High-definition television (HDTV) provides a resolution that is substantially higher than that of standard-definition television.
HDTV may be transmitted in various formats:
– 1080p – 1920×1080p: 2,073,600 pixels (~2.07 megapixels) per frame
– 1080i – 1920×1080i: 1,036,800 pixels (~1.04 MP) per field or 2,073,600 pixels (~2.07 MP) per frame A non-standard CEA resolution exists in some countries such as 1440×1080i: 777,600 pixels (~0.78 MP) per field or 1,555,200 pixels (~1.56 MP) per frame
– 720p – 1280×720p: 921,600 pixels (~0.92 MP) per frame
The letter “p” here stands for progressive scan while “i” indicates interlaced.
When transmitted at two megapixels per frame, HDTV provides about five times as many pixels as SD (standard-definition television).
HDTV broadcast systems are identified with three major parameters:
– Frame size in pixels is defined as number of horizontal pixels × number of vertical pixels, for example 1280 × 720 or 1920 × 1080. Often the number of horizontal pixels is implied from context and is omitted, as in the case of 720p and 1080p.
– Scanning system is identified with the letter p for progressive scanning or i for interlaced scanning.
– Frame rate is identified as number of video frames per second. For interlaced systems the number of frames per second should be specified, but it is not uncommon to see the field rate incorrectly used instead.
If all three parameters are used, they are specified in the following form: [frame size][scanning system][frame or field rate] or [frame size]/[frame or field rate][scanning system]. Often, frame size or frame rate can be dropped if its value is implied from context. In this case the remaining numeric parameter is specified first, followed by the scanning system.
For example, 1920×1080p25 identifies progressive scanning format with 25 frames per second, each frame being 1,920 pixels wide and 1,080 pixels high. The 1080i25 or 1080i50 notation identifies interlaced scanning format with 25 frames (50 fields) per second, each frame being 1,920 pixels wide and 1,080 pixels high. The 1080i30 or 1080i60 notation identifies interlaced scanning format with 30 frames (60 fields) per second, each frame being 1,920 pixels wide and 1,080 pixels high. The 720p60 notation identifies progressive scanning format with 60 frames per second, each frame being 720 pixels high; 1,280 pixels horizontally are implied.
50 Hz systems support three scanning rates: 50i, 25p and 50p. 60 Hz systems support a much wider set of frame rates: 59.94i, 60i, 23.976p, 24p, 29.97p, 30p, 59.94p and 60p. In the days of standard definition television, the fractional rates were often rounded up to whole numbers, e.g. 23.976p was often called 24p, or 59.94i was often called 60i. 60 Hz high definition television supports both fractional and slightly different integer rates, therefore strict usage of notation is required to avoid ambiguity. Nevertheless, 29.97i/59.94i is almost universally called 60i, likewise 23.976p is called 24p.
For commercial naming of a product, the frame rate is often dropped and is implied from context (e.g., a 1080i television set). A frame rate can also be specified without a resolution. For example, 24p means 24 progressive scan frames per second, and 50i means 25 interlaced frames per second.
There is no single standard for HDTV colour support. Colours are typically broadcast using a (10-bits per channel) YUV colour space but, depending on the underlying image generating technologies of the receiver, are then subsequently converted to a RGB colour space using standardized algorithms. When transmitted directly through the Internet, the colours are typically pre-converted to 8-bit RGB channels for additional storage savings with the assumption that it will only be viewed only on a (sRGB) computer screen. As an added benefit to the original broadcasters, the losses of the pre-conversion essentially make these files unsuitable for professional TV re-broadcasting.
At a minimum, HDTV has twice the linear resolution of standard-definition television (SDTV), thus showing greater detail than either analogue television or regular DVD. The technical standards for broadcasting HDTV also handle the 16:9 aspect ratio images without using letterboxing or anamorphic stretching, thus increasing the effective image resolution.
A very high resolution source may require more bandwidth than available in order to be transmitted without loss of fidelity. The lossy compression that is used in all digital HDTV storage and transmission systems will distort the received picture, when compared to the uncompressed source.