The official start date for terrestrial DTV broadcasting was 1, November 1998. While some of the horses were out of the gate a few days early broadcasting the launch of the space shuttle, we are actually still building the track. The construction is so close to where the horses are running that we may temporally be loosing sight of our direction. Selling DTV to the consumer is a case in point.

Check out what’s being sold at retail as "DTV certified" or "DTV compatible" or "DTV ready." Knowing a bit about the real DTV system, you might find yourself wondering how some of these sets could possibly be connected with DTV when they are nothing more than ordinary TVs. Does an S-video input and 1.33:1 (4:3) aspect ratio somehow qualify a set for a DTV nod from retailers?

One could argue that they aren’t far off, after all many DTV set-top boxes will provide a down converted S-video or even a composite video output. If current TV sets are good enough to be considered DTV compatible, what added value is there to DTV that could justify the broadcast and program production community’s additional investment to make it happen? Should we try to create the kind of difference in digital versus analog that existed between color and black and white? If DTV is to have added value should we encourage displays of it to be dramatically better?

There is a need to define what constitutes a DTV display in a way that the added value of the system can be clearly demonstrated. The Consumer Electronics Manufacturers Association (CEMA) and the Advanced Television Standards Committee (ATSC) are already headed in this direction with a basic DTV certification program. The problem, as we see it, is that since DTV exists on so many levels that one level of certification really doesn’t tell the consumer enough for them to make an intelligent purchase. CEMA and the ATSC have a laudable goal and that’s to encourage the consumer to buy into this new system. We’d like to see a certification system that not only encourages them to buy into the system, but gets them into it at a level that will support the expanded capabilities that can be delivered.

DTV isn’t easy to define. Some in the industry believe it’s a replacement for our analog transmission system. That’s only the beginning. One of its purposes is conservation of bandwidth. More information can fit into a given space in the digital domain. We may choose to waist that space by upconverting conventional resolution pictures to higher rates, requiring all of the digital space allocated to the new system, or we could broadcast sources at their native rate and use the rest of the space for something else. On the other side we’ve just seriously expanded the canvas with which the artist can convey a message. How much do we want to short change that expanded message in a receiver with limited capability?

The ATSC has defined 18 to 36 scan rates and two aspect ratios, depending on who’s counting, so that the broadcaster can pick the right rate and shape for the program being transmitted. The FCC has wisely chosen not to limit the system to so few scan rates or aspect ratios. After all, in many minds, the ATSC left out at least 540p (progressive) and 600p, let alone a common version of 480I (interlace). As the quality of MPEG compression improves we might be adding 768p to the list.

For those asking about 1080p programming, we certainly don’t expect it to be a viable format for distribution in the consumer world. The cost of a display device that can take advantage of this format is and will be out of the range of most high-end consumers for a long time to come.

The original ATSC system called for two aspect ratios, 1.33:1 and 1.78:1 (16:9). Maybe 0.77:1 is correct for program information that is in the shape of a piece of paper. You’ll find many people in the motion picture industry that will argue for all sorts of other aspect ratios. Among them, 1.37:1, 1.66:1, 1.85:1, 2.0:1, 2.2:1, and 2.35:1 (2:40:1). Have I left any out? Probably.

Considering all of this, is it probable to come up with one specification for DTV certification that would cover all of this, or even the more confined scope of the ATSC rates and aspect ratios? What’s really needed is a division of certification specifications. In figuring out what those should be, we might do well to go back to the 1980’s proposed analog transition from standard definition TV to HDTV. You may remember that was back when our high definition system was going to be analog and maybe even compatible with the current NTSC system.

Following the analog lead, the new certification system would start at a Standard Definition (SDTV) level. It would include the 480i system we now use for the majority of our analog transmission. The interlaced PAL system might also be included, something around 580i. The next step up would be Improved Definition (IDTV). We’d put 480p into that category. It would be followed by Extended Definition (EDTV). We’d include a range from 540p, to say 720p, encompassing a progressive version of PAL and 1080i. Near the top quality category would be High Definition (HDTV). It would include 1080i, 720p, 768p and maybe even higher scan rates. Since we see 1080p being out of reach for all but a handful of consumers in the next five or more years, that rate would belong to some future specification.

