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An Overview of TV Resolution

HDTV-Resolution is the primary focal point in understanding the unique picture quality of High Definition Television. Before HDTV, viewers had no reason to be concerned with TV resolution; all TV's displayed the same (single) resolution. However, with the advent of Digital High Definition Television and the introduction of HDTV-Resolution everything has changed.

It is important to note that this brief, overview is not intended to be a comprehensive review of the subject. In discussing HDTV-Resolution, as with virtually any topic related to Digital-HDTV, the challenge is to transpose the relevant points, from within a highly-complex technology, to a level of understanding that is useful to the consumer.

The following, therefore, is offered as a highly, simplified view of HDTV-Resolution...
By providing a closer look at what TV resolution is, how it is measured and how it relates to TV picture quality, the non-tech consumer can get a better understanding of how completely different the HDTV experience is.

All Televisions are not created equal. This is quickly discernable in the wide-ranging disparity observed when comparing the Picture Quality displayed by different TV sets.
There are, of course, a number of other factors that also contribute to the picture quality of individual televisions.

What is TV Resolution?

• The number of lines per (mm or inch) both vertically and horizontally.
• The number of line-pairs per (mm or inch) - vertically and horizontally.
• The number of lines per total-display - Lines per Picture Height - LPH.
  

The TV Picture

To understand the significance of resolution in Digital-HDTV, let's begin with the television we know - the traditional, NTSC-analog TV. It will help to take a closer look at how the images we see on the TV screen are formed.

In pictures made on film - 'movie-pictures' - images are projected on to the screen as a complete picture, in a single action. Creating video or television pictures is quite different.
The pictures and completed images we see on a traditional CRT (Cathode Ray Tube) TV screen, are really a series of horizontal and vertical lines; (visualize Horizontal Rows and Vertical Columns) with these 'rows' and 'columns' consisting of "tiny" dots or dashes.

The 'rows' are commonly called "Scan Lines" since they are applied to the screen as the 'electron gun' scans from left to right, and top to bottom. (This application of scan lines is sometimes described as 'painting' or 'drawing' the scan lines)

The 'electron gun,' located at the rear of the picture tube, is basically a wire filament that becomes heated due to resistance as electric current is applied. The heating action causes electrons to collect around the filament. By applying a high, positive voltage, the negatively charged electrons are accelerated away from the filament, and towards the phosphor-coated, interior surface of the picture tube. The accelerating electrons are concentrated in a narrow beam which strikes the coated surface of the picture tube, causing the phosphor to glow in that focused area..

Where a picture's scene is darker, the electron beam is 'weaker' - or less intense - and the phosphor 'glow' is less. Where a scene in the picture is brighter, the electron beam is more intense, and the phosphor 'glow' is brighter.

The electron beam scans across the surface of the picture tube, in direct coordination with how the original scene was scanned by the sensor in the television camera. As the scan lines vary - changing from bright, to dark, with many intensities in between - images are formed on the TV screen.

As stated above, the "resolution" of a TV is how well it is able to distinguish between the alternating light and dark lines, when these are spaced close together. If the lines are too close together (exceeding the resolution limits) they will appear merged - being neither dark, nor light, but blurred into 'muddy' shades of gray.

What All the Numbers Mean

In NTSC television, the electron beam scans (525) horizontal lines (rows) across the screen - starting with line-number (1) at the top-left of the screen, and ending at the bottom with line-number (525).

However, NOT ALL of the (525) scan lines are visible on the screen. Some loss of lines occurs while the electron beam moves from the bottom of the screen to go back to the top, and start a new scanning sequence. Also, some of the "non-visible" scan lines contain transmitting and display data - information the TV uses to create the display. Thus the total number of visible scan lines that appear on the screen is reduced to about (480 lines).

Since TV viewers are primarily concerned with just the visible scan lines, and also as a way to maintain some consistency when discussing TV resolutions, the common reference for NTSC-Analog TV resolution is 480-i; the (i) indicates that the scanning method used is "interlaced." (More about scanning methods later.)

Vertical and Horizontal Resolution

The "vertical resolution" of NTSC TV refers to the total number of lines (rows) scanned from left to right across the screen - BUT Counted from Top to Bottom, or Vertically. This number is set by the NTSC TV 'Standard' (ie: 520 lines - 480 'visible' lines).
This Vertical Resolution number is static - it doesn't change.
Therefore, the Vertical Resolution is the same for ALL TV's manufactured to meet a specified Standard.

