Television and Video Resolution

TV and Video Resolution

We believe that this is the most correct easiest to understand description of horizontal and vertical resolution for television and video. Information has been gathered from a variety of sources and is continually being updated.

There are at least three major subtopics for resolution,

1. For the TV set itself, its ability to reproduce fine detail in terms of how small a spot or how thin a line it can make on the screen, for example 500 lines (or alternating light and dark dots) of resolution.

2. For the transmission (e.g. broadcast) or storage (e.g. DVD) format, the amount of detail that can be retained or conveyed, for example 720 by 480 pixels in a rectangular frame,

3. How effectively the method of recording (video camera) captures live subject detail (Kell Factors).

All of this contributes to the final measure of resolution, what you see in terms of venetian blinds, picket fences, or polka dot clothing.

Resolution Factoids, or, In a Nutshell

Click on colored words to get details about that subtopic.

While all (NTSC) TV sets and program material (broadcasts, tapes, disks, etc.) use the same 525 scan lines, the advertised resolution (240, 425, 500, etc.) refers to the horizontal resolution which is the number of side by side dots that can be reproduced within any one scan line.

The number of scan lines is the limit for vertical resolution, the number of dots in a column that can be reproduced. It is pretty much the same for all decent NTSC TV sets of moderate or large size.

If the video signal was processed digitally at any time, there will be a horizontal pixel count which is is an upper limit for horizontal resolution.

Both horizontal and vertical resolution also depend on how tiny a spot can be produced on the screen.

The finite number of scan lines and, for digital video, the number of pixels horizontally also limit the smoothness of diagonal lines and the resolving of position of fine details.

When evaluating a TV set, DVD player, etc. for purchase, view several programs. It is possible for the program material to be deficient in resolution.

Traditionally, "lines of resolution" is correctly measured across the largest circle that fits in the space you are talking about. However some advertisers exaggerate using the entire screen width or using some electronic formula that only covers part of the circuitry. Modern standards stating resolution or screen dimensions in "pixels" (dots) reflect the entire screen width or height.

For NTSC, the picture occupies approximately 480 of the 525 scan lines. For broadcasts the portion of a scan line that is visible can hold up to about 440 dots so a grid 480 high by 440 wide represents the maximum amount of picture detail possible.

Advertised resolution applies to blacks, grays, and whites only. Color resolution is much less. The human eye and brain are less sensitive to lack of color within fine detail.

"Lines of resolution" for video include both the black lines and the white spaces between them in a test pattern but not the gaps if any between scan lines or between phosphor dots/stripes.

Dot pitch of the screen also limits resolution and applies to rear projection TV sets as well. Pixels do not correspond to phosphor dots or stripes on a picture tube or ribs on a rear projection screen..

The more brightness and color changes, representing picture detail, the greater the circuitry bandwidth and broadcast channel space needed. Horizontal resolution is the first to be affected by bandwidth limitations.

In terms of accurately reproducing the program material, there are three general areas where resolution loss may occur:

1. During capture of the picture information by the camera and electronic processing of the fresh video signal,
2. In the process of storing the picture information on disk or tape, or transmitting it over the air on via satellite or "cable",
3. During playback, in the TV set, also in VCR and disk player electronics.

The video frame can stand for any screen aspect ratio chosen in advance before the subject matter is televised or recorded, although only 4:3 and 16:9 aspect ratios have been standardized so far for NTSC.

Pixels do not have to be square. The ratio of pixels horizontally to pixels vertically does not imply any specific aspect ratio.

The number of lines of resolution vertically is smaller than the number of scan lines; for digital video the number of lines of horizontal resolution is smaller than the stated number of pixels across. This is because the scan lines or dot positions can straddle subject detail so as to reproduce nothing. Experts disagree on the ratio of lines of resolution to scan lines or pixel positions, some say as little as 70%.

If, during interlaced scanning, the even lines land on top of the odd lines (line pairing) instead of being in between, vertical resolution is lost.

Progressive scan conversion can synthesize but cannot recover details missed because the source was not progressively scanned originally, its main purpose is to reduce flicker by repainting the picture on the screen faster.

