HD Guru Basics: About HDTV Resolution
Resolution is one of the most misunderstood HDTV topics. Here are the basics.
Item 1- Resolution: One Term, Two Meanings
When you see the term Ã¢â‚¬Å“HDTV resolutionÃ¢â‚¬Â it will be referring to one of two types: Broadcast resolution or a displayÃ¢â‚¬â„¢s Ã¢â‚¬Å“native resolutionÃ¢â‚¬Â.
Item 2 -Broadcast Resolution
The two high definition resolutions being broadcast today are 720p and 1080i. All high definition sets can accept both types of signals. The resolution you will actually see displayed on the TV screen is covered under Ã¢â‚¬Å“native resolutionÃ¢â‚¬Â in section 4.
720p and1080i Defined
720p is a standard defined as producing a high definition image with 1280 picture elements (pixels) horizontally and 720 lines (also can be called pixels) measured vertically. The Ã¢â‚¬Å“pÃ¢â‚¬Â means progressive. Progressive is defined as a complete frame (think of it like a frame of motion picture film) being sent to the display every sixtieth of second. This means you see new frames sixty times per second. This resolution is commonly written as 720p (60) or 720 60p. They all mean the same. The aspect ratio or proportion of a 720p image is 16 x9, also called widescreen.
2B- 1080i 1080i is the standard defined as producing and image with 1920 picture elements (pixels) horizontally and 1080 lines (pixels) measured vertically in the 16:9 aspect ratio. The Ã¢â‚¬Å“iÃ¢â‚¬Â means interlaced. You see new frames 30 times per second.
To explain interlace, imagine a sheet of paper 16 inches long and 9 inches wide, with a still image of the sea shore printed on it. Inserting this piece of paper in a shredder it cut the paper into 1080 strips (but intact). Each strip is 1 to 1080 starting at the top of the paper. With the sheet fully numbered sliding just the 540 even numbered strips of paper to the right and leaving the 540 odd numbered ones intact would result in having two sets of strips of the seashore image side by side, each set one containing exactly one half of the picture information.
This is what interlacing does. All the odd number lines of resolution making up the image are transmitted sequentially, the first set of odd lines in 1/60 of a second. This is called field one. The next is the even numbered ones transmitted as field two. The two fields are combined by the TV inserting the even field lines of image between the lines of the odd numbered image information from field one. The result; a complete image seen every 1/30th of a second. (1/60th of a second odd lines + 1/60th of a second even numbered = new image ever 1/30th of a second). A complete image is called a frame (as in a frame of movie film). 1080i is sometimes written as 1080i30 (for 30 frames per second) and sometimes written as 1080i 60 (for sixty fields a second) they both mean the same. It is highest resolution broadcast standard.
Item 3 Ã¢â‚¬â€œHDTVs Displays: Two separate groups Interlaced and Progressive
3A- The interlaced group is made up of HDTVs are made up of
a) CRT direct view televisions (also called a picture tube TV, they are made in 16:9 widescreen and old style 4:3 screen aspect ratios);
b) CRT rear projection, which use three picture tubes (red, blue and green) to form a full color projected 1080i interlaced image. Unlike digital TVs (see below) CRTs can change their scan rate to match 480i broadcast and change again to match a 1080i broadcast signal. This multiscan ability is unique to CRT technology.
c) The 42Ã¢â‚¬Â plasma TVs made by Hitachi (theyÃ¢â‚¬â„¢re called AliS) are the only other interlaced HDTVs.
3B -All other TVs produce progressive images placing a new complete frame on the screen every 1/60th of a second. This group is made up of three types:
a) LCD flat panels,
b) Plasma (except the aforementioned Hitachi),
c) Microdisplay projectors (DLP, LCoS and LCD),
Iem 4- Native Resolution
The term native resolution means the resolution that a given TV produces. All CRT displays are Ã¢â‚¬Å“analogÃ¢â‚¬Â They scan an electron beam across the face of the tube.
Direct view CRT HDTVs range from 800 to about 1000 pixels resolution across. The vertical resolution of direct view HDTVs when fed a high definition signal is always 1080 lines. Most direct view CRT HDTVs can also produce vertical resolution of 480 lines when fed a standard definition (480i) signal.
CRT rear projectors range from about 800 to around 1200 pixels resolution across. All CRT rear projectors HD vertical resolution is 1080. CRT rear projectors can also change their Ã¢â‚¬Å“scan rateÃ¢â‚¬Â to produce a standard definition vertical resolution of 480 lines
All Progressive displays are Ã¢â‚¬Å“digitalÃ¢â‚¬Â and all digital displays (except the interlaced 42Ã¢â‚¬Â Hitachi above) are progressive. Digital displays are microdisplay projection, flat panel LCD and all Plasma. All digital display devices have a fixed vertical and horizontal resolution. To be called a High definition display, it must have at least 720 pixels (lines) of native vertical resolution.
The most common HD Ã¢â‚¬Å“native matrixÃ¢â‚¬Â resolutions for digital displays are 1280 x 720, 1024 x 768, (1365) or 1366 x768 and 1920 x 1080. There is no standard and there are sets on the market that have native resolutions of 1024 x 1080, 1280 x 768 and other matrixes.
Item 5- Getting Broadcast Resolution and Rate to Match a Display
If a broadcast signal doesnÃ¢â‚¬â„¢t match the native resolution of a display it must be processed and converted to match. Uniquely, as previously stated, CRTs based devices can adjust their scan rate to 480i for native standard definition signals requiring no scaling conversion. This is why people say that standard definition looks best on a CRT. No conversion means there are conversion artifacts that degrade the image. However, all current CRT projectors and direct views must perform conversion to turn 720p broadcasts signal into 1080i for display.
All digital HDTV displays must convert broadcast signals to match the displayÃ¢â‚¬â„¢s native resolution. All sets require a chip called a scaler that is used to do the conversion. Scalers (also called signal processors) are dedicated computing chips that use complex mathematical computations to make adjustments for motion and other factors. All digital displays (except the Hitachi 42Ã¢â‚¬Â) must also convert interlaced signals (480i/1080i) into progressive signals.
As a rule of thumb, many (but not all) inexpensive non-brand name televisions as well as a number of name brand sets use poor scalers to save money. The HD Guru has been evaluating HDTV internal deinterlacer and scaler performance. You can find his latest tests at hometheatermag.com and soon within the blog.
Item 6 – Wrap up
CRTs produced the best standard definition image because they do not need to convert standard definition interlaced signals into progressive high definition for display*. All digital displays must convert standard (480i) definition and all 1080i signals into progressive (called deinterlacing) as well as up/down convert broadcast signals to match the native resolution of the display**. This conversion is called scaling.
Item 7 – The Bottom line Unlike what is written in many of the store advertisements, not all HDTV have 720p or 1080p resolutions. Resolution is just one factor in determining which HDTV to purchase. Viewing distance goes hand and hand with screen resolution, if you are too far from a display your eyes will not be able to resolve all of the screen resolution. Check out my HDTV Viewing Chart to find the right screen size for your viewing distance.
*(Except 42Ã¢â‚¬Â Hitachi plasma)
** There are a number of HDTVs with native 1280×720 resolution. These native 720p HDTVs still must scale 720p broadcasts to make the image larger (called overscan). Overscan is employed to prevent artifacts from being seen at the edge of the display
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