Your HDTV Set Might Not Be High Definition

Overview

This dirty little secret is finally coming out into the open.

If your HDTV set is fundamentally 720p, it might deliver 1080i shows not in high definition.If your HDTV set is 1080i, it or your set top tuner box might deliver 720p shows not in high definition. In either case the vertical resolution could be as low as 540 lines.

There are even a few "HDTV" sets and set top HDTV tuner boxes that deliver only 540 lines of vertical resolution, sub-HDTV quality, for all HDTV programs.

Add on devices (external de-interlacers) to improve resolution are available. If you change your mind later and want to use an add-on device, your TV must allow an input that matches or nearly matches the screen resolution. For the newest 1080p TV's you would need a 1080p input jack.

Actually many HDTV sets are 1080i and do not accept 720p per se. This is common and normal, but you then have to be concerned about the quality of the set top tuner box, which must convert 720p programs to 1080i.

TV sets that accept HDTV material but that have less than 700 scan lines are not high definition to begin with.

A few 4:3 aspect ratio HDTV sets display SDTV shows in 1080i format while displaying HDTV using only 810 scan lines.

We don't know what brands and models of TV sets and set top boxes are affected. Test patterns are needed to verify this.

If you are able to get ahold of the December 2005 issue of The Perfect Vision magazine, there is an article by Gary Merson on this subject you may be interested in.

Updated 12/8/05

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At First Glance

Don't get me wrong, even close to the bottom line HDTV sets will give you more than your old (standard definition) TV set. Then, maybe, your old TV does not give you all of what SDTV can offer either.

When you walk into a TV store, you will definitely be "wowed" or mesmerized or enraptured by the higher quality of HDTV over standard definition TV (SDTV) even if the TV set you are thinking of buying at has the deficiences described in this discussion.

HDTV provides much finer color changes. There can be usually at least 300 color changes across a 30 inch direct view CRT set, or usuall at least 400 color changes across a 50 inch rear projection set. Compare that with no more than fifty color changes across the screen for standard definition NTSC regardless of the kind of TV or size of the screen. Theoretically there can be as many as 640 color changes across the screen for 720p or 960 color changes across for 1080i HDTV where the screen itself usually lessens this resolution.

With HDTV you don't see the scan lines as readily when the screen is large. Traditional TV's show up to 480 scan lines and the odd lines and even lines pulsate alternately slightly (due to interlacing and picture tube characteristics). Depending on the action on the screen you may notice the individual scan lines particularly if you wear glasses. HDTV either has 720 scan lines that do not pulsate alternately (the picture is non-interlaced) or has 1080 scan lines.

1080i vs. 720p

Casual viewers and even critical viewers not specfically looking for deficiencies regard 1080i and 720p as providing approximately equal overall picture quality. The 720p gives better reproduction of fast motion, most noticed in sports programs, while 1080i as you would guess gives a more detailed picture. As far as program material goes, you do not have a choice.

CRT HDTV sets, which still give the best shadow detail (via black levels) of all the video technologies, are more likely to be 1080i while HDTV sets using the other technologies (LCD, DLP, etc.) are more likely to be 720p.

U. S. HDTV (ATSC) broadcasts come in three broad classifications. We will use their real names, 1080i, 1080p, and 720p, in this discussion. Actually, today's 1080p broadcasts are all delivered to the TV as 1080i if not 720p.

(ATSC TV formats 480i and 480p are standard definition, not high definition.)

Meanwhile TV sets for HDTV come in four broad classifications.

"EDTV" -- "HDTV" sets with 480 to 540 rows of pixels, or visible scan lines, are not high definition, even though they accept HDTV video signals. We will refer to them as 540p. Some folks prefer the term EDTV for "extended definition".

720p -- Many LCD, DLP, or plasma sets that are really HDTV have 720 to 768 rows of pixels and we will refer to them collectively as 720p.