Redoubling as a quadrupler might do can synthesize but cannot recover details that scanning the source originally at that faster scan rate right may have recorded. This just repeats each scan line to fill the tiny gaps between scan lines, and/or blends adjacent scan lines to reduce jagged edges.

Today's cable TV systems carry mostly NTSC broadcast quality but they will carry video signals with much more resolution when more customers have HDTV quality TV sets.

If a computer monitor can display 640 x 480 and 1024 x 768, then it can display 720 x 480 or even 1024 x 480.

The rule of 80 lines of resolution for every megahertz of bandwidth (for NTSC) is measured from the carrier frequency to one side only, i.e. the larger sideband when dealing with modulated signals.

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Video glossary

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Quick Description of Vertical and Horizontal Resolution

The more scan lines we have, the better the vertical resolution is possible. That is, the more dots in a column, or thin lines lying flat in a stack, can be reproduced and distinguished. When portions of several scan lines in a row are the same at a particular place on the screen we are reproducing coarse detail. When adjacent scan lines differ, we are reproducing fine vertical detail. Since each video standard has a fixed number of scan lines (525 for NTSC, 625 for PAL), vertical resolution is the same for all TV sets of the same standard except the poorest quality, defective, or very small sets.

Advertised resolution is the horizontal resolution and it varies with the quality of the TV set. Side by side thin upright lines, whose cross section is a row of dots, are used to measure horizontal resolution. To resolve horizontal detail, the electron beam changes to make dots (and dashes) of different colors, along each scan line. The finer the detail we need to reproduce, the tinier a spot the electron beam must be able to make.

The conglomeration of all those dots and dashes on all the scan lines equals the picture.

Since the electron beam "paints" the screen at a constant speed, the finer the horizontal detail (smaller dots), the faster the changes must be. The faster the changes, the higher the frequency the video signal must be able to attain. Since signal frequency is one of the limiting factors for video information storage and transmission, horizontal resolution is the first to be affected.

The Circle Rule

When measuring "lines of resolution" for TV sets or monitors, a reference distance needs to be specified. Traditionally, video resolution is measured across the largest circle (not an ellipse) that fits in the space we are talking about. All the technical books on video say this, although using different words. On a standard TV screen such a circle would span 3/4 of the screen width. For U.S. HDTV (16:9 aspect ratio) the circle would span a little over half the screen width. Since the circle exactly fits the screen height (assuming no overscan), the phrase "picture height" stands for a distance equal to the diameter of the circle and is also used when talking about horizontal resolution. The circle rule also applies to film.

Under this rule, if a standard (4:3) TV set can reproduce 800 dots alternating black and white across the screen width using an input signal representing same, it is said to have 600 lines of horizontal resolution. However if the source material had just 330 lines of resolution you will see just 330 lines of resolution. A 16:9 aspect ratio TV that can reproduce just 800 dots across its screen would have 450 lines of horizontal resolution.

Technical term: Cycles per picture height -- One cycle of a simple video signal waveform consists of an "up" which makes a dark dot or dash and a "down" which makes a light dot or dash. At the 4.2 MHz maximum usable NTSC broadcast video frequency, at most 165 cycles can occur while the electron beam is drawing inside the circle we were talking about above so we can say that the horizontal resolution is 165 cycles per picture height. The term "cycles per picture width" is also used but less often. Incidentally one cycle corresponds to one "line pair" in photographer's terminology.
For film "lines of resolution" is typically measured as per millimeter. We cannot easily use "lines per inch" to describe video resolution because we would expect two TV sets of the same quality but with different sized screens to have about the same horizontal resolution, and exactly the same the vertical resolution. This has led to the phrase "per picture height".

When pictures are transmitted or stored digitally, each scan line has to be broken up into dots called pixels. Both the vertical and full width horizontal pixel counts are specified, representing a rectangular grid. A common computer video resolution is called VGA and is stated as 640 by 480 pixels. Here the aspect ratio does not have to be known or stated. For analog video (broadcasts, VHS tape, laser disks) we can think of the horizontal pixel count as the maximum amount of the finest details that can be put on a line and still be distinguished. The video source may have a given resolution in pixels but if the TV cannot make spots small enough, what should be individual closely spaced dots will be a continuous blur..

