Getting To Grips With HD

Few people with any interest in technology can have failed to notice the recent buzz that is being created about the term “High Definition” or HD. Many products now come with the HD Ready tag, but what exactly is HD and how will it change our viewing of television and movies?

What is HD?

First and foremost, High Definition as a format is entirely digital (DTV) and therefore provides a picture which is superior to that of the analogue signals that we’ve been used to for many years; snow, ghosting and other such interference are a thing of the past. A digital signal sends the information as a series of ones and zeros rather than an analogue wave. This means the digital signal can also send a larger amount of information than analogue which allows for the greatest advantage of HD – a much higher screen resolution – 720 or 1080 visible scan lines rather than the SD standard of 576 (PAL).

A television picture is made up hundreds of coloured pixels (picture elements) which when combined together create the image that we see. The greater the number of pixels in a given area, the higher the resolution and therefore the sharper the picture appears. Standard Definition screens have 576 visible lines of pixels from top to bottom (therefore 576 scan lines) and 768 pixels from left to right which gives the aspect ratio of 4:3 that for many years was the standard. The total number of pixels is therefore 768 x 576 = 42,368.  A true HD picture has 1920 x 1080 = 2,073,600 pixels, clearly a much higher resolution. However it is a little more complicated as, due to bandwidth restraints with the analogue signal, the SD picture actually has 576 interlaced scan lines meaning that only half (288) of the horizontal lines are scanned on the screen every 1/50th of a second. The eye does not readily detect this (although it may appear to “shimmer” a little) and sees the full frame displayed. The resolution is therefore referred to as 576i. A non-interlaced picture is referred to as progressive and shows the full number of scan lines on every frame and therefore produces a steadier and more pleasing picture.

Due to the lower bandwidth usage, currently High Definition TV (HDTV) will be available in 1080i or 720p (progressive). Whilst at first glance it may seem that 1080i would produce the superior picture of the two resolutions due to its higher overall pixel count, this is not necessarily the case. Because 720p displays a full frame of 720 lines at a time (usually every 1/25th of a second, this is termed 720p/25) it is better at displaying fast moving images such as sports broadcasts without any of the blurring that can result from an interlaced image, whilst 1080i has the higher resolution but only shows 540 of its scan lines at any one time (usually every 50th of a second, this is termed 1080i/50) and is better suited to showing slow moving images and fine detail. 1080p encoded films are being produced for both HD DVD and Blu-ray (see below) and offer the best quality picture available, although players which can actually show these at their full quality are currently very thin on the ground.

In viewing terms this rise in resolution equates to a vastly superior picture with stunningly sharp detail. The increased quality means that colour is greatly enhanced and vibrant. HD is also widescreen as standard, the viewing area having an aspect ratio of 16:9 compared to the traditional analogue ratio of 4:3. This provides a screen aspect closer to that of the cinema (2.35:1) allowing the viewing of movies as they were intended and a ratio which is more natural to the human eye. On the audio side, HD is capable of providing CD quality 5.1 surround sound for that real “centre of the action” feel.

What do I need?

Most importantly you will need a display capable of receiving and showing the HD images. An HD image viewed on a standard definition screen will appear superior to a non HD image, however to truly get the best from the HD signal you will require an HD Ready screen.

To benefit from the full audio experience that HD can provide you will need a 5.1 surround sound system home theatre set up, including a processor, amplifier and matching speakers. At present some but by no means all HD broadcasts take advantage of HD’s surround sound capabilities.

Whilst broadcasting in High Definition is currently limited in the UK, progress is being made. In June 2006 Sky released their HDTV set-top box in time for the World Cup and the BBC made their HD coverage of the event available to those lucky enough to get hold of a Sky box in time as part of their HDTV trial. Sky now has a number of channels available for subscribers to Sky HD. Telewest have also launched their HD service which is available to those subscribers who can receive it.

