HDTV Projector Resolution Specifications Explained: Bandwidth, Nyquist Limits, and Format Comparisons

HDTV & Front Projection Systems (part 2)

By Greg Rogers

For instance, a projector may specify an optical resolution of 1200 TVL, a video resolution of 1000 TVL and a RGB resolution of 1600x1200. In this case, the RGB resolution of 1600 pixels across the entire screen width would imply a video resolution of 1600 * 75% = 1200 TVL, the same as the optical resolution of the lens. But the best estimate of the actual maximum resolution on video pictures would be the lower of the specifications, or 1000 TVL. Furthermore, the specifications will assume the best possible setup of focus, astigmatism and convergence at a minimum acceptable light output to reduce the displayed spot size. Projector resolution is a subjective measurement since it relies on a judgment call of when lines are resolvable or not. You can see the difficulty of this measurement for yourself by using your own judgment to specify the resolution of a monitor or projector using the resolution wedges included on the Video Essentials laserdisc or DVD. You will also see that the resolution will vary depending on the contrast setting, which alters the light output from the CRTs and therefore the spot size. Some projectors will hold their spot size with increases in brightness while others will defocus more quickly. Two projectors with the same subjective resolution at a low brightness level may measure much differently at higher and more realistic brightness levels. So keep this in mind when considering projector resolution specifications and get a direct comparison demonstration whenever possible. Not withstanding the caveats expressed above, what is the maximum resolution performance of the HDTV formats? The 720P format has 1280 pixels across the width of the active picture area. The 1080I format has 1920 pixels from edge to edge of the picture area. Could an HDTV picture ever display that number of alternating black and while lines? Theoretically yes, but practically no. The video frequency content of the luminance signal must be limited to below the Nyquist frequency of the pixel sampling rate to avoid aliasing. If you look at the chart you will see that the pixel sampling rate is 74.25 MHz for both HDTV formats. The Nyquist frequency is one-half of the pixel sampling rate or 37.125 MHz. If the pixels were alternating black and white, to form black and white vertical lines, then the frequency of the video signal would be exactly 1/2 of 74.25 MHz or 37.125 MHz. But to avoid any frequency components from exceeding this limit, the video signal is filtered prior to MPEG encoding. The filters can not have infinitely steep frequency band rejection, so they must filter out some frequencies considerably below the Nyquist limit. The ATSC standard specifies a virtually flat (+/- 0.05 dB) video frequency response to 30 MHz. A practical filter flat to 30 MHz might roll off into the video background noise within about 10% of that frequency. So I have chosen 33 MHz as the limiting bandwidth (the point where no additional high frequency detail can be seen). The 10% transition region is consistent with the performance observed for current DVD filtering. Limiting Resolution per Picture Width = (Limiting Bandwidth / Nyquist Frequency) * Active Pixels per Picture Width Limiting Resolution per Picture Height (4:3 Picture) = Limiting Resolution per Picture Width * 3/4 Limiting Resolution per Picture Height (16:9 Picture) = Limiting Resolution per Picture Width * 9/16 The limiting resolution for each format is shown in the table below. Horizontal Resolution Feature NTSC Composite Digital DVD ITU- R-601 DVD Line Doubled DVD Line Quadrupled ATSC 720P ATSC 1080I ATSC 1080I Line Doubled Frame Rate (Hz) 29.97 29.97 59.94 59.94 60 30 60 Field Rate (interlaced only) (Hz) 59.94 59.94 Progressive Progressive Progressive 59.94 Progressive Active Picture Horizontal Pixels 768 720 720 720 1280 1920 1920 Pixel Clock Rate (MHz) 14.318 13.5 27.0 54.0 74.25 74.25 148.5 Video Signal Bandwidth      +/-0.05 dB BW (MHz)       Limiting BW (MHz) - 4.2 RF - 5.3 LD 5.75 6.3 11.5 12.6 23 25.2 30.0 33 30.0 33 60.0 66 Limiting Horiz Resolution per Picture Width (TV Lines) 448 RF 567 LD 667 667 667 1138 1707 1707 Limiting Horiz Resolution/PH (TVL for 4:3 display) 336 RF 424 LD 506 506 506 853 1280 1280 Limiting Horiz Resolution/PH (TVL for 16:9 display) N/A 380 380 380 640 960 960 Projector Bandwidth (-3 dB) (MHz) 10 10 20 40 50 50 100 A 4:3 display with a true 1000 lines of horizontal resolution (1333 lines across the full width) would have a CRT spot size, optical resolution and video bandwidth sufficient for 720P, but not enough