High Definition video is a new standard for digital video broadcast, which allows for a much sharper and more realistic picture than the current television broadcasting systems, such a PAL, NTSC or SECAM, can produce.
First developed by LG and introduced to the States during the 1990’s by the Digital HDTV Grand Alliance, HD achieves its higher quality by increasing the number of lines that make up the television picture, sent from the broadcast station.
According to the body governing Digital TV Format Standards, the Advanced Television Systems Committee (ATSC), any system that with a resolution of 720, or more, vertical lines is considered High Definition.
In the context of High Definition, the following notation can be observed:
[Vertical Lines]
[Progressive Frames / Interlaced Fields]
[Frames/Fields per Second]
For example:
A HD signal expressed as 720p60, translates to a screen resolution of 1280 x 720 pixels, with progressive encoding running at 60 frames a second.
Likewise, a signal expressed as 1080i50, translates to a resolution of 1920 x 1080 pixels, with interlaced encoding running at 25 fields per second (50 frames).
Despite this informative notation, HD signals are commonly expressed with only the number of vertical lines and the encoding method; 720p, 1080i, 1080p.
HDTV’s are also able to handle 16:9 True Widescreen aspect ratio pictures without using letter boxing – a technique utilizing the alteration of a 4:3 aspect ratio - thus further increasing the effective resolution for such content.
High Definition VS Standard Definition
TV broadcast resolution is defined by the physical size of the image transmitted by the broadcaster. The larger the image size, or resolution - specified as number of pixels (dots) wide by number of lines high - the more detail the image can contain.
Until fairly recently, there was only one type of broadcast resolution available in the UK - what we now call Standard Definition or SD. This is the broadcast we are all use to as it is the system used by both analogue and digital TV in the UK. The resolution of an SD broadcast is 720 x 576 (576 lines of 720 pixels each).
UK PAL broadcasts are considered to have 625 lines of vertical resolution, however, only 576 of these are visible, the remainder being used for text services such as Ceefax and Teletext, control messages, and monitoring. Likewise, the American NTSC system is known to run at 525 vertical lines, although only 480 of these make up the visible picture.
Typically, the out of frame, line 23 is used as an encryption control method in Japan, with future applications in Sky and Digital TV in the UK.
Interlacing and Progressive Scan
SD broadcasts use a technique called Interlacing, indicated by the letter “i” in 576i, which breaks down the video into frames, which then splits each frame into two fields. The first field contains all the odd lines, whilst the other field contains all the even lines. The TV, therefore, only ever displays half of the picture at any one time although, with 50 fields being displayed every second, this is sufficient to trick your eye into believing its seeing a full frame.
Two Interlaced fields forming one full frame
With the advent of HD come two further broadcast resolutions.
720p
720p offers a resolution of 1280 x 720. The p refers to the fact that this system uses a progressive broadcast. Meaning that each complete frame is broadcast. In this case the TV displays the entire image all of the time.
1080i
The second HD broadcast resolution offers an image size of 1920 x 1080, broadcast using the interlaced method.
1080p
Although not strictly a broadcast standard this offers 1920 x 1080 resolution in the progressive mode. There are no plans for 1080p transmissions for at least 10years and no hardware is currently capable of recording 1080p.
HD Connections
There are various differing types of connection that can be used for carrying HD video:
Component
Component cable features 3 phono-to-phono analogue leads that carry only analogue video and do not include any robust copy protection. Component connectivity will carry no audio and have a maximum HD specification of 1080i.
DVI-D / DVI-I
Digital Visual Interface Digital / Analogue & Digital, is used mainly by computer based devices, although some manufacturers have used it on TVs. DVI carries only digital video signals, so audio would need to be sent via other means, usually two phono leads. DVI will support High Definition (up to 1080i) but requires support of the High Definition Copy Protection (HDCP) security method in order to do so.
HDMI
High Definition Multimedia Interface, or HDMI, is the digital equivalent of the SCART lead. It carries both video and, up to, 8 channels of audio. HDMI is the preferred method for connecting HD devices. HDMI will support the full 1080p signal and comes complete with integrated HDCP backing.
HD Ready
To eliminate confusion amongst consumers, a HD Ready logo has been developed to identify all TVs that meet the following minimum criteria:
- Must provide a minimum vertical resolution of 720 lines.
- It must feature either HDMI or a DVI input that supports HDCP
- It must have a component video input for compatibility with analogue HD sources, such as HDV camcorders.
Recording
While it is possible to record HDTV to Digital-VHS, the current preferred method is to use a hard drive. Given the extremely large bandwidths and high amounts of data required when working with HD, standard DVD recorders fail to provide a suitable length of recording time. To this end, two new, disc based, storage mediums have emerged – BluRay and HD-DVD. Each providing capacities in excess of 60 Gigabytes.
Availability
Whilst Japan has had the earliest HDTV system in place, from as long ago as 1979, the UK’s first HDTV service began in December 2005 with the cable TV company Telewest (now part of NTL), offering HDTV programs to customers in the London area. At which time the BBC, having a vast array of HD content, began trials of High Definition over the Freeview digital TV service.
By May 2006, High Definition had entered mainstream broadcasting with the launch of Sky’s HDTV service. The Sky service is currently available (June/06) through the, £300, Sky HD box and a £10 additional subscription fee.
Contrary to popular belief, High Definition is available and has been for some time. It should be noted that in order to view a High Definition picture, you require a suitable source, these can include, HD Camcorders, Games Consoles (PS3 release date 16th November 2006), Sky HDTV, Cable or a Computer. Whilst many HD Ready TV’s sport a standard higher resolution (typically 1mp over 0.8mp), you will not get a HD quality image from a standard analogue or digital broadcast.
1080p is likely to be used be devices such as the PS3 and offers ultimate HD experience currently available. It is almost a certainty that broadcast HD will not enter the realm of 1080p, rather settling for the lower end HD formats 720p and 1080i in order to save on bandwidth and increase the number of services available.