (Power line networks impose a high frequency data signal over the standard 50Hz AC mains power supply. While attractive in principle, poor wiring and interference have proven problematic.) A developing G.hn standard incorporates power line connections alongside telephone and coaxial cabling to deliver multimedia, voice over IP (VOIP) and other digital services around the home. Reports indicate data rates of 200Mbps over power lines and double that for coaxial cabling. Although adequate for compressed 720p, these speeds are some way short of what is required for uncompressed 1080p. A number of other power line communications groups have recently joined the HomeGrid Forum to promote the new standard and ensure interoperability. G.hn is a 'place holder' title, with the final name yet to be agreed.

Buying in to HD video networks

Manufacturers' claims regarding transmission of HD video must be examined carefully - outcomes will depend on the video format chosen and the image quality that is deemed as acceptable by the user, which will largely be governed by the level of compression applied. While Ethernet networks may be capable of transmitting the large quantities of data involved, they will rapidly become overwhelmed if too many users generate too much high quality video. Although there are a number of wireless and alternative cabled technologies available, there is no widely agreed standard and each has some significant drawbacks or has yet to be proven in the field.

References

HD/HQ and widescreen options on embedded videos
http://uk.youtube.com/blog?entry=ZNk9ZtV62cc

Trust assessment of 'Canvas' proposals http://www.bbc.co.uk/bbctrust/consult/open_consultations/canvas.html

Femtocells(TechNews 09/08. Digital living room edition not available online.) http://emergingtechnologies.becta.org.uk/index.php?section=etn&rid=14138

Canvas and the connected home http://www.bbc.co.uk/blogs/technology/2008/12/canvas_and_the_connected_home.html

Understanding HD Formats http://www.microsoft.com/windows/windowsmedia/howto/articles/UnderstandingHDFormats.aspx

Wi-Fi Alliance
http://www.wi-fi.org

Wireless USB from the USB-IF
http://www.usb.org/developers/wusb

WHDI
http://www.whdi.org

WirelessHD
http://www.wirelesshd.org

Connecting with HDMI
http://www.hdmi.org/consumer/how_to_connect.aspx

Sigma Designs CoAir http://www.sigmadesigns.com/public/Products/coair/coair.html

New standard promises HDTV over home networks http://www.vnunet.com/vnunet/news/2232740/standard-promises-hdtv-home

ITU-T G.hn Specification Achieves Key Milestone... http://www.homegridforum.org/news_events/pr/12_15_08

Technology organisations align to support United Nations' ITU-T G.hn standard
http://www.homegridforum.org/news_events/pr/02_25_09