Is this all we need to know when defining the quality of a display device? Could we build a certification system for TV sets based just on their scan capability? No, not really. Other factors, such as horizontal resolution, proper decoding of the component video signals, gray scale tracking, the aspect ratio of the display itself and correct colors of red, green, and blue are equally important. Here’s where things get a little more difficult. Let’s take displayed picture resolution in particular. Horizontal resolution is probably the same for 480i and 480p, just as it could be equal for 1080i or 1080p. Does the "p" versus "I" look enough better to place the "p" in a higher category? Absolutely! The vertical resolution in the "p" version is enough better than the "I" signal to bump the "p" up to the higher category.

How important is horizontal resolution in any of these categories? There are a number of considerations to be taken into account when answering that question, but horizontal resolution isn’t as important as it is credited in most circles. When it comes to direct view sets, light output capability is inversely proportional to horizontal resolution. As resolution goes up, light output goes down. It will go down to the point where if we were to attempt to clearly reproduce 1920 lines across the width of a 32 inch wide screen, the set won’t produce enough useable light output to be considered for any certification.

The same light output versus resolution consideration isn’t necessarily true for CRT-based projectors. Often times higher line rate capabilities in the source signal will produce better performance in the CRT. Yet there are significant limitations here as well. As the CRT size gets larger and the focus capability gets better, the performance increase is in picture detail, not more light output as manufactures would have you believe. You’ll have to keep the screen size down, even on 9-inch CRT-type projectors, in order to obtain the performance capability of the tube. While we’re on the subject of picture performance from a projector, the screen material plays an equally important roll in picture quality. That must also be part of any real DTV certification specification. Any of the fresnel, lenticular screens we’ve encountered would not fit into the ED or HD categories.

You might think that we’d have to consider a separate category for CRT-based projectors in either a front or rear screen configuration. In reality, direct view sets wouldn’t qualify for the HD category, so projectors have that to themselves by default.

While still on the subject of picture resolution, there are additional factors to consider in a direct view display device. The distance of the viewer from the picture is important. Anything more than one or two picture heights away from a 30-inch wide set and you’ll be hard pressed to see half of the 1920 lines no matter what the capability of the set. The conclusion here is that while horizontal resolution capability can’t be ignored, it’s not critical that it even approaches the capability of the high definition source signals for most of the categories.

As for the matrix decoding, gray scale tracking and the primary colors of the display factors become more significant as you get beyond SDTV. They belong near the top of the list for consideration in the advanced categories. Since the matrix is important to color quality we should also cover the other two important parameters of color fidelity, the primary colors of red, green and blue, and gray scale tracking.

There is a set of primary colors defined for the higher resolution capability of the DTV system. It is slightly different from that defined for standard definition, a color set that really hasn’t been widely available in the consumer market. Certainly at an ED or HD level, the color of red, green, and blue should closely conform to the Society of Motion Picture and Television Engineers’ (SMPTE) definitions for the 1080i and 720p systems. (They are both the same.)

As for gray scale tracking, it’s as critical as the correct colors of red, green and blue, and must be easily calibrated. What does that mean? Access to the controls should be easy for the person trained in properly setting a gray scale. The resolution of the controls should be good enough to make accurate calibration easy. Once the set is calibrated at the high and low points of the gray scale, it should also track gray within a certain specification. The number commonly used in the broadcast world is 6 CIELUV. That’s three times the minimum perceptible difference in color. We would assign specifications to DTV categories according to their position on the quality scale.

What about the shape of the set? SDTV and IDTV could both be 1.33:1, with the IDTV set having the capability of displaying a 1.78:1 image somewhere in the 1.33:1 area of the display. Display devices in the EDTV or HDTV categories should be widescreen, a 1.78:1 aspect ratio if we stick to the ATSC criteria.

