Is It Really HDTV
Here are some things the TV set manufacturers and TV store salesman don't tell you and that could mean your new HDTV does not give you a high definition picture.
Updated 10/9/00
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Overview
In the year 2000, HDTV is generally defined as delivering a picture with about one million "pixels". As an example, an excellent regular (NTSC; analog) TV picture can reproduce a fine checkerboard pattern with 440 alternating black and white dots across and 480 alternating black and white dots vertically for a total of about 210 thousand dots (pixels). DVD can show (assuming your TV is good enough) 720 across by 480 high for a total of 338 thousand dots.
The two primary HDTV formats in the U.S. are 1080i (1920 across by 1080 high, or just over 2 million pixels), and 720p (1280 across by 720 high, or 922 thousand pixels). Shortcomings of TV sets and/or the video production and broadcast process tend to affect the horizontal resolution first, so if the horizontal resolution is cut in half (to about 1000 pixels across), 1080i still has about a million pixels while 720p is cut back to just under half a million.
"Someone commented on the Internet that a home theater should give the viewer an experience comparable to going to the movies as opposed to watching television.But with all the complaints we see on Internet forums and hear at gatherings of home theater enthusiasts, it seems as if, while shopping for a "home theater display device", all you can get for a reasonable price is television set quality."
In a Nutshell, or Table of Contents
Here are things to think about when shopping for an HDTV set or other video equipment.
What To Expect -- We have reason to believe that 1000 x 1000 is going to be the standard of performance for high end home theater HDTV equipment.
Dirty Secret #1-- Equipment that does not handle the bandwidth needed for HDTV.
1a -- Each piece of equipment might have enough bandwidth but connected together they don't.1b -- Bandwidth of the strongest link, not the weakest link, being advertised
Dirty Secret #2 -- HDTV provides a wide 16:9 picture but the TV's full resolution is available only in a 4:3 shape.
2a -- 16:9 pictures that suffer loss of horizontal resolution.
Dirty Secret #3 -- HDTV sets lacking HDTV quality.
3a -- Convergence errors.
3c -- Equipment makers not honoring warranties by claiming the equipment is within specifications.
Questions you need to but really can't easily ask
How Do People View HDTV?
In the year 2000 it appears as if there are five categories, or grades, of equipment.
By 1/600'th we mean the smallest dots or details take up 1/600'th the screen width. The specification might still be called 1920 x 1080 which is true in that any dot may occupy one of 1920 different horizontal positions giving smoother diagonal lines than when using a specification of 640 x 480. But three tiny dots each 1/600'th the screen width, dark, light, dark, occupying consecutive positions on a scale of 1 to 1920, will blur together.
1. Existing TV Set
Uses HDTV tuner in a set top box feeding the antenna jack, video in (composite) jack, or S-video jack of TV. Signal is converted to 480i. Resolution is the same as the existing TV which may be anywhere from 1/600 by 480i (typical high end pre-HDTV) to 1/300 across by 480i high (typical 19 inch portable). Bandwidth, for comparison, ranges from about 6 down to 3 MHz.
2. 480p, Minimal Digital Ready TV (DTV)
.Uses HDTV tuner in set top box downconverting the signal to 480p to feed the TV using Y,Pb,Pr component video or RGB. The TV is the typical large progressive scan TV available nowadays intended to go with progressive scan DVD players or for displaying computer video in auditoriums and where readability of fine text is not critical. Bandwidth would be around 12 to 15 MHz giving a resolution of around 1/700 x 480p.
3. Consumer grade HDTV ready
The TV set is painting all 1080 scan lines on the picture tube(s) but suffers from the usual scan line overlap due to the electron beams' being too fat, and from reduced horizontal resolution due to insufficient bandwidth and also the electron beams' being too fat. RPTV's and FPTV's with five or six inch CRT's would be in this category. Typical resolution: 1/800 x 540p.
4. Home Theater grade HDTV
Equipment is capable of displaying 1080I and perhaps 720p in every respect except bandwidth which is half of the theoretical requirement. This causes the horizontal resolution to be about 1000 pixels across, about half of the 1920 pixels provided. This is actually about the minimum needed to fit the definition of HDTV and it seems as if there is a performance level plateau 1/1000+- across x 1080i high. A/V receivers with 20 to 25 MHz bandwidth are becoming common and fit this category.