Published Resolution is for Black and White

(Applies to transmission and storage and also to displaying of video)

All horizontal resolution figures published for video, if not otherwise qualified, apply to blacks, grays, and whites only. All of this has to do with maximum signal frequency and limits on what will fit into the broadcast channel or on VHS tape, etc. The luminance signal, also called luma, or Y, contains all the information needed to produces the video picture but in black and white. It is transmitted or recorded with enough bandwidth to carry the desired or maximum possible horizontal resolution of the subject material. Color is broadcast using a second signal (called the chrominance, chroma, or C signal). It has much less bandwidth, in some cases as low as 0.5 MHz allowing only 40 lines (broad stripes) of horizontal resolution. Small horizontal details (dots along a scan line) therefore cannot be of alternating contrasting colors. The human eye and brain do not notice loss of color in the finer details of a motion picture as much as they sense blurring of the light and dark content.

Improved color resolution is a significant factor in the quality of digital TV programs such as on DVD.

More on color resolution

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Both the Black Lines and White Spaces Count

Resolution is measured using charts with closely spaced dots, parallel lines or approximately parallel lines, the more that can be distinguished in a given area, the better the resolution. In order to see a series of dark lines we must have light spaces in between them. For film, "lines of resolution" count only the dark lines, not the spaces in between. Because video has much lower resolution than film, video people count both the dark lines and the light spaces as "lines" in order to have larger numbers to talk about. The term "television line" or "TVL" thus refers to scan lines vertically or the smallest possible dots on a line horizontally. In response to this, film folks have resorted to using the phrase "line pair" to stand for a dark line together with the light space to one side. Sometimes the scan lines are thin enough that dark gaps are seen between them. These gaps, and also the dark boundaries between red, green, and blue bars or dots on the picture tube face, do not count as lines of resolution.

An Example (NTSC broadcasting)

To show a movie (motion picture) in the theater, still pictures are flashed on the screen one after another, 24 frames of the film strip per second. For TV programs it is 30 frames per second. Once we start discussing things that repeat many times per second we must bring the words cycles, megahertz, and frequency into our discussion. (Hertz is short for "cycles per second".)

The first number that comes to mind when talking about NTSC video is 525, the number of horizontal stripes or scan lines drawn across the screen to construct the picture. The scan lines are drawn as follows: 240* odd lines of picture information, 22* black lines, 240* even lines of picture information, 23* black lines. This is all repeated 30+ times a second. The "black" lines contain information to keep the TV set scanning in synchronism with the broadcast and allow time to get the electron beam back to the top of the screen. They also contain coded information for closed caption text, commercial identification and tracking, etc.

To create picture detail the electron beam makes tiny dots and dashes as it draws each scan line. With 525 scan lines redrawn 30 times a second, millions of these dots and dashes are needed every second. Electronically the NTSC broadcast video signal uses frequencies up to 4.2* million cycles per second (MHz). Every cycle has an "up" that represents a dark dot and a "down" that represents a light dot, thus 4.2 MHz can represent 8.4 million dots per second. To make coarser detail (dashes) the cycles are stretched out, or represent lower frequencies. To make shades of gray the ups and downs are not so high or deep. At this rate we can get a maximum of 533* dots per scan line. Not all of these dots can be used for picture detail. In the early days of TV, the last seventeen percent of each scan line had to be made black to hold synchronizing information and to get the electron beam back to the left side to draw the next line. The remaining 83% of a scan line holds 442 dots which is the maximum amount of horizontal detail we can put into the picture.

Applying the circle rule, 330 of the dots fit across the largest circle that in turn fits in the screen, thus giving the commonly published horizontal resolution of 330 lines for NTSC broadcast TV programs. While the TV set may be capable of higher resolution (making even smaller sized dots as needed for the best playback of DVD) the most resolution that will be seen for received TV broadcasts is 330 lines.

