HDTV & Front Projection Systems
A special thanks to CyberTheater Productions LLC to for granting permission to
re-post this informative article.
| "I want to buy a front projector, so what specs must it have for HDTV?"
This is the most common question that I've been asked in the last several months. I find it particularly interesting because until recently I rarely received questions about front projectors. This sudden and dramatic interest in front projection is a result of two important technological revolutions in Home Theater - DVD and HDTV. DVD has been with us for a year now and HDTV is well ... just around the corner. Really! We now anticipate the launch of broadcast HDTV in late 1998. For the Big Screen Home Theater enthusiast, it is time to consider the impact of HDTV when selecting equipment. The decision to purchase a front projection system is a major financial investment. You should be concerned that any purchase now be compatible with HDTV requirements in the future. CRT front projection and HDTV are a perfect synergistic coupling of technology. Front projection will be the best way to see the picture improvements delivered by HDTV, and HDTV will provide the high resolution source that a large screen front projector craves. In this article I will explain the front projection system requirements necessary to be compatible with HDTV in the future. The operative word here is 'future'. It is obviously impossible to predict every decision that the video broadcasting and the video equipment manufacturing industries might make over the next 5-8 years as HDTV technology matures. But I believe we can now specify the range of projection requirements that should serve us well during this evolution. DVD & Front Projection Why has DVD created such an interest in front projection? Primarily because of the DVD 16:9 format, confusingly called the anamorphic format by the manufacturers. The 16:9 format provides 33% more vertical resolution than the conventional 4:3 letterbox format that is so popular on laserdiscs. But to get this major improvement in picture resolution you must have a 16:9 display device. A 16:9 format picture will be horizontally squeezed and distorted on a conventional 4:3 display. Nearly all CRT based front projection systems provide the ability to display pictures in multiple aspect ratios including 16:9 and 4:3, so they are ideally suited to take advantage of DVD's key quality enhancement feature. The only current LCD projector that can display both 4:3 and 16:9 pictures is the Sony W400Q. But that projector can not display progressive HDTV signals and its LCD pixel resolution is far less than specified for the HDTV formats. Unfortunately only about half of today's DVDs use the 16:9 format, most of the remaining widescreen movies use the 4:3 letterbox format. It really disappoints me that several studios are refusing to provide their movies in the highest quality consumer format currently available. It is even more distressing that those studios stand to gain from this omission in the future. They will get to sell the same DVD titles over again in 16:9 when their original customers inevitably acquire 16:9 TVs. This smacks as the same kind of opportunistic marketing that filled laserdisc collections with duplicate titles - first P&S and later a letterboxed version. So I especially applaud Columbia Tri-Star (all of their widescreen releases have been magnificently transferred to the 16:9 format) and Warner (predominately providing 16:9 transfers) for their policy of superb support for the quality and success of DVD, and the best interests of their customers. The other reason that DVD has driven a desire for front projection pictures is that most of the quality limitations of laserdiscs, which looked marginal when enlarged to room filling screen sizes, have been eliminated with DVD. This includes the traditional problems of cross-luma and cross-color Y/C separation artifacts (dot crawl, hanging dots, rainbow moire patterns), excessive chroma noise and low color resolution (requires DVD's component video signals). Add to those advantages the 16:9 format's 33% improvement in vertical resolution. Now that we have a source that stands up to critical review when expanded to fill a large screen, it is no wonder that more enthusiasts want to create a video Home Theater environment with the visual and emotional impact that can only be achieved today by front projection. DVD 16:9 Format NoteThe DVD standard provides a pixel array of 720 horizontal pixels by 480 vertical pixels for the active picture area. DVD producers can choose to either map a 4:3 image area or a 16:9 image area onto the 720x480 pixel array. If a 4:3 image area is used, then widescreen movies will be letterboxed within the 4:3 pixel image area (equivalent to the analog technique used for letterboxed laserdiscs) and many of the pixels will be wasted for black bands at the top and bottom of the movie's picture area. If a 16:9 image area is chosen, then the widescreen movie will occupy a much larger proportion of the pixel area and far fewer pixels will be wasted in much smaller black bands. In fact, since a 16:9 image area occupies 3/4 of the 4:3 picture's height, only 360 vertical pixels are used for the same picture area that has 480 vertical pixels in the 16:9 format. Therefore, the 16:9 format will always have 33% more vertical resolution than the 4:3 letterboxed format regardless of the actual widescreen movie aspect ratio (1.85, 2.35, etc.). Both the 4:3 and 16:9 DVD formats have 480 vertical pixels (i.e. scan lines) in the picture area, so they both fill the entire