1080i -- Most interlaced CRT TV sets that are really HDTV have 810, 960, or 1080 visible scan lines and we will refer to them collectively as 1080i.

1080p -- LCD, LCoS, DLP, and plasma sets with 1024 to 1080 rows of pixels are all non-interlaced and we will refer to them collectively as 1080p. There are also a few CRT 1080p sets.

(In this discussion we are talking only about the scan lines that contain picture material, a.k.a. active scan lines.)

Converting from 720p to 1080i

A 720p show has video frames of 720 scan lines each every (approx.) 1/60'th of a second. The 1080i TV displays fields (half pictures) of 540 scan lines every 1/60'th of a second. The straightforward way of converting the video is to convert each incoming 720 scan line frame into 540 scan lines. If the same formula is used for each incoming frame, that is, disregarding whether we are creating an odd field or an even field, the vertical resolution of the picture drops to 540 even though the picture tube is finally painted with scan lines occupying 1080 unique positions. Every two scan lines on the picture tube would contain the same content.

To preserve the 720 lines of vertical resolution, different scan lines must be picked and saved from the 720 line frame that will become an odd 1080i field compared with the 720 line frame that will become an even 1080i field.

Click here for some sample math.

Converting from 1080i to 720p

From a 1080i HDTV show, every (approx.) 1/60'th of a second a field of 540 scan lines is received, first a field of all of the odd lines and then a field of all of the even lines. Meanwhile the LCD, plasma, etc. system or picture tube circuits expect a frame of 720 scan lines in each 1/60'th second time period.

To "scale" the 540 scan line field into a 720 line frame the 540 incoming scan lines are spaced out as evenly as possible leaving 180 scattered positions in the finished frame empty. Then, in the simplest case, each gap is filled by a repetition of the scan line above it. More sophisticated methods may blend the line above and the line below each gap.

Regardless of the method used to fill the gaps, if each 720 line video frame is constructed the same way, with no regard to whether the source field was odd or even, the overall vertical resolution becomes no more than 540 lines. Scan lines for each incoming 1080i field, whether from an odd field or an even field, would be inserted into the same 540 unique vertical positions in the frame and the 180 gaps would always be in the same places.

In order to achieve a vertical resolution of 720 lines, the 540 scan lines from an incoming odd 1080i field must be inserted into the 720 line video frame in different positions compared with the 540 scan lines from an incoming even 1080i field although still spread out as evenly as possible. (The 180 gaps would be in different places also.)

The question arises, with the gaps in different positions for the frames constructed from the even 1080i fields, would it be better to leave the previous frame's content behind in each gap rather than repeat the new scan line just above. Video experts have debated long and hard on this question and a slight majority feels that filling every scan line with new content is better. Both repeating a scan line or blending together any two scan lines(1)  blurs vertical detail thus giving a softer picture compared with having unique original material in each scan line position. Meanwhile if we leave behind a scan line that was there from the previous frame(2), there may be visible motion artifacts (ghosting of subject motion) in the finished picture.

What is really needed is high quality de-interlacing. Then the 720 scan lines needed are picked out from a full 1080 scan line frame each time.

Click here for some sample math.

(1) A process whose result is similar to "bob" or "interpolative" de-interlacing.

(2) A process whose result is similar to "weave" de-interlacing.

Converting 1080i to 1080p

When the display element "has 1080i resolution" and is non-interlaced as are all LCD, DLP, LCoS, and plasma displays, incoming 1080i needs to be de-interlaced. In the simplest sense, de-interlacing consists of making full 1080 scan line frames from 540 scan line odd and even fields. The result is called 1080p. Currently no broadcasts or other program materal (DVD, etc.) is presented to TV sets as 1080p.

We have the same problems encountered in the 1080i to 720p conversion. If the full frames containing even scan lines are constructed in (exactly) the same fashion as frames containing odd scan lines, the vertical resolution drops to 540 lines which is the resolution of a single field. In other words, if the incoming 1080i material is treated as if it were 540p instead of 1080i, the de-interlaced result will have just 540 lines of resolution.