Trying to broadcast more dots on a scan line would make parts of the video signal go above 4.2 MHz and interfere with the sound portion of the TV channel which is positioned at 4.5 MHz. The circuits (filters) that make sure the video signal does not contaminate of the audio signal unfortunately usually weaken the portion of the valid video signal near 4.2 MHz. This makes it impossible to get the tiniest details that could be broadcast to be really black or white; they end up being dark gray and light gray instead.

Technical term: Bandwidth -- The difference between the maximum and the minimum frequencies of a band of frequencies that a circuit or system can pass with no more than 50% attenuation (weakening). If the bandwidth does not start at zero, two of the parameters lower bound, upper bound, and width, must be specified.

Some sources state that 4.0 MHz is the maximum usable video frequency, since typical video amplifiers in the early days couldn't get up to 4.2 MHz while still cutting off by 4.5 MHz. This permits an even 8 million dots per second or 508 dots per scan line. Also, if the transmitting electronics weakened the 4.2 MHz portions of the video signal by 50% and the receiving electronics weakened it another 50%, each set of electronics by itself meeting the definition of a 4.2 MHz bandwidth, by the time the picture got to the screen, the tiniest dots could still be too much grayed out to be distinguished.

* Numbers marked with an asterisk are approximate. These numbers and also the exact location of the 525'th line may vary slightly depending on the subject matter and with the make and model of equipment used.

+ To make this text clearer, numbers marked with a plus sign have been rounded from actual precise values that have lots of decimal places.

For NTSC interlaced video, the horizontal resolution is 80 (79) times the video circuitry bandwidth in megahertz, for example 3.0 MHz which is typical for a cheap TV allows about 240 lines of resolution.

Digital vs. Analog -- the Extended Kell Factor

Picture detail must be on the scan lines, not between.. This rule is an example of a digital representation. For video that was processed digitally at any time in its life, each scan line is divided into evenly spaced pixels and picture details must also land squarely on these pixel positions to resolve the maximum detail. It is as if you imagine looking at something through a coarse silk screen or window screen and have to choose just one color for each hole, and then paint the scene on canvas one dot at a time corresponding to the holes. For computer generated video, as in some cartoons, details can be as small as one pixel consistently. But during televising of live subjects, if a white spot wanted to end up between scan lines, it would be represented as a white spot on the line above, a white spot on the line below, or faked as gray spots on both the line above and the line below, depending on the strategy used by the equipment. In the last case it took two scan lines to represent one detail so the total number of distinguishable details spanning a given distance in a column (and in a row for digital video) would be less.

If the scan lines straddled televised subject matter consisting of stacked black and white stripes, such as a Venetian blind in the background, the result could be a total gray blur.

High definition TV provides more scan lines and more pixels on a scan line to choose from, thus improving the resolution of the position of details. Smoother diagonal edges and sharper thin diagonal lines are seen even if the miminum spot size is larger than one pixel which is still true for today's HDTV sets.

Technical Term: Sampling -- In picture reproduction, sampling is the act of taking a finite number of spots from the original scene and having them stand for the entire picture. In the case of DVD recording, almost 350 thousand evenly spaced spots (samples) are taken, from an imaginary grid 720* dots wide and 480 dots high covering the picture.

For ordinary TV broadcasting, horizontal picture detail can occur anywhere along a scan line without having to line up with grid positions. This is an example of an analog representation.

Digital video techniques, DTV, DVD, etc. represent horizontal detail digitally as well, so detail blurring out due to straddling the grid (pixel) positions becomes a problem horizontally just as it does vertically for all forms of video. A white picket fence in the background could end up as a gray blur. Or if the pickets closely but don't exactly match the pixel grid, the fence will be clear in some spots and blurred in others. This effect is an example of a moire pattern.