5. Ultimate Deluxe, or commercial theater grade
Equipment comes close to if not actually displaying everything HDTV has to offer. Some TV sets may de-interlace the 1080i to be 1080p. Very little of this equipment (36 MHz bandwidth for 1080i, 72 MHz for 1080p) is readily available in stores, those pieces which are available are very pricey
Most of you reading this article would have grade 4 above as a goal. Grade 3 is not really HDTV, resolution wise, and most of us don't want to spend the money for grade 5. Grade 4, incidently, is what we get with half the required 36 MHz bandwidth. We do have the capability to display 1080i with about 1000 pixels across. This fits the definition of HDTV and still gives a very smooth picture compared with today's DVD which delivers about 700 pixels across by 480 high.
You do have to buy components each with a little more than the desired bandwidth so that, when they are all connected together, the bandwidth of the entire signal path is what you want.
Unfortunately half the required bandwidth does not work well for 720p. This is why 720p has not caught on. The specification for 720p is 1280 pixels across by 720 high. Using 18 MHz bandwidth, which is half of specifications, the horizontal resolution, like that of 1080i, is cut in half which comes out here to 640 pixels across.
We believe that 720p should also resolve 1000 pixels across. Until the bandwidth issue is resolved, probably by going to digital interconnects such as Firewire (R), we probably won't get 1000 pixels across for resolution. .
And now the dirty secrets that stores and manufacturers usually don't tell you about.
Most video equipment does not handle the bandwidth needed for HDTV.
Both 1080i and 720p require about 36 MHz of video bandwidth within the TV set (and also for the output stages of the tuner or decoder and for the cables feeding the TV). Contrast that to 13.5 MHz for the progressive scan DVD player output and 6.75 MHz for a "regular" DVD player output where 6 MHz even is more typically found.
Half the bandwidth means half the horizontal resolution, and so on.
Even though each piece of equipment may have a sufficient bandwidth, when connected together, the video signal path provided may not have sufficient bandwidth.
To better understand bandwidth, you need an understanding of frequency response. As an idea of inferior frequency response, imagine a piano or organ on which the keys at the far left and far right play much more softly than the keys in the middle. (Usually it is the lower and upper limits where frequency response drops off.) The greater the deficiency in response to high frequencies a video component has, the poorer it will pass along or reproduce fine horizontal details.
The bandwidth, if truthfully advertised, extends to the point where the signal strength falls to half, which strength is expressed as being "down 3 dB". If the video has to pass through two components (perhaps an A/V receiver plus one cable) each of which has its "3 dB down point" in the same place, only half of a half, or 25%, of those video frequencies will get through.
It is common for a respectable bandwidth to be advertised but it applies only to part of the signal path, notably the main video amplifier for a TV set. Overlooked is the bandwidth of other circuitry such as the video preamplifier just inside the component video jack. Since bandwidth directly affects horizontal resolution, the advertised figure is sometimes expressed as lines of resolution.
It is the higher frequencies that are more affected by noise, such as from nearby power cables, or even nearby video cables. As a cable carries anything, there are electrical and magnetic fields around it. If these fields find their way into another cable, the result is noise that may show up as snow or smeared colors or ghosts. All video cables have shields, cylindrical shells that surround the "hot" conductor that also serve as the "ground" conductor. The shield both reduces the fields radiated out of a cable and reduce the fields penetrating the cable from outside. Electric and magnetic fields are also found in an A/V receiver where the video signals run through circuit board traces that may be close enough to one another to pick up noise from each other. Finally, the signal, starting with the higher frequencies, is attenuated (tends to disappear) when the conductor carrying it is physically near other conductors particularly grounded conductors. This is due to capacitance which is too complex a topic to expand upon here. The amount of attenuation, or loss, varies with what separates the conductors, air, plastic, wood, etc. and also the length of the cable or other conductor.
The number of pixels available for a wide screen picture is greatly reduced on some TV sets.
TV sets with a 4:3 aspect ratio screen are still common. While 1080i HDTV has 1080 visible scan lines, most HDTV sets continue to display the 1080 scan lines only as evenly spread up and down the entire screen.
Both 1080i and 720p normally provide 16:9 aspect ratio pictures. In order to fit the entire 16:9 picture on a 4:3 screen, the picture must be shrunk so it occupies 3/4 of the screen height. This means that for 1080i, 810 scan lines will be inside the picture, and for 720p, 540 scan lines will be inside the picture. So 1080i and 720p are no longer displayed with 1080 and 720 scan lines, respectively!