to completely resolve the horizontal resolution of 1080I. In terms of specifications used for 4:3 displays, the horizontal resolution would have to be at least 1707 * 3/4 = 1280 TVL to resolve all of the practical horizontal resolution information delivered by the 1080I format. The resolution would have to be 1440 TVL (1920 * 0.75) on a 4:3 display to fully resolve the theoretical maximum resolution of the 1080I format. The 1280 TVL is beyond any current direct view CRT monitor, including Sony's professional BVM HDTV monitors. This is also beyond most CRT projectors. But CRT projectors with a true 1000 TV lines or more of horizontal resolution are still going to look just wonderful with an HDTV signal. Video (RGB) Bandwidth Bandwidth is closely related to horizontal resolution. Regardless of the CRT spot size and optical characteristics of the lenses, the RGB signals must arrive at the CRTs with sufficient bandwidth to modulate the beam current at a rate (frequency) that creates the high resolution light to dark transitions of the image. The frequency response of the projector's electronics will degrade the frequency bandwidth of the HDTV signals unless the projector bandwidth is considerably higher than the HDTV signal bandwidth. If that happens, horizontal picture resolution will be lost. The projector manufacturer's horizontal resolution specification should include any limitations from the projector's bandwidth. However, this is not always the case. So the video bandwidth, or more precisely the RGB bandwidth (video bandwidth is sometimes taken to mean the bandwidth through a composite or Y/C input), should be checked as an independent requirement. Since both 1080I and 720P use the same sample rate, 74.25 MHz, they have the same specification for a flat frequency response to 30 MHz. The projector frequency response should also be flat within a fraction of a dB to 30 MHz. This means a projector bandwidth (which is usually specified at - 3 dB) of about 50 MHz should be adequate. However, each projector will have a somewhat different frequency response curve, so unless the bandwidth is 60 MHz or more you might want to ask the manufacturer for the response at 30-35 MHz. To use a projector with an HD 1080I line doubler the frequency response of the projector should be flat to 60 MHz. This would require a projector with about a 100 MHz bandwidth. Vertical Resolution There are two factors that make the vertical resolution difference between the NTSC and ATSC formats even larger than it may appear. The ATSC numbers refer to the active picture area only. NTSC's 525 scan lines include about 42 lines that occur during the vertical blanking interval. This leaves about 483 lines for the active picture area. Secondly, the ATSC 720P and 1080I formats are 16:9 formats. If a 16:9 area is letterboxed within a 4:3 NTSC picture, as it often is on laserdiscs or some DVDs, then there are only about 360 scan lines within the 16:9 image area. So the ATSC 720P format has twice the vertical resolution of a 4:3 letterboxed picture. The DVD's 16:9 ('anamorphic') format provides a 33% improvement in vertical resolution over the 4:3 letterboxed format. The 16:9 format has 480 horizontal scan lines in the 16:9 image. This is equivalent to a vertical resolution of 640 lines (480 * 4/3) for a 4:3 display height. The 720P HDTV format provides an additional 50% improvement over the DVD 16:9 format. Moreover, the 720P ATSC format is progressive with 720 scan lines per frame repeated 60 times per second. Laserdiscs and DVDs display 60 interlaced fields per second. Each field contains only half of the total frame scan lines. A 16:9 image area has only 180 scan lines per field using the 4:3 letterboxed format and 240 scan lines using the 16:9 format. The low vertical resolution makes interlacing artifacts especially obvious whenever there is slight motion between fields. The ATSC 1080I format is also interlaced, but it has three times the vertical resolution of the 4:3 letterbox format. The 1080I format puts a difficult demand on any display device. As I just indicated, it requires three times the vertical resolution of a SD 4:3 picture. For 1080I, this is equivalent to displaying 1440 scan lines on the full height of a 4:3 display (480 * 3 = 1080 * 4/3 = 1440). Very few projectors will have a spot size small enough to actually resolve each line individually, the basic definition of vertical resolution. You might notice that the theoretical maximum 1080I horizontal resolution expressed in pixels/PH is 1080 (1920 * 9/16). This is the same as the nominal vertical resolution. When the horizontal and vertical resolutions (both expressed in picture heights) are the same, they are