vertical height of any NTSC TV. Since the standard width to height ratio of conventional TVs is 4:3, the 4:3 letterbox format will be correctly proportioned on the display. A 4:3 TV display is too narrow compared to its height for the 16:9 format DVD, so the picture will be horizontally squeezed and everything will appear too tall and skinny. You must have a 16:9 display to take advantage of the 16:9 format's extra vertical resolution and get a properly proportioned picture. CRT front projectors provide the multiple aspect ratio capability to display 16:9 format DVDs and also to switch to 4:3 for older non-widescreen movies and other conventional NTSC video sources. HDTV StandardsThe history of HDTV development in the U.S. is a long, long story. In this article I would like to avoid re-telling that tale and begin instead with where we are today, or at least where we appear to be. The FCC has specified a standard for terrestrial over-the-air broadcast transmission from local stations to consumer's homes. That standard describes how the HDTV signals are to be transmitted and received, but it does not mandate the picture format to be transmitted. There is no single picture format that will be used for the production or distribution of HDTV sources to consumers. Instead, each provider of HDTV source material is free to choose a format that they prefer. This seems to invite HD anarchy and inevitable format wars. Fortunately, things are not quite that dismal. Prior to FCC final approval, the Advanced Television Systems Committee (ATSC) published their recommended ATSC Digital Television Standard that included a list of 18 picture compression formats that encompass a wide range of digital television (DTV) performance. The ATSC formats span a range considered to be Standard Definition TV (SDTV) to High Definition TV (HDTV). The ATSC formats are shown in the table below.
Table Note: The picture rates can either be the ones given in the table or any of them can be decreased by the factor 1000/1001. This is intended to provide easier compatibility with the current NTSC system, which uses 60 / 1.001 = 59.94 Hz. At this time it appears that the picture rates given in the table will be used. I have used these later to calculate projector requirements because they are the worse case. In an eleventh hour decision these formats were not officially adopted by the FCC, but the major television networks and the consumer TV manufacturers have decided to follow the ATSC formats. It is believed that most consumer TV manufacturers will include support for receiving any of the ATSC formats in each HDTV or HDTV receiver. Notice that these are actually compression formats. They describe the MPEG encoded video that is being transmitted (or stored). Once the video is decoded in a DTV receiver, it doesn't have to be displayed in the same format as its original source. The DTV may convert the video into another format for display. (However, just like line doublers, these conversions may introduce artifacts.) Many of the TV manufacturers have said that their HDTVs will convert any incoming format into 1920x1080 @60I (abbreviated 1080I) for display. It is also possible that some TV monitors will convert everything into 1280x720 @60P (abbreviated 720P) for display. The broadcast networks are split on which compression format they will use for HDTV transmissions. CBS and PBS have announced that they plan to broadcast 1080I, while ABC has announced it will support 720P. This ensures that some format conversions will be required by an HDTV that has only a single display format. Further, the networks have discussed using their DTV channels to broadcast multiple SDTV programs using a multiplexed system during daytime hours. Since the SDTV formats need only 1/4 to 1/6 of the bits required to compress the HDTV formats, 4-6 SDTV programs may be simultaneously broadcast in place of a single HDTV program. This is an opportunity to generate additional advertising revenue to help pay for the expensive transition to DTV. Multiple SDTV channels are thought to be economically viable because of the high cost of true HDTV monitors. Many consumers may opt to buy set-top boxes that receive the multiplexed digital SDTV transmissions and also downconvert HDTV transmissions to SDTV. These boxes would convert the digital signals to analog for compatibility with today's 4:3 NTSC TVs. The multiplexed SDTV channels would be about the quality of today's digital satellite systems. The downconverted HDTV transmissions could have about the same quality as DVDs. True HDTV monitors will provide options for upconverting various SDTV formats to pseudo HDTV formats and crossconverting between HDTV formats like 720P and 1080I. Professional quality broadcast equipment that perform those conversions have price tags in the multiple tens of thousands of dollars and even into six figures. So it's not unreasonable to expect that some artifacts will occur during format conversions in HDTV monitors, even though the initial products are expected to be in the $5-10K range. The best option would be a multi-scan monitor that could display DTV in the native formats without scan line or pixel conversion. Unfortunately, multi-scan capability hasn't been announced for any of the first generation HDTVs. This opportunity may be limited to front projection systems initially. HDTV Signal Sources A key question is what HDTV sources will be available for Home Theater. So far I have discussed only one source, over-the-air broadcasts from local network or independent stations. We can be reasonably certain this will occur because the