We will call the highly subjective ratio of "lines of resolution" (analog) to scan lines or pixels (digital) an Extended Kell factor. This takes into account pixel straddling and also such reproduction shortcomings as thin black gaps between scan lines. The larger the better although it can never exceed 1.0 -- you cannot resolve more details than there are pixels. Experts disagree to this day as to what an Extended Kell factor for good video equipment should be. The only published figure for a Kell factor so far is 0.7 and it dates back to television in the 1950's. Using a Kell factor of 0.7, the 480 scan lines give 337 lines of vertical resolution. This is roughly the same as the broadcast horizontal resolution which relationship was a design goal. (The Extended Kell factor would be the same or less since it takes more deficiencies into account than the original Kell factor).

To convert between pixels and lines of resolution we must apply the circle rule in the longer direction (horizontally); after that we always apply the Extended Kell factor vertically, and we apply the Extended Kell factor horizontally for digital video only. Therefore:

To convert pixels wide to horizontal lines of resolution we divide by the aspect ratio and multiply by the Extended Kell factor.

To convert horizontal lines of resolution to needed pixels we multiply by the aspect ratio and divide by the Extended Kell factor.

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The NTSC Broadcast Channel

Each U.S. TV station is allotted a six megahertz portion of the frequency spectrum to broadcast in. For example Channel 2 spans 54 to 60 MHz.

To get the 4.2 MHz video signal into the channel, it is "modulated" using a carrier frequency, 55.25 MHz in the case of Channel 2. What then happens is that the 0 to 4.2 MHz video frequencies become 55.25 to 59.45 MHz frequencies respectively. What also happens is that a mirror image, frequencies 55.25 down to 51.05 MHz, appears. The total of 8.4 MHz of bandwidth obviously exceeds the channel boundaries. If, when television was first developed, we simply chopped off the lower sideband 51.05 to 55.25 MHz, the electronics needed to process the rest of the signal were too complex and expensive. But keeping a portion of the lower sideband, 55.25 down to 54 MHz, made the electronics reasonably simple and that is what is done.

More.

Computer and Digital Video Resolution

For transmission and storage of video in digital form (which includes U.S. HDTV and video in a computer prior to reaching the monitor), each scan line must be broken up into small dots (picture elements; pixels). Resolution is stated in pixels horizontally and vertically spanning the entire screen width and height, for example 1920 x 1080, or 640 x 480. Picture details can never be smaller than one pixel, thus for 640 x 480, a picture detail cannot be narrower than 1/640'th the screen width or 1/480'th the screen height.

For displaying the video, the minimum spot size imposed by the monitor and also affected by the bandwidth of the cables and amplifiers still applies. Although the smallest picture detail in 1080i HDTV is 1/1920'th the screen width, very few HDTV ready TV sets today can make a spot smaller than a tenth of a percent of the screen width which is two 1080i HDTV pixels. Some cannot make a spot smaller than 1/640'th the screen width or three pixels. So the TV may be able to display details in 1920 x 1080 unique positions (2.07 million pixels) but might not be able to resolve details in any three consecutive positions at the same time.

While NTSC broadcasts have about 440 maximum pixels across the screen (MPA) and NTSC laser disks have about 565, at those limits what would be black and white dots have nearly faded to gray within the source material. Crisp resolution is probably more like 400 MPA for broadcasts and 500 for laser disk. These are transmission and storage limits that vary slightly with the makes and models of equipment. But "pixels" in analog video are not constrained to "pixel positions" as in digital video. To make up for this, digital video must have a few more pixels per scan line to equal analog video in appearance. U.S. digital TV includes two formats referred to as standard definition TV, and with resolution just a little better than NTSC. They have 640 x 480 and 704 x 480 pixels respectively.

HDTV still has the advantage over NTSC because there are enough horizontal and more vertical positions to choose from.

If your computer monitor with a CRT is handling regular VGA (640 x 480) and you feed it a 720 x 480 DVD video signal (with the same frame rate of 60 per second), you still see the whole picture. In other words monitor does not stretch the first 640 pixels to fill the screen and crop off the rest. If the smallest spot the monitor could make was 1/640'th the screen width, the finest details from the DVD feed (1/720'th the screen width) would just be blurred together but would be in the correct places on the screen. (You may have to adjust the horizontal and vertical size controls to stretch the picture to the proper proportions.)