The "horizontal resolution," (number of vertical lines or columns) however, is variable.
The horizontal resolution of television, and other video displays, is dependent upon the quality of the video signal's source.
As an example - the horizontal resolution of VHS tape is (about) 240 lines; broadcast TV (about) 330 lines, laserdisc (about) 420 lines; and DVD (about) 480 lines.

A TV's resolution can be reported as the result of counting the total number of picture elements (pixels) per scan line, across the entire screen-width, multiplied by the total number of scan lines. However, TV screen-sizes vary, making an equal comparison of different displays more complex. TV's also differ technically, functionally and in component quality; this results in additional complications.

An alternative method is to count the number of pixels that fit within a prescribed circle, having a diameter equal to the screen height. Known as LPH - Lines per Picture Height - this is the 'correct' method in determining TV resolution.

As this shows, along with other, similar variables, the accuracy of a 'stated' horizontal resolution for a particular display, may depend on who is doing the 'stating' ...

However, for the purpose of this overview of HDTV-Resolution, the primary point regarding horizontal resolution, is that it is variable. Unlike vertical resolution which is 'fixed,' horizontal resolution can differ from one TV display to another.

SDTV and HDTV

In general, it is common practice to list a television's vertical resolution without referencing the horizontal resolution when indicating the display capability. Since the buyer is primarily interested in knowing whether or not the TV can display SDTV (Standard Definition) pictures (only), or is able to display both SDTV and HDTV picture quality, listing the vertical resolution alone is generally sufficient.

Digital TV consists of (18) specified formats accepted by the ATSC (Advanced Television Standards Committee). Of the eighteen, only those formats that apply to consumer television viewing are of interest to this present discussion. This includes SDTV (Standard Definition TV) and HDTV (High Definition Television). The vertical resolution for these formats is set by the ATSC Standard.

Note: There is an 'alternative' Digital TV Classification - "EDTV" (Enhanced Digital TV). However, since it merely denotes digital televisions that meet the SDTV Standards, and have the same attributes of 'high-end' SDTV. the value of this designation may be questionable.

The resolution set by the SDTV Standard is 480 (i/p) - visible scan lines - either (i) interlaced or (p) progressive scan.

The Standard for HDTV-Resolution is (any) one (1) of three (3) specified resolutions:
720p, 1080i and 1080p. Again, the numbers refer to visible scan lines, and interlaced or progressive scan, as applicable.

Different Views of SDTV and HDTV-Resolution

Digital TV - SDTV:

• 480i - 704x480 interlaced
• 480p - 704x480 progressive

Digital-HDTV:

• 720p - 1280x720 progressive
• 1080i - 1920x1080 interlaced
• 1080p - 1920x1080 progressive

More Accurate - Listing the resolution and frame rate:

• 480i - The picture is 704x480 - (60/2 interlaced frames per second)
  = 30 complete frames per second.
• 480p - The picture is 704x480 - 60 complete frames per second.
• 720p - The picture is 1280x720 - 60 complete frames per second.
• 1080i - The picture is 1920x1080 - (60/2 interlaced frames per second)
  = 30 complete frames per second.
• 1080p - The picture is 1920x1080 - 60 complete frames per second.

Comparison of Digital TV and HDTV

Note: Although "non-CRT" television sets use different technologies in creating the screen images, and there are other factors involved with how they display the picture, in the end, HDTV-Resolutions are still (720p) and (1080i) - or higher.

Because it is important, yet so often confused, it bears repeating: Vertical Resolution refers to the lines (rows) that are applied (scanned) across the screen, from left to right; these are counted from top to bottom, or vertically - thus the designation, Vertical Resolution.

Similarly, Horizontal Resolution refers to the lines (columns) going from top to bottom, which are counted across the width of the display, or horizontally - and referenced as Horizontal Resolution.

Some additional notes of interest:

Among the many issues that feed the relentless mis/dis-information surrounding Digital-HDTV, is the prevalent misconception that "Digital TV" and "HDTV" are synonymous. The chronic mis-use of these two, distinct classifications - even by some within the 'consumer-electronics-industry,' and many apparently un-informed retail-sales people and media-reporters, only adds to the insidious confusion that has so bewildered consumers.

The FCC mandate is for TV Broadcasts to transition from NTSC Analog TV to ATSC Digital TV. Although Television Broadcasters have been encouraged to transmit Digital-HDTV Signals,they are NOT Required to use the Digital TV spectrum to broadcast High Definition Television Signals.
So, while Digital TV is an established reality, the future of High Definition Television - HDTV - remains uncertain; there are NO Guarantees.

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