government is insisting that the networks broadcast HDTV. But I wouldn't place any bets on how many hours of HDTV programming we are likely to get in 1998 or 1999. CBS recently announced that they planned about 5 hours of HDTV programming per WEEK starting in the fall of 1998. I haven't seen numbers from the other networks yet. So we can expect something for the early adopters to view and show off to their friends, but only time will tell how quickly the programming will grow. HDTV Broadcast Receiverhe first standalone receiver/decoders for broadcast HDTV, with RGB outputs to feed a front projector, are expected before the end of 1998. Everyone would like to know what price and what features to expect. At this point we must speculate on features, but prices in the $3000-$6000 region have been mentioned. This seems quite high since a standalone receiver is little different than a subset of the electronics in a complete HDTV. But manufacturers often view projector owners as a privileged species and price products at whatever they think the market will bear. Zenith and Panasonic have been mentioned as two possible suppliers of early HDTV receivers.As far as features are concerned, we can certainly expect RGB outputs since every projector with sufficient performance to support HD signals would have them. Since most projectors have only a single set of RGB inputs, I hope that a set of high quality, active RGB pass-thru inputs would be included for an existing line doubler or another HDTV source. Some line doublers also have active RGB pass-thru inputs, so the HDTV receiver and other future HDTV sources could be daisy-chained together. It is important that pass-thru inputs be active and not passive at these video frequencies. Ultimately, I would expect YPbPr component video to become the standard for interconnecting HDTV sources and not RGB. But since many projectors don't have YPbPr inputs there needs to be at least one component that can directly drive the projector's RGB inputs and provide YPbPr switching for other products. The only YPbPr to RGB converter product that I currently know about is the Extron CVC 100 at $895. So there will be an opportunity in the future for a moderately priced, multiple input switcher with YPbPr to RGB conversion. My key expectation for any HDTV Broadcast Receiver is that it output both 720P and 1080I formats without having to convert between them. It must also receive the multiplexed digital SDTV broadcasts. I would expect it to output the SDTV programs as progressive frames in the 704x480 @60P format, as well as providing several options for converting them to pseudo-HD formats. That's what I would like, now we will wait to see what the first products deliver. We can also expect Dolby Digital audio outputs since this is a standard feature of the ATSC TV standard. However, it is unknown at this time if any of the networks will support Dolby Digital 5.1 channel surround sound. It is possible that they will initially broadcast only Dolby Digital 2.0 channels with Dolby matrix encoding for Pro Logic systems because this is more compatible with their existing audio equipment. HDTV by Digital SatelliteDirecTV has announced their intention to broadcast two pay-per-view HDTV channels in 1998. In order to receive those channels it will be necessary to purchase a new DSS HD Receiver. No pricing or feature information is available yet. Due to the nature of DSS, I would expect any new receivers to support all current channels and features, as well as new HDTV channels. I also hope they will begin broadcasting Dolby Digital 5.1 when these new receivers are available. But they haven't announced those plans yet. The same concerns about inputs and outputs apply to the satellite receivers. We must hope for adequate pass-thru inputs and RGB outputs as discussed previously. DirecTV has discussed using a variation of the ATSC 1080I format. Instead of using 1920 horizontal pixels per line, they would use only 1280. This would reduce the total amount of pixel data by 1/3 and therefore reduce the encoded bit rate for HDTV proportionately. Since the ATSC 1080I format bit rate is specified as 19.4 Mbps (includes audio), DirecTV would be able to use bit rates of about 13 Mbps with equivalent MPEG compression. Since their existing SDTV format uses an average of about 3.5 Mbps, they would save digital transmission bandwidth for about two SD channels by this format change. As we shall see below, the reduction in horizontal resolution would likely be insignificant on all but the best possible display systems. Cable NetworksHBO is the first premium cable network to announce that they plan to broadcast two channels of HDTV this year. They have also announced that they plan to support the 1080I format. It remains to be seen which cable companies will choose to deliver these channels. Again it will take a special cable HD set-top decoder box to receive these signals. I hope the cable companies will provide adequate flexibility to drive our RGB inputs with minimal format conversions in a set-top decoder. Another possibility would be for USSB, which broadcasts the premium movie networks on DSS, to add the HBO HDTV channels using the same new HD DSS receivers discussed above. They haven't made any announcements yet. Other HDTV SourcesOther possible HDTV sources include other cable TV networks, a high definition DVD format, and a digital tape format similar to D-VHS. So far the cable TV companies have shown less interest in HDTV. They seem to be moving toward digital SDTV using proprietary formats for higher channel density. I would expect the cable companies