There are several more important points that should be spelled out prior to certification. The display device can easily be limited by its input(s) and signal processing. Where an external tuner is required we might only be capable of declaring a display device as DTV ready. If a DTV tuner is part of the package its capabilities will also have to be considered. Then there’s audio. Part of higher end certification for display devices with a built-in DTV tuner might include a serial digital audio output for external processing of the audio. In any event, a high-end display device should not be part of the acoustic reproduction capability.

Looking at the video input, it is our experience that a Y, Pb, Pr connection is critical to obtaining good picture quality, plus having an advantage in setup. A black and white signal is needed to help assure proper calibration of black and white levels, gray scale and convergence. That’s easy to obtain if you can disconnect Pb and Pr. If the input to the set is serial digital, an internal capability of shutting off the Pb and Pr must be provided. If the display device only provides for an RGB input, as is the case with most high-end video projectors, a Y, Pb, Pr to RGB converter must be part of the specification for high-end DTV certification.

We’ve already alluded to the fact that the DTV system is dynamic. Specifications set today will most likely have to be changed in the near future. As an example, the standards for data transmission have yet to be set so we can’t specify how a current generation set is to handle that part of DTV. We might even have to date the specifications, calling them SDTV ‘99 or HDTV ‘01. There is another reason for wanting these specifications dated. It is our firm belief that in the future all display devices will run at their own rate, and at just one rate, totally independent of incoming signals. We feel that the cost of high quality translation from any incoming format to the ideal rate for the individual display will be far less than building a multi-sync display device. This approach is required for fixed array displays that are currently on the market and will be beneficial if used on variable rate-capable display devices.

The DTV tuner specifications are obviously important to the resulting picture quality displayed on any of these sets. A separate certification process will have to be set up for them.

Application

Enough of the background. Let’s see what happens if we apply what we’ve discussed. At this point we’re providing a first draft, something designed to stimulate discussion. We fully recognize that getting to some of the specification found in the EDTV and HDTV parameters isn’t going to happen in current stand-alone equipment. We do feel that combinations of equipment could be assembled that would meet these specifications. Comments are welcome.


SDTV &emdash; Standard Definition Television:

Since these are "DTV Certified" specifications for SDTV the criteria is going to be more stringent than might be found for a regular TV set, yet not much beyond the current capability of good sets that are already available.

Scan rate capability need only cover our current analog interlaced TV system of 525 lines at 59.94 Hz otherwise known as 480i in the DTV system. All color decoder and sync specifications of being able to handle VHS in standard play as well as fast forward and rewind apply. Should the PAL rate of 625/50 be considered? That might depend on the market, but probably not.

A display aspect ratio of 1.33:1 would be acceptable, but not limited to that shape. An aspect ratio of 1.78:1 would also qualify. What about other aspect ratios? How much unnecessary confusion do you want to add to DTV? At the moment we would only accept 1.33:1 or 1.78:1.

This set would not be required to have multiple aspect ratio capability in the first generation, relying on the DTV set-top receiver to do the aspect ratio conversion.

In a few years, the SDTV category might be upgraded, requiring a 1.78:1 aspect ratio display. A 1.33:1 screen shape might not be acceptable for any category of DTV certification in the future.

Analog Y, Pb, Pr input(s). A flat Y frequency response out to 6.5 MHz ± 0.5 dB. A Pb and Pr response out to 3.0 MHz ± 0.5 dB.

Accurate translation (decoding) from Y, Pb, Pr to RGB using the NTSC matrix.

The RGB path response inside the set should be flat out to 10 MHz ± 0.5 dB.

For compatibility with conventional analog signals the set must contain a composite and S-video input with an accurate color decode capability. An adaptive comb filter would be required in this application.

Any SVM circuits that might be included in the set have to have an easy way of shutting them off when in the DTV display mode.

Easily calibrated gray scale, capability of tracking D65 (6500° Kelvin) to at least 12 CIELUV. The resolution of the calibration controls should be good enough that the calibration points can be set within 1 CIELUV of the target color of gray.

Display light output capability of 25 foot-Lamberts without blooming.

The choice of colors for red, green, and blue will most likely be left to the manufacturer for the first generation of these set. We would suggest something close to SMPTE C colors and insist upon it for the second generation of SDTV specifications.