To preserve more vertical resolution, the full frames constructed from even scan lines must be constructed differently from the full frames constructed from odd scan lines. The difference in construction could be as simple as inserting each incoming scan line into the completed frame one scan line position lower for the even scan lines.This method preserves about 2/3 of the vertical resolution, 720 lines of resolution in a 1080 line frame(1).

Still more vertical resolution is preserved if the even positions are left with their former contents when lines from an odd field are inserted, and the odd positions are left with their former contents when lines from an even field are inserted(2). In this example, nearly the entire 1080 lines of vertical resolution are preserved. As in the 1080i to 720p conversion, the drawback to this method is that blurring is more noticeable if subject motion has occurred relative to the two fields in question.

Click here for some sample math.

Click here for more on de-interlacing.

(1) The preceding two processes represent the "bob" or "interpolative" method of de-interlacing.

(2) The "weave" method of de-interlacing.

HDTV Set Top Tuner Boxes

This discussion also applies to the control box, or receiver, for cable TV or satellite TV, if some of the channels you can receive are HDTV.

Most HDTV viewers nowadays have a converter box that probably cost about USD $200. to $500. This box almost always converts any 720p broadcast to 1080i. It is completely unknown what the quality of the conversion is unless details are published.

If the TV is of the 720p classification, it will convert any incoming 1080i video feed to (or back to) 720p. If the broadcast was 720p to begin with and delivered to the TV as 1080i, it has now undergone two conversions. We can only imagine how much quality was lost.

Built In HDTV (ATSC) Tuners

Having a built in HDTV tuner does not guarantee that the TV set is free of the problems we have described. If you decide later that you need an external de-interlacer to improve the HDTV picture quality you will have to revert to using an external set top box tuner. Use the "native" output mode of the set top box, i.e. have the box output 720p for 720p shows and 1080i for 1080i shows, letting the de-interlacer do the necessary processing.

Only 810 Scan Lines

All U.S. HDTV has a 16:9 aspect ratio. There are still a few HDTV sets that have a 4:3 screen and display 1080i in a 4:3 aspect ratio. Standard definition shows are presented using all 1080i scan lines (although still as standard definition). To show the 16:9 picture the picture is shrunk slightly (letterboxed) leaving unused black borders at the top and bottom of the screen. To find out whether scan lines are indeed wasted at the top and bottom, turn the contrast way down and then turn the brightness way up. If the top and bottom black bars turn gray (if not already) then the TV is using only 810 scan lines for the picture. To avoid damaging the set, turn the brightness down before turning the contrast back up no more than halfway for a direct view tube or plasma set and no more than a third of the way for a CRT projection set.

1080p TV Set, 1080i Quality

When the 1080p TV set has no 1080p input, just 1080i (and/or 720p and/or 480p) inputs, you are stuck with the conversion to 1080p that the manufacturer provides.  (All LCD, DLP, plasma, and LCoS sets must perform the conversion.) This might not be of concern today but upgrading would require replacing the entire TV set.

As of late 2005 less than 1% of HDTV sets made have "high quality" conversion to 1080p or 720p. All of the methods described in the examples below are not "high quality" methods.

External converters (de-interlacer and scaler built into a single unit) are currently available for high quality conversion to 720p and 1080p. Currently the lowest priced units are the Lumagen DVI for $1000. (conversion to 720p) and the Lumagen HDP for $1500. (conversion to 720p and to 1080p). Units with the Realta (TM) chipset (conversion to 720p and 1080p) begin at about $2900. There is also the Faroudja DVP1080 that costs about $5000.

To be prepared for future upgrades including the use of external scalers, videophiles shopping for a 1080p TV set should wait if necessary to get a set that has 1080p@60 input with HDCP compliance.