to accelerate their HDTV activities only if they feel significant HD competitive pressure from the satellite providers. An HD-DVD format is probably out on the horizon somewhere, but I realistically don't look for it to happen for at least 5 years. The technology for higher density and higher bit rate playback will probably be around long before a high definition DVD format could overcome the copy protection issues that nearly sunk the initial introduction. Besides, there will be no economic incentive to bring HDTV pre-recorded movies to market until the number of HDTVs reaches a significant percentage of US homes. And that could take much longer than 5 years. An HD digital video tape format could be brought to market fairly quickly based on technology similar to D-VHS, which is currently used to record the digital bitstream from SD digital satellite systems. Satellite based HDTV broadcasts could also be time-shifted using an extension of this technology. However, the movie studios could force the HD satellite broadcasts to be copy protected to prevent this type of digital copying. I wouldn't expect to see pre-recorded HDTV tapes using this system. They would suffer from the same problems as HD-DVD, no installed base of HDTV monitors and the same copy protection issues. Besides, if copy protection were used to prevent taping, then an HD-VHS extension would have no advantage over HD-DVD at all. It would be more costly, bulkier, less reliable, etc. Recordable DVD technology should also be an alternative within the time frame of HD-DVD. In summary, I don't expect to see any pre-recorded HDTV sources in the next 5 years. I believe we will see a gradual increase in network HDTV programming over-the-air, via digital satellite and eventually by cable. I would also expect a modest number of pay-per-view HDTV movies delivered via satellite and cable. So why should someone with a front projection system, or thinking of getting a front projection system, worry about HDTV compatibility at all? If you are one of us, then you really don't have to ask this question. It is our nature. We are the early adopters. We are the Home Theater enthusiasts that always push the envelope. If anyone is going to have HDTV in their homes, then dog-gone-it, it's going to be us. So now let's see what we need to make it happen in a front projector. HDTV Display ParametersThe table below shows the display parameters associated with several video formats that can be used with a front projection system.
Digital Video Pixel Array NoteAll of the displayed picture information is contained in the active pixel area. There are additional horizontal pixel periods at the end of each scan line that are not seen but provide time for the right-to-left horizontal retrace of the CRT beam during the horizontal blanking period. There are also additional vertical pixel periods (corresponding to scan lines) that are not seen during the vertical blanking interval, which provide time for the bottom-to-top vertical retrace of the CRT beam. The concept of an active pixel area and a total pixel area is used with all digital video formats. The total number of pixels and the number of active pixels are very important to determining the projector requirements as shown later in this article.In column (1) I have included the basic NTSC interlaced picture parameters as a reference. NTSC is a composite video format that can be stored in analog form or in digital form. To make the comparisons easier I have shown the parameters associated with storing the composite video in a digital form. Although the video on a laserdisc is stored as RF modulated analog composite video, it is usually mastered from a D-2 digital composite VTR source. Column (2) captures the display parameters associated with DVD digital video. The DVD video format is very close to ITU-R-601 specification for SDTV digital video. The biggest difference is that DVD uses the distribution quality 4:2:0 video format instead of the 4:2:2 video format that is used for studio quality video. The vertical resolution of the 4:2:0 color information is one-half of that used in studio quality video. The same 4:2:0 format is also used for all the ATSC HDTV formats. Columns (3) and (4) show the effect of line doubling or line quadrupling on the display parameters for DVD video. If laserdisc video is line doubled or quadrupled, it will first be digitized to nearly the same parameters as shown in column (2) for the DVD, except that its limiting video bandwidth will remain lower at 5.3 MHz. It would exhibit the same characteristics as shown in columns (3) and (4) except that the video bandwidth is doubled to 10.6 MHz, or quadrupled to about 21.2 MHz. Columns (5) and (6) show the display parameters associated with the two most common ATSC formats, 720P and 1080I. Although HDTV video can be compressed as 24 frame per second (FPS) progressive video or 30 FPS progressive video it can not be displayed in those formats without objectionable flicker. These formats will normally be converted to either 60 FPS progressive video or 30 FPS interlaced video (60 fields per second) for display. It would also be technically straightforward to convert 24 FPS progressive video to 72 FPS. But since that isn't an ATSC format I consider it fairly unlikely. Column (7) is not an official ATSC format. I would expect however, that HDTV line doubling products will be created to convert 1080I to 1080P, especially for use with film source video. The purpose of this conversion, as with SD line doublers, is not to increase resolution, but to eliminate interlacing artifacts such as line twitter.