Displayed horizontal resolution of 530 lines across the full width of the screen. This translates to about 400 horizontal lines per picture height for a 1.33:1 picture; the analog TV specification for horizontal resolution. We are using a full picture width in our specification because it is independent of aspect ratio. Inexpensive TV sets can now do from 380 to about 420 lines/picture height, yet we find only a few expensive sets reaching as high as 450 lines. (This despite claims of 700, 800, or even 900 lines of resolution on the part of some manufacturers.)

Separate setup for brightness, contrast, and gray scale for the composite or S-video inputs versus the component inputs.


IDTV &emdash; Improved Definition Television:

Display scan rate capability starts at 480p. While the set will be able to accept lower rates, they must be upconverted for display. This set may or may not include a PAL progressive capability. It would not be required to go any higher than either of these two rates.

A minimum displayed horizontal resolution of 560 lines across the full width of the screen.

An aspect ratio of 1.33:1 would be acceptable, but not limited to that shape. An aspect ratio of 1.78:1 would be encouraged in the first generation of this specification and mandatory in the second generation of the specification.

Capability of displaying a 1.78:1 image inside the 1.33:1 picture area if the set is a 1.33:1 aspect ratio. If the set is a 1.78:1 aspect ratio we would require the capability of a 1.33:1 image in the center of the 1.78:1 image area. In addition, there would have to be a capability of displaying a letterboxed 1.78:1 image out to the full width of the screen. Individual brightness and contrast memories would be necessary for each aspect ratio displayed.

Y, Pb, Pr input. RGB or VGA is optional on the first generation. The Y, Pb, Pr decode matrix would be NTSC. A flat Y frequency response out to 6.5 MHz ± 0.5 dB and a Pb and Pr response out to 3.0 MHz ± 0.5 dB for the 480i input would be required. If the component input could also accept a progressive signal, the frequency response specification would have to be doubled. The RGB or VGA input response would be flat in each channel to 20 MHz ± 0.5 dB all the way to the imaging device.

For compatibility with conventional analog signals the set would contain a composite and S-video input. Accurate color decoding of composite and S-video would be necessary. A high quality adaptive comb filter would be required for going between composite and S-video. The set would contain an internal processor to convert the 480i signals to 480p. This same processor could be used to convert any DTV 480i source to 480p. That means the processor would require a component as well as an S-video and composite input capability. The component path would conform to the bandwidth requirements stated in the SDTV specifications. We would encourage the manufacturer to include two-third pull-down recognition in this video processor in the first generation with it being required in the second generation.

Matrix decode capability for 480i and 480p only. (They are the same thing.) Conversion from higher rates, including the change in matrix, is to take place outside this set.

Capability of turning any SVM circuits off in any display of DTV signals.

Easily calibrated gray scale, capability of tracking D65 (6500° Kelvin) to no greater than a 10 CIELUV variation from 0.5 foot-Lamberts to full brightness. Resolution of the controls at the calibration points should be less than 1 CIELUV. A minimum of a second choice of color temperature must be made available. It needs to have a range that could easily reach 5400° Kelvin. The color temperature memory would be selectable from the front panel. It needs to be assigned to individual input memories so that when the user selects a particular input, the correct color temperature will also be selected. We will encourage at least four memories for color temperature, any of which can be assigned to a particular input in the first generation, requiring it in the second generation.

Light output capability of 25 foot-Lamberts without blooming.

Separate setup for brightness, contrast, geometry, blanking and gray scale controls for each displayed aspect ratio. Minimum of two choices for gray scale, each of which could be individually calibrated and assigned to particular memories. What would be included in geometry? Certainly height and width plus any other controls needed to insure reasonable geometry at each aspect ratio. In a direct view set, it is our experience that separate vertical linearity memory would be necessary for the two display aspect ratios.

Colors for red, green and blue to approach SMPTE C colors.

EDTV &emdash; Extended Definition Television:

In the initial stages of specifying this set we would call for a multi-scan capability in CRT-based display devices. It would start at 480p and run up to 768p. That range would include 1080i, which is just above 480p. Careful attention would have to be paid to the ringing in the picture caused by the short retrace time of the 1080i signal.