Simple 720p to 1080i Conversion Math

Let's consider just the first one percent of the scan lines which consist of seven scan lines of video source that become ten scan lines of the finished picture. The first column below represents a finished odd 1080i field, the second column represents a finished even 1080i field. The best we can do is shown below. Note that all seven scan lines are utilized and that the odd field and even field contain different contents..

Result:
Odd  Even
1
    2
2
    3
4
    4
5
    6
6
    7

On the other hand, a simpler conversion method constructs both odd and even fields in the same fashion. Note that only five of the seven incoming scan lines are used for a lower quality result.

Result:
Odd  Even
1
    1
2
    2
4
    4
5
    5
7
    7

Simple 1080i to 720p Conversion Math

Again, just the first one percent of the picture is enough to demonstrate how picture quality can vary. Here we have ten scan lines of video source (5 odd and 5 even as successive fields) that become the first seven scan lines of the finished picture, two frames shown. Obviously some of the source lines need to be repeated to fill all seven positions but note that which source lines are repeated differs for the even source scan lines compared with the odd source scan lines in this example. On the screen, the scan lines in each row blend together optically as the results of odd and even fields alternate but we do have seven different blends making each scan line unique.

Result:
First Second
1    2 
3    2
3    4
5    6
7    6
7    8
9    10

The simpler method treats both the incoming odd field and the incoming even field the same way, repeating the same source scan lines at the same times. Again, on the screen, the scan lines in each row blend together but here we only have five different blends for a vertical resolution of five instead of seven.

Result:
First Second
1    2 
3    4
3    4
5    6
7    8
7    8
9    10

Simple 1080i to 1080p Conversion Math

To obtain the maximum vertical resolution the even field content must be staggered downward by one scan line in the respective completed frame (also referred to as a one scan line phase shift). Again, optical blending occurs on the screen but we have ten unique blends. Two completed frames are shown in this example.

Result:
First Second
1   (black)
1    2
3    2
3    4
5    4
5    6
7    6
7    8
9    8
9    10

If we construct full frames in the same fashion from both odd and even incoming fields, i.e. without the stagger, we end up with just five unique scan lines in every group of ten producing a halving of the vertical resolution, 1080i comes out looking like 540p.

Result:
First Second
1    2
1    2
3    4
3    4
5    6
5    6
7    8
7    8
9    10
9    10

Skimping some more, the manufacturer might even discard the even fields and output each frame constructed from the odd field twice. Apparently some examples of this have been found.

Horizontal Resolution

The number of picture details that can be resolved across the screen is theoretically 1280 for 720p and 1920 for 1080i. For (standard definition) DVD the number is 720; for (NTSC) SDTV the number is at best 440. Numerous factors limit horizontal resolution, the most common being the quality of the source material, the quality of the cables, the quality of the electronics, the display element (picture tube, LCD panel, etc.), and the screen for projection TV.

Again, it takes test patterns, or perhaps scenes from a movie you have watched numerous times, to evaluate horizontal resolution.

Direct view CRT's have what is referred to as "dot pitch" or the spacing between two of the red, green, and blue dots or stripes of the same color on the screen surface. Even for today's HDTV sets the dot pitch is sometimes as large as one millimeter, maybe a little less in the middle of the screen and a little more at the sides. For a 32 inch TV the screen is 25-1/2 inches or about 650 mm wide. In order for a picture detail to be "reasonably white" it has to span at least two of the three color dots or stripes or 2/3 of the dot pitch. So with a 650 color triplets 1 mm wide each there can be 975 pixels alternating dark and light distinguishable across the screen. For color details at least two dots or stripes of each color are needed for one picture detail so with 650 color triplets there can be about 325 color details going across the screen.

Rear projection TV sets have a "rib pitch" which can be seen if you are standing a few inches from the screen. We feel that it takes at least three ribs to represent two pixels dark versus light or luminance-wise, and at least two ribs to represent one fine color detail.