called "square pixel formats". Both the 1080I and 720P ATSC formats are square pixel formats. Vertical Resolution Feature NTSC Composite Digital DVD ITU- R-601 DVD Line  Doubled DVD Line  Quadrupled ATSC  720P ATSC  1080I ATSC  1080I  Line Doubled Frame Rate (Hz) 29.97 29.97 59.94 59.94 60 30 60 Field Rate (interlaced only) (Hz) 59.94 59.94 Progressive Progressive Progressive 59.94 Progressive Active Picture Lines/Frame 483 480 480 960 720 1080 1080 Active Picture Lines/Field 241.5 240 Progressive Progressive Progressive 540 Progressive Vertical Resolution Lines per Picture Height for a 4:3 Display   * 16:9 'Anamorphic' Format 483 480 640 * 480 640 * 960 1280 * 960 1440 1440 I don't believe any manufacturer specifies the vertical resolution of their projectors as a separate line item. As I mentioned earlier, the RGB or computer resolutions that are sometimes specified, do not necessarily mean that each and every pixel can be resolved. So I would use caution in drawing too many conclusions from that data unless the manufacturer specifies that they are resolvable pixel resolutions. In most cases the projector's vertical resolution for a 4:3 picture will be approximately the same as the horizontal resolution in TV Lines (referenced to a 4:3 picture height). This assumes that the spot size is approximately round (this will require good adjustment of astigmatism and focus) and that the horizontal resolution is not limited by the RGB bandwidth. Only a handful of projectors will have sufficiently small spot sizes to truly display the full vertical resolution of the 1080I format. On the positive side, some modest overlapping of the scan lines will help hide the interlacing artifacts from this format. There is a very fine balancing act going on here that makes for the interesting debate between proponents of the 720P and 1080I formats. The 1080I format should produce a 50% improvement in horizontal and vertical resolution over 720P, but CRT and optical limitations in projectors (and aperture-grille and shadow-mask limitations in direct view TVs) will limit that significantly. But those same limitations partially obscure the visibility of the fine line-twitter and other interlace artifacts of the 1080I format. So the debate goes on between proponents of the 1080I and the 720P formats as the networks and others choose up sides for round one of HDTV. Returning to the projector requirements, any projectors with conventional 4:3 horizontal resolutions of 1000 TV lines or more should display about all of the vertical and horizontal resolution that 720P has to offer. Projectors with 9" CRTs and premium lenses that can fully (or very nearly) resolve the full 1080I resolution will suffer more from the visibility of interlacing artifacts. These projectors will benefit from HD line doublers and should have the performance requirements discussed earlier to support them. Two final notes on vertical resolution. First, the perceivable vertical resolution is significantly lower than the number of scan lines would indicate. This is related to the fact that the scan lines establish a discrete vertical sampling structure with finite scan line width (this is known as the aperture effect), as well as the fact that the scan lines may be interlaced. The measured loss of vertical resolution varies with the physical parameters of the display as well as the type of image used for evaluation. Several experimental evaluations of this phenomena have occurred over the years with varying results for perceived vertical resolution ranging from about 0.5-0.8 of the nominal scan line count. This is known as the Kell Factor and the figure most often used is 0.7. Second, to help eliminate interline flicker on interlaced video, it is a common practice to vertically filter the output of the camera for video sources, or the telecine during film to video transfers. This filtering reduces interline flicker and shimmering by broadening the height of fine details to occupy more than a single scan line. Between the Kell Factor and deliberate vertical filtering, the perceived vertical resolution of an interlaced image may end up being little more than 50% of the nominal resolution found by counting the frame scan lines. In that case, the projectors spot size and optical resolution would seldom be a limiting factor. Keep these last two considerations in mind when observing the nominal vertical resolution requirements given in the tables. Summary of HDTV Parameters & Projector Requirements Display Parameters for DTV Formats Feature NTSC Composite Digital DVD ITU- R-601 DVD Line Doubled DVD Line Quadrupled ATSC 720P ATSC 1080I ATSC 1080I Line Doubled Frame Rate (Hz) 29.97 29.97 59.94 59.94 60 30 60 