When it comes to watching movies in HD, two different and mutually incompatible formats are currently competing to become the storage medium standard, these are HD DVD and Blu-ray. Both are capable of holding far more information and therefore hours of footage than the current DVD format due to their use of blue lasers (hence Blu-ray) to read the information from the disc rather than the standard red ones currently used in DVD playback. Blue laser light has a shorter wavelength than red (wavelengths as low as 400nm compared to between 630 to 650nm for red lasers) and therefore more data can be stored on a single disc. Disc capacities are shown below:

Blu-ray

Pre-recorded playback	- single layer: 25GB – dual layer: 50GB
Home recorded	- single layer: 25GB – dual layer: 50GB

HD DVD

Pre-recorded playback	- single layer: 15GB – dual layer: 30GB
Home recorded	- single layer: 20GB – dual layer: 32GB

Both formats have a transfer rate of 36Mbps (Megabits per second).

Whilst HD DVD uses similar technology to a DVD disc that uses a laser to read from the underneath of the middle layer of the disc, the laser in a Blu-ray player reads from the bottom layer of a Blu-ray disc, possibly making the data layer more prone to damage, although a scratchproof layer has been developed to avoid this. Also, due to HD DVD discs being physically similar to DVD’s the cost of converting the manufacturing plants will be significantly lower than for Blu-ray discs which demand a whole new manufacturing process. This may be a large factor in the take up of either technology.

HD DVD was developed by Toshiba (who launched the HD DVD player in March) which has the backing of a number of film studios. Blu-ray was developed by the Blu-ray Disc Association which includes some big players such as Sony, Samsung, Pioneer, Philips and Panasonic and also has the backing of a number of other film studios. However some of the studios intend to release movies on both formats. Devices are also being developed which can handle the playback of both formats. It remains to be seen which medium if any will become the standard.

A new interface standard has also been developed to carry the digital signal between devices; this is HDMI (High Definition Multimedia Interface). Whereas previously when viewing a DVD on an LCD screen for example, a digital signal is sent from the DVD player then converted to analogue, then converted back to digital again by the LCD display device. This causes a loss of quality in the conversion process. HDMI is entirely digital and allows an uncompressed, clean digital signal to flow from the digital source to the digital display device with no interruption. This produces the best possible image. However HDMI not only carries the display signal but can carry up to 8 channels of audio meaning that only one lead is needed between devices. HDMI leads can also send signals in both directions which means that the display can also “talk” to the source. This means that the source can recognise the display characteristics (using EDID, Extended Display Identification Data) and deliver the content in its most effective format. It also means that it will become possible to easily control all products from just the one remote.

Due to the fact that HDMI allows the transfer of precise digital information it would open the way for piracy on a grand scale due to the ability to create exact copies of the original source material. Due to this an encryption standard has been developed called HDCP (Hi-Bandwidth Digital Content Protection) which has been adopted by the Motion Picture Association of America. This is designed to eliminate the reproduction of copyrighted material. What this means is that if you wish to view content which is encrypted with HDCP, both your player and your display device must be HDCP compliant. Therefore it is highly advisable that any HD devices that you purchase are HDMI/HDCP compatible/compliant.

Blu-ray and HD DVD use AACS (Advanced Access Content System) copyright protection which works to encrypt the information and only allow copies to be made and used on home media servers, hard drives or authorised media and not distributed via the internet or other sources. Within AACS, an ICT (Image Constraint Token) can be set by the movie studios when the disc is mastered, this means that should someone attempt to view (or duplicate) a movie with equipment which is not HDMI/HDCP compatible/compliant the player will detect this and down-convert the resolution from 1920 x 1080 to 960 x 540 pixels which is intended to prevent the piracy of full resolution HD movies.

One other way to view HD footage is via your personal computer. If running Windows XP or later it is possible to watch video using WMV HD (Windows Media Video High Definition) which uses VC-1 (Video Compression 1) compression. Because computer displays are progressive, this allows viewing at 720p or 1080p, assuming your monitor has a high enough resolution to display these formats and your computer meets the minimum requirements for playback. 5.1 surround sound is supported and existing 5.1 computer speaker systems can be used for this purpose.

Conclusion

In conclusion, it is clear that HD marks a huge step forward for the audio visual experience, which has been likened to that of the change from black and white to colour television. However whilst HD is essentially currently in its infancy, it also requires some knowledge of the technologies involved to ensure that the very best results are achieved and that forward compatibility is ensured. With the correct knowledge and equipment the future looks very good for superb quality audio visual entertainment.