For analog parts of the video signal path which include component video cables, lesser quality in electronics or cables shows up first as loss of horizontal resolution. It is useful to understand resolution in terms of fraction of the screen width for example 1/640'th. Despite degraded horizontal resolution picture details can sometimes still be positioned in (for 1080i) 1920 different increments across the screen except the picture details might not be reproduced narrow enough to not blur together.

The 720p format is more sensitive to bandwidth shortcomings than 1080i. If the bandwidth is half of the needed 37 MHz, the horizontal resolution is halved, picture details for 720p can then be no narrower than 1/640'th the screen width as opposed to 1/1280'th. Whereas for 1080i picture details restricted to 1/960'th the screen width as opposed to 1/1920'th the screen width don't produce as noticeable a degradation.

Click here for more on horizontal resolution.

What Are 1080p Broadcasts?

Non-interlaced video material with 1080 active (picture material containing) scan lines comes in three varieties in the U.S. (and other countries that use NTSC): 24 frames per second, 30 (29.97)  frames per second, and 60 (59.94) frames per second. The first two are included in the ATSC HDTV standard. When tuned in by the TV tuner or external set top box, these formats are converted to 1080i (interlaced, with 30 full frames per second) and/or to 720p. The currently allocated broadcast channels have insufficient transmission capacity (bandwidth) to carry 1080p@60. Video signals in 1080p@60 format are encountered by the consumer nowadays only as the output of external de-interlacers.

Good HDTV de-interlacers such as the ones we mentioned, will produce 1080p@60 from 1080i that comes close to what an original 1080p@60 source might have in picture quality.

What Is 540p?

Subject wise, 540p captures or televises subject matter using 540 vertical positions on the video camera capture element to obtain 540 active scan lines per non-interlaced video frame. Simple upconversion of SDTV sources to 1080i yields 540p subject wise. Optional additional processing to make diagonals appear less jagged may result in 1080 unique scan lines per two 540p frames but the vertical resolution is still no more than that of the source, 480 lines. The analog transmission format of 540p is almost identical to 1080i and most HDTV sets accept both 540p and 1080i component video without any difficulty. On a TV set the 540p setting, if present, juxtaposes the normally staggered odd and even1080i fields painted on the picture tube faces and works the same way regardless of whether the subject matter or incoming video signal or both were 1080i or 540p or standard definition. This 540p setting may reduce flicker but must be switched back to 1080i before evaluating a TV set for inferior vertical resolution..

A Few Terms

ATSC -- Advanced Television Systems Committee, and also the specifications said committee developed for HDTV in the U.S.

CRT -- Cathode Ray Tube, the traditional TV picture tube, for a projection TV set there are three of the  tubes that are smaller and mounted inside with lenses and mirrors.

DLP -- Texas Instruments' trademark for video display technology consisting of microscopic mirrors for individual pixels, that individually tilt to direct light at the screen (via lenses and mirrors) or off in a different direction, different duty cycles providing gray scale.

EDTV -- Extended definition television -- Another name for SDTV, generally used for non-interlaced displays.

HDTV -- Refers to TV pictures having roughly a million if not more picture details, or pixels.

LCD -- Liquid Crystal Display, tiny spots become transparent or quite opaque to provide light to dark gradations.

LCoS -- Liquid Crystal on Silicon, same as LCD except that the spots are on a mirror and light has to pass through them twice allowing for a darker "black".

NTSC -- National Television Systems Committee, and also the specifications said committee developed in the 1950's for standard definition TV in the U.S.

Plasma -- Direct view video panel with thin closely spaced gas filled phosphor coated tubes segments of which are made to glow forming pixels on the panel which serves as the viewing screen.

SDTV -- Standard definition television, consisting of pictures with approximately the amount of detail as pre-HDTV broadcasts, namely having about 500 scan lines and about 500 picture details on each scan line.

Click here for more terminology.

Video hints
More on de-interlacing
More on video resolution
More on color resolution
More on analog bandwidth
More on scaling
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All parts (c) copyright 2003-2005, Allan W. Jayne, Jr. unless otherwise noted or other origin stated.

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