Field Rate (interlaced only) (Hz) 59.94 59.94 Progressive Progressive Progressive 59.94 Progressive Aspect Ratio 4:3 4:3/16:9 4:3/16:9 4:3/16:9 16:9 16:9 16:9 Active Picture Horizontal Pixels 768 720 720 720 1280 1920 1920 Total Horizontal Pixels 910 858 858 858 1650 2200 2200 Active Picture Lines/Frame 483 480 480 960 720 1080 1080 Total Lines/Frame 525 525 525 1050 750 1125 1125 Horizontal Scan Rate (KHz) 15.734 15.734 31.468 62.937 45.0 33.75 67.5 Pixel Clock Rate (MHz) 14.318 13.5 27.0 54.0 74.25 74.25 148.5 Horizontal Retrace Time (uS) 9.92 10.22 5.11 2.55 4.98 3.77 1.89 Video Signal Bandwidth      +/-0.05 dB BW (MHz)       Limiting BW (MHz) - 4.2 RF - 5.3 LD 5.75 6.3 11.5 12.6 23 25.2 30.0 33 30.0 33 60.0 66 Projector Requirements for DTV Formats Feature NTSC Composite Digital DVD ITU- R-601 DVD Line  Doubled DVD Line  Quadrupled ATSC 720P ATSC 1080I ATSC 1080I Line Doubled Horizontal Scan Rate (KHz) 15.734 15.734 31.468 62.937 45.0 33.75 67.5 Horizontal Retrace Time (uS) 9.92 10.22 5.11 2.55 4.98 3.77 1.89 Vertical Retrace Time (uS) 1335 1430 1430 1430 667 667 667 Projector Bandwidth (-3 dB) (MHz) 10 10 20 40 50 50 100 Limiting Horiz Resolution per Picture Width (TV Lines) 448 RF 567 LD 667 667 667 1138 1707 1707 Limiting Horiz Resolution/PH (TVL for 4:3 display) 336 RF 424 LD 506 506 506 853 1280 1280 Limiting Horiz Resolution/PH (TVL for 16:9 display) N/A 380 380 380 640 960 960 Vertical Resolution Lines per Picture Height for a 4:3 Display   * 16:9 'Anamorphic' Format 483 480 640 * 480 640 * 960 1280 * 960 1440 1440 Projector Specifications Important The following specifications for currently available projector models were assembled from the manufacturer's web pages. In many cases the information on their web pages is incomplete. There are also conflicting specifications on different web pages of the same manufacturer. In that case, I used the lower performance specification unless I thought there was an obvious error. I apologize for any errors in this table and invite manufacturers to submit corrections and additional information. Different manufacturers may use different methods for measuring or stating the same specifications. Therefore, the specifications may not be directly comparable. This is particularly true in the case of the resolution specifications. Readers should always contact the projector manufacturers directly to verify and understand their specifications when making purchasing comparisons and decisions. I've included several projectors in the table that are no longer current production models but are commonly available as used/refurbished products from dealers that advertise on the web. The specifications for those models were obtained from dealer web sites and private contacts. Refurbished products may include replacement components that are different than originally specified, and therefore the performance of these products may be better (or worse) than the original models. You should obtain a performance guarantee from your dealer. CyberTheater assumes no responsiblility for the accuracy of the following specifications. Projector Specifications See notes at end of table Price CRT Light Output lumens peak Horiz Scan Rate (Khz) Horiz Retrace Time (uS) Vert Retrace Time (uS) Band width (-3dB) (Mhz) RGB Resolution 4:3 Display Nominal or Resolvable Limiting Horiz Resolution TVL/PH 4:3 Display Lim Horiz Res/PW ---- Vertical Res/PH 4:3 Display ATSC 720P Requirements 45 4.98 667 50   853 1138 ---- 960 ATSC 1080I Requirements 33.75 3.77 667 50   1280 1707 ---- 1440 Line doubled 1080P Requirements 67.5 1.89 667 100   1280 1707 ---- 1440 Barco Graphics 1208s/ 1209s 8"/9" EMF 1000/ 1050 135 1.25 200 120 Barco Graphics 808s 8" EMF 1050 110 2.5 200 120 1600x 1200 Barco Data 808s 8" EMF 1250 75 3.3 300 75 1180x 900 Barco Vision1609 HDTV/ QDTV 9" 1600/ 1050 64 3.3 300 60 CAL Cinevision IDP 950 7" EMF 800 33.75     30   CAL Cinevision IDP 1050 7" ESF 850 70     75 1600x 1200 Electrohome Marquee 9500LC 9" EMF 1300/ 260 ANSI 130 1.8 300 120 2500x 2000 Addr 1500x 1200 Ansi Electrohome Marquee 8110/8500/ 8500LC 8" EMF 210/ 225/ 235 ANSI 110/ 130/ 130 1.8 300 100 2500x 2000 Addr 1350x 1100 Ansi Electrohome ECP Plus 4500/4501 7" ESF 1005 150 ANSI 90 2.5 (5.0 below 37 KHz) 300 70 1600x 1280 Addr 1280x 1024 Resolvable 1024 Electrohome ECP Plus 3500/3501 7" ESF 1005 150 ANSI 72 3.6 300 60 1280x 1024 Resolvable 1024 Electrohome ECP 4100 (Used Only) 7" ESF 650 80 2.5 (5.0 below 37 KHz) 300 70 1280x 1024 1000 Electrohome ECP 3100 (Used Only) 7" ESF 650 50 or 58 3.6 300 60 1280x 1024 NEC 6PG XTRA 7" EMF 1000 69 2.5 350 100 1280x 1024 1200 NEC XG75/ XG75A 8" 1100 230 ANSI 75 2.6 270 110 1500 NEC XG110LC 8" 1200 240 ANSI 110 2.6 270 120 2500x 