Suppose that 1000 pixels across can be resolved (quite good for a year 2000 HDTV ready set). With 810 scan lines making up the picture we have 810 thousand (4/5 of a million) pixels.
Fortunately TV set makers and stores will soon be required to tell us how many scan lines are being used for the wide screen pictures.
In converting (downconverting) the 1080 scan lines of source material to fill in the 810 scan lines of the 16:9 space provided, the horizontal resolution may be compromised.
Downconversion requires converting the incoming video signal, which is analog, to digital. What this means is that each scan line is chopped up into pixels. They don't tell you how many pixels are used. If fewer than 1920 pixels are used here, there is no way that all 1920 pixels of incoming 1080i picture information can be preserved regardless of the bandwidth of the video amplifiers. Actually more than 1920 pixels of digital memory are needed since the digitizing process here might not chop up the signal matching the original pixels of the source and that straddling effect will further soften the picture. That subject is another program.
Downconversion may cause noticeable flickering of objects moving up and down.
Normally the way to downconvert as needed here, one out of every four scan lines is dropped or blended with a neighboring scan line. But this is done for just the odd scan lines of 1080i, and then done again for just the odd scan lines of 1080i. The end result is that in many cases two consecutive scan lines out of every eight scan lines are dropped. (Click here for more details on this subject.)
Manufacturing and calibration tolerances at the factory often leave the TV set delivering performance well below that needed for HDTV. There is variation from one set to the next of the same model.
In this writer's opinion, if convergence is off by more than half a scan line near the middle of the screen and more than one scan line anywhere on the screen, the picture quality is substandard. By a one scan line error we mean that what should be a single white scan line should be no more than two scan lines wide taking into account all three colors red, green, and blue. It must be remembered that when dealing with HDTV, an error of one scan line is a tenth of one percent.
Horizontal resolution is also limited by dot pitch
All direct view set screens have thin red, green, and blue stripes (or small dots), typically 0.8 mm from one red stripe to the next, with thin black separating borders. All rear projection sets have thin vertical ribs, typically 0.7 mm wide, on their screens that divide the picture into thin vertical stripes with thin black lines between stripes. Both of these structures restrict the horizontal resolution.
A 32 inch direct view TV screen is 25-1/2 inches wide. With a dot pitch of 0.8mm there are about 800 dot or stripe triplets. In order that a white dot not appear too discolored it must be big enough to span (any) two color stripes. So the screen has 1200 stripe pairs and at best a resolution of 1200 pixels across.
A 60 inch rear projection TV screen is 48 inches wide. With a 0.7mm rib pitch there are about 1750 ribs. But if two tiny white dots land between ribs, it is not possible to see the dark space in between so the dots blur together. Therefore the resolution is somewhat less than 1750 pixels across. The resolution is estimated to be about 0.7 the number of ribs, or 1225.
Some TV set makers define their specifications and manufacturing tolerances to permit deficiencies that bring the overall resolution of the set well below HDTV standards.
We have seen in Internet forums stories of complaints by TV set owners where the manufacturer claims that the deficiency is within the specifications for that make and model. Included are convergence errors as well as discolored areas on the screen or tiny specks referred to as dead pixels.
Difficult Questions For Experts
These are the kinds of questions you would have to ask in order to find out about bandwidth and other technical details. I don't expect the typical salesperson to understand them let alone know the answers. Contributors to various Internet forums on video have reported that from time to time, salespersons getting questions such as these have become unhelpful.
1. (For cables, A/V receivers) What is the video frequency response plus or minus one dB?
2. How many scan lines are used to reproduce a 16:9 aspect ratio picture for 1080i (... for 720p, ... for 480p)? (The answer will usually be either 1080 or 810 for 1080i, either 540 or 720 for 720p, and either 360 or 480 for 480p)
3. What are the dot pitches for screen center and screen edge (direct view), or What is the lenticular screen rib pitch (rear projection)?
4. We don't want to recommend buying a TV that displays the 1080i picture with only 810 scan lines but if you really want to, you could research this question: What is the pixel width of the conversion process to 16:9 aspect ratio?
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All parts (c) copyright 2000, Allan W. Jayne, Jr. unless otherwise noted or other origin stated.
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