If your CRT based (analog) monitor also resolves 1024 x 768 ultra VGA (most do), its minimum spot size is small enough to resolve all 720 pixels horizontally for DVD video which is displayed using the monitor's 640 x 480 mode. But if the video is going through the computer, the latter's video card might restrict the resolution to 640 x 480 using some scaling formulas but again not cropping off the right side of the picture. The video signal entering the monitor is analog. Although the 640 or 720 pixels could be determined through analysis, an analog monitor does not care and does not do such analysis but still displays the picture correctly. You can think of the monitor's "resolution" as "anything by 480", "anything by 768", etc. limited by the smallest spot it can produce.

We cannot overlook the frame rate (frames per second) for the display device. For TV sets, both NTSC and HDTV, it is approx. 60 fields per second for interlaced and 60 frames per second for progressive scan. All VGA computer monitors display 640 x 480 (anything x 480) at 60 frames per second, which is "regular VGA" and is the same frame rate and scan line rate (scan rate) as 480p digital TV. Most have several to several dozen other scan rates such as anything x 768 x 60 fps, anything x 600 x 85 fps, and/or anything x 480 x 70 fps, but they generally cannot accept "any vertical pixel count in between". Unfortunately 1280 x 720 and 1920 x 1080, which are the U.S. HDTV formats, are usually not included.

Digital monitors, such as those with LCD screens and monitors with digital input (e.g. FireWire) may or may not handle incoming video in their 640 x 480 mode/format/scan rate but with 720 pixels per scan line.

Note: There is also a computer standard 720 x 480 which is the not the same as the regular VGA, it has a different scan rate from line doublers used with DVD players which deliver the 720 x 480 frame using the regular VGA scan rate.

Progressive Scan

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Line Doubling

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What Detracts From Resolution?

1. The video camera, its lens, its video sensor (CCD, iconoscope, etc.) and its electronics may be less than top notch.

2. The video signals were excessively bandwidth limited during video production and recording.

3. Too much data compression was used.

4. The comb filter in the TV set, laser disk player, or S-VHS VCR was less than top notch.

5. The video signal was comb filtered, re-composited, and comb filtered again.

6. The electron beam is too fat relative to the size of the screen. Scan lines then run into each other.

7. During interlaced scanning the even lines land on top of the odd lines instead of being in between.

8. The dot pitch is large relative to the size of the screen, typically a problem with screens smaller than ten inches.

Limited signal bandwidth is not a factor affecting vertical resolution. Since all the detail on any scan line has to be completed before drawing of the next scan line begins, restricting the bandwidth so much as to affect vertical resolution will have wiped out 100% of the horizontal resolution.

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Dot Pitch

(Affects resolution of the TV or monitor)

In this writer's opinion, the smallest dot of detail for the purposes of computing resolution must completely span at least two, any two, of the three phosphor color dots (or stripes) red, green, and blue on a direct view screen. If a detail dot was so small as to span just one phosphor color, what should be a white dot could end up pure red, pure green, or pure blue. If it spanned two color dots it would at least be a lighter shade, magenta, yellow, or cyan. If a dot spanned half of a red stripe, all of the adjacent green stripe, and half of the adjacent blue stripe, it qualifies as spanning two whole stripes.

Medium sized direct view screens (about 20 inches) typically have a dot (color dot or stripe) pitch of 0.75 mm, sometimes larger, rarely smaller. Large screens typically have a dot pitch of 0.9 mm or more, rarely smaller.

Consider a 25 inch standard picture tube with a dot pitch of 0.8 mm. Its horizontal resolution is 715.

(The screen width is 20 inches, times 25.4 mm per inch equals 508 mm wide. Divided by 0.8 mm dot pitch equals 635 phosphor stripe triplets. Divided by 2/3 equals 952 pairs of phosphor stripes, any two colors in a pair. Divide by the aspect ratio 4/3 to apply the circle rule yields 715 lines of resolution.)