Projector requirements can be separated into two categories. First are the minimum requirements to display the HDTV format signals. These include Horizontal Scan Rates, Horizontal Retrace Time and Vertical Retrace Time. If these requirements are not met, then the projector will not sync to the signals and no stable picture will be produced. Second are the requirements necessary to achieve the maximum quality delivered by the HDTV format. Even if the second requirements are not fully met, the picture quality from an HDTV source should still exceed that delivered by SDTV sources. Horizontal Scan Rate & Horizontal Retrace Time
Horizontal Scan Rate = Pixel Clock Rate / Total Horizontal Pixels The maximum Horizontal Scan Rate for the ATSC HD formats occurs with 720P at 45.0 KHz. This is far above the capability of some projectors that work with SD line doublers (about 31.5 kHz from column 3), but compatible with projectors that support SD line quadruplers. The 1080I format also slightly exceeds the scan rate of SD line doublers. Therefore, some line doubled projectors may not be HDTV compatible, check this spec carefully. An HD 1080I line doubler would require a scan rate of 67.5 KHz which exceeds the requirements for SD line quadruplers (about 63 kHz). There are projectors that are only spec'd to 64 KHz, so this is a potential problem to consider. Horizontal Retrace Time = (Total Horizontal Pixels - Active Horizontal Pixels) / Pixel Clock Rate The CRT beam must retrace from right to left across the screen in the time between the last active pixel on the scan line and the first active pixel on the following scan line. The 1080I retrace time is much shorter (shorter is more difficult) than the time permitted for a line doubled picture. So this requirement (3.77 uS) should be carefully checked against the projector specifications. Some projectors capable of line doubled video may not be capable of ATSC HDTV for this reason. A 1080I line doubler would create a horizontal retrace problem that very few projectors could handle. The 1.89 uS retrace time is shorter than the time specified for all but a handful of products such as the Barco 1208s/1209s, the Runco DTV1000 (based on the Barco 1209s), the Electrohome Marquee projectors and the Vidikron Vision One /Two (based on the Electrohome Marquee). With this problem in mind, any manufacturer of a high definition line doubler would be wise to rescale the ratio of the active pixel width to the total pixels in a line to increase the horizontal retrace time. For instance, to achieve a 2.5 uS retrace time for projectors that are compatible with today's quadruplers, the active pixels could be digitally resampled to occupy 1828 pixels of the 2200 pixel line. The projector's horizontal size adjustment could then be used to set the proper display width. Vertical Retrace Time
Vertical Retrace Time = (Total Lines - Active Lines) / Horizontal Scan Rate This specification shouldn't be much of a problem. All of the HDTV formats require the same 667uS vertical retrace time. Any projector that provides the necessary horizontal scan rates should have no trouble with vertical retrace. Common vertical retrace specifications for projectors are about 300-450 uS. HDTV Resolution Requirements Resolution can be considered a 'soft' specification (ok, pun intended). Any projector that satisfies the 'hard' specs will produce an HDTV picture. But how 'high definition' the picture will be, depends on the resolution performance of the projector. This is an especially difficult requirement to assess since the projector specs are open to interpretation and are also susceptible to a bit of specsmanship. It may even be difficult to get resolution specifications from the manufacturers. In the end, the absolute performance of each projector should be determined by actual HDTV test patterns and video material played from HDTV sources. But it is reasonable to assume that the projectors that exhibit the best resolution with line doubled or line quadrupled 16:9 format DVDs will also look the best with HDTV. Horizontal Resolution Video horizontal resolution is a measure of the number of black and white vertical lines that can be individually discerned in a given distance across the display. By convention, video resolution is measured in picture heights, whether it is vertical resolution or horizontal resolution. So the horizontal resolution is the number of resolvable vertical lines across a width of the display equal to the picture height. For a 4:3 display this is equivalent to 75% of the resolvable lines across the full width of the display. In reality, the incremental horizontal resolution varies at different points on the screen, falling off at the sides and corners as the CRT focus typically degrades in those areas. Although front projection systems can be adjusted for many different aspect ratios, video resolution in TV Lines (TVL) is generally quoted for a 4:3 display unless otherwise stated. Since this provides the largest picture height relative to picture width, it also yields the largest and best sounding resolution numbers. Some projectors will quote a TVL number identified as the optical resolution or related to the lens. This normally refers to the performance limit of the lens alone and doesn't include other limiting factors such as CRT spot size or video bandwidth. So look carefully for another horizontal or video resolution figure in TVL. Another resolution number often quoted for a projector is the RGB, pixel, or computer display resolution. This number is usually expressed as a pixel array size such as 1600x1200 where the first number refers to the horizontal pixels, and the second number the vertical pixels. The numbers quoted are usually the largest standard computer pixel array size that the projector can reasonably display. In effect it is the maximum recommended resolution that a computer should use when displaying on the projector. It doesn't necessarily mean that the projector will be able to clearly display each and every pixel without overlapping the surrounding pixels |