2000 Addr 1500 NEC XG135LC $24K 8" 1200 240 ANSI 135 2.6 270 150 2500x 2000 Addr 1500 Runco DTV-852 w/Line Dbler $15K 7" EMF 850 50 Bypass doubler 50 900 Runco IDP-980/950 $19K/ 8" EMF 1100 65 2.5 350 80 1600x 1200 1000 Runco IDP-980 Ultra $23K 8" EMF 1100 89 2.5 350 100 1600x 1200 1000 Runco DTV-991/ Ultra $25K/ $33K 8" EMF 1200 75/ 110 2.6 270 110 1600x 1200 2500x 2000 Runco DTV-1000 $48K 9" EMF 1250 135 1.25 200 120 2500x 2000 Sony VPL W400Q $5K Three 1.35" 16:9 LCDs 400 ANSI 1080I 480I only 800x480 4:3 1068x480 16:9 600 Sony VPH D50Q 7" 800 64 75 1280x 1024 1000 Sony VPH G70Q 8" 1200 110 120 1700x 1200 1100 Vidikron Vision One 9" 130 Vidikron VPH 50 HD 64 Vidikron VPF 40 HD 55 Zenith Pro900 $12.6K 800 50 1920x 1080     Zenith Pro2000 w/Line Dbler $20.5K EMF 190 ANSI 1600x 1200     Projector Table Notes: CRT EMF - Electromagnetic Focus (EMF) generally provides a better spot size and more picture detail than Electrostatic Focusing. ESF - Electrostatic Focus Light Output Peak Lumens - This is often measured by illuminating an area of about 10% the screen size in the center of the display. This measurement ignores the reduction in brightness at the sides and corners of the screen. ANSI Lumens - Tested per an ANSI (American National Standards Institute) specification. The measurement is made by dividing the screen into 9 blocks, taking a reading in the center of each, and averaging the readings. This number is much lower than the peak lumen measurement (usually around 20-25%). RGB Resolution Nominal - This is the projector manufacturer's highest recommended computer resolution setting for use with their product. These are subjective recommendations and shouldn't be used for precise comparisons between products from different manufacturers. Addr - Addressable, same as Nominal above. ANSI (RGB/Computer Display) Resolution - This ANSI specification averages the measured resolution in nine different rectangular zones across the display and expresses the result in equivalent pixels across the picture width or height. The contrast between alternating black and white pixels is called the depth of modulation. The measurements are made by increasing the number of black and white lines (pixels) in the rectangular measurement zones until the depth of modulation drops below 30% of the large area contrast ratio. Limiting Horizontal Resolution (for a 4:3 Display) These specifications are almost always expressed in terms of TV Lines (alternating black and white vertical lines) across a display width equal to the picture height. This measurement is subjective, but if carefully made using normal picture operating conditions, it can be quite representative of actual performance.   Projector Table Notes: CRT EMF - Electromagnetic Focus (EMF) generally provides a better spot size and more picture detail than Electrostatic Focusing. ESF - Electrostatic Focus Light Output Peak Lumens - This is often measured by illuminating an area of about 10% the screen size in the center of the display. This measurement ignores the reduction in brightness at the sides and corners of the screen. ANSI Lumens - Tested per an ANSI (American National Standards Institute) specification. The measurement is made by dividing the screen into 9 blocks, taking a reading in the center of each, and averaging the readings. This number is much lower than the peak lumen measurement (usually around 20-25%). RGB Resolution Nominal - This is the projector manufacturer's highest recommended computer resolution setting for use with their product. These are subjective recommendations and shouldn't be used for precise comparisons between products from different manufacturers. Addr - Addressable, same as Nominal above. ANSI (RGB/Computer Display) Resolution - This ANSI specification averages the measured resolution in nine different rectangular zones across the display and expresses the result in equivalent pixels across the picture width or height. The contrast between alternating black and white pixels is called the depth of modulation. The measurements are made by increasing the number of black and white lines (pixels) in the rectangular measurement zones until the depth of modulation drops below 30% of the large area contrast ratio. Limiting Horizontal Resolution (for a 4:3 Display) These specifications are almost always expressed in terms of TV Lines (alternating black and white vertical lines) across a display width equal to the picture height. This measurement is subjective, but if carefully made using normal picture operating conditions, it can be quite representative of actual performance. A special thanks to CyberTheater Productions LLC to for granting permission to post this informative article.