This writer has seen 25" TV sets with dot pitch more like 1.0 mm and larger sets with even larger dot pitch. Dot pitch may be larger at the sides of the screen compared with in the middle.

Most rear projection TV sets also have a "dot pitch", which is the spacing between the tiny vertical ribs of the integral lenticular lens assembly that is part of the screen. If you look closely, perhaps using a magnifying glass, you will see that the picture has been broken into thin vertical stripes. In May 1999 we did a quick survey and found ten TV sets, somewhat randomly chosen, with 35 ribs per inch (0.70 mm pitch) and one with 32 ribs per inch (0.75 mm). Any rib can be any color; the color has already been given to the light beams down inside the projection optics.

Consider a 50 inch 4:3 aspect ratio RPTV screen with 35 ribs per inch. Its horizontal resolution is 735 to 1050 depending on who you are talking to. Not quite enough for displaying 720p (1280 x 720) HDTV to its absolute maximum but it will display all of the color content (transmitted as half the luminance resolution) which will satisfy many viewers.

The screen width is 40 inches, times 35 ribs per inch equals 1400 ribs. Divide by the aspect ratio 4/3 to get lines of resolution, 1050. Because two pixels' worth of picture detail could land on the same rib and be blurred together, some experts say you have to compensate by multiplying by an Extended Kell factor of less than 1.0. A common Kell factor is 0.7 which yields the alternate resolution of 735 lines stated above. The RPTV resolution is the smallest of its screen resolution, its CRT (gun) resolution, and its electronics' resolution.

RPTV's in general seem less sharp than direct view sets because of convergence errors. But even with correct convergence the picture has been magnified so much that the limited picture detail of the video signal is easily seen. This is especially noticed in home use where the set was purchased in order for viewers to sit closer to a bigger picture.

Pixels as stated for digital video programs do not correspond to and are not related to the dot pitch for a direct view screen or the rib pitch for a rear projection screen. But an LCD or a DLP projector and a plasma screen have pixel counts all their own. When the pixel count of the projector unit or plasma screen is much greater than the pixel count and number of scan lines for the source material, scaling is done and there is no loss of resolution. But if the pixel counts are approximately the same, there is loss of resolution when the pixels of the source material do not line up with the pixels in the projector unit or on the screen.

Pixel Straddling

(Affects resolution of the recording of live subjects, also the net resolution after scaling of the picture to different pixel dimension.)

Some cartoons are generated directly on computer screens. For them, picture details may be consistently lined up with the pixels, and for a 720 x 480 pixel DVD frame, the resolution can be consistently 720 x 480.

For the televising or recording of live subjects, the scan lines and (for digital video) the pixels horizontally will not consistently match up with fine details in the subject material. This can be seen if the camera is panned across details that are as fine as the pixels, those details will change from sharp to blurred to sharp again. For this reason, there are some people who will say that a 720 x 480 video frame has far less than 480 lines of resolution vertically or 720 pixels across horizontally.

As we mentioned earlier, a pixel or a small stretch of a scan line must be all one color. Going back to an earlier example of looking at a scene through a silk screen and painting it on canvas one dot at a time, if there is both black and white showing through the same hole in the screen, you have to choose the in between color of gray.

In the diagrams below we show some examples of pixel straddling in the horizontal direction. A moire pattern is the result of picture details going in and out of straddling (in and out of phase with the pixels) over a large area.

Of course we can use twice as many pixels or scan lines to eliminate the straddling but sooner or later the bandwidth needed for the video signal is too large to fit in the allotted broadcast channel or the amount of data is too large to fit on the DVD.

Pixel Straddling Examples (greatly enlarged)

Original source is on top, video representation is on bottom. The red and white dashes along the top denote the pixel positions.

Aliasing and Averaging

Aliasing means "mimicking of something else". The left diagram above shows an attempt to reproduce fifteen evenly spaced stripes such as a far away picket fence, using sixteen pixels. The right diagram shows an attempt to reproduce seventeen stripes using sixteen pixels. Notice that the video representation, however blurry, is the same for both. So by looking at the result you cannot tell whether the source material was 15 stripes or 17.