It is our feeling that down the road ID and ED sets will only scan at a single progressive rate. Second or third generation of specifications for these categories would include an internal scaler that would convert incoming signals to the specific rate required by the display.

Fixed array display devices entering this qualification would now have to have processors that would convert the incoming signal to the display configuration of the device. Processor quality specifications would have to be included.

Input connections would include a composite 75 ohm BNC for NTSC and PAL, an S-video connector, a component input on three 75 ohm BNCs, and RGB plus Hand V sync on five 75ohm BNCs, plus a VGA-type connection. We would encourage at least two component and two RGB connections. The two RGB input connections could be divided into one for the BNC connectors and a second at the VGA connection.

The set would contain an internal signal processor for 480i source signals. Conversion could be to any rate between 480p and 768p. The converter would be required to recognize two-third pull-down in film original material. The NTSC and S-video decoder would have to be color accurate. We would require at least a high quality 2D adaptive comb filter and would encourage a 3D adaptive comb filter.

The set would have to contain a matrix decode capability for both 480 interlace and progressive signals and the higher rates as specified by SMPTE. We would encourage automatic detection of the input rate for the proper selection of the decode matrix.

An ability to shut Pb and Pr off for component video calibration. An ability to display blue only for NTSC decoder calibration.

Luminance bandwidth of the entire component and RGB path would have to be flat out to 40 MHz, ± 0.5 dB. Pb and Pr response would be flat to 20 MHz ± 0.5 dB. If a frequency detection method is used to differentiate between the 480 interlace and progressive rates from the higher rates, the 480i signal path into the processor could be band limited to 7.0 MHz for luminance and 3.5 MHz for the two color channels. The 480p signal would not be band limited, as is optional for the 480i input, but sent to the proper decode matrix. The circuits might be kept simple if the line processor, converting 480i to a progressive signal, were done in the component domain. The output could them be fed to the 480p matrix decoder.

The red, green, and blue colors of the display would closely conform to SMPTE specifications for 720p. (That same specification also applies to 1080i and 1080p.)

The shape of the picture would be 1.78:1

The set would contain full multiple aspect ratio capability. We would insist on four but encourage a minimum of six options of memory for aspect ratios. We would require good geometry setup for a 1.33:1 picture in the center of the 1.78:1 display, an overscanned 1.33:1 image, a linear 1.66:1 aspect ratio; letterboxed, a linear 1.78:1 image; letterboxed and a 1.78:1 image edge to edge of the raster, sometimes know as the anamorphic format. It’s this 1.78:1 edge to edge position that would most likely be used for the 1080i and 720p rates. Memories for each would have to contain individual information for brightness, contrast, color temperature, and picture shape and geometry.

Separate memories would have to be provided for each input rate. As much as there would be multiple aspect ratio capability, it would exist individually for each rate. It would also exist individually for each input connection on the back of the set.

Monitors that are 32 inches wide or smaller would provide 25 foot-Lamberts of light output prior to any noticeable blooming in the picture. Flat field uniformity would be within 12 CIELUV for color and no more than 45 percent fall-off from center to edge in luminance. These measurements would be checked over a 60° angle from the center of the picture, both horizontally and vertically.

Light output could drop to as low as 10 foot-Lamberts as the screen width approached 72 inches. The 12 CIELUV and 45 percent fall-off specification would still apply to the larger screens. Measurements would be made over a 60° angle from the center of the picture.

Gray scale tracking would have to be within 10 CIELUV, with a resolution of less than 1 CIELUV at the calibration points.

Horizontal resolution across the width of the screen would have to be at least 800 lines for a 27-inch wide screen size set and approach 1200 lines in a 72-inch wide image.

We don’t yet know the numbers for quality of geometry and convergence, but need to convey that they would be tight.


HDTV &emdash; High Definition Television:

Input and processing requirements of EDTV. We would extend the higher scan rate input bandwidth flat out to 50 MHz, encouraging and internal RGB bandwidth to be flat out to at least 80 MHz.

Scan rate capability would include 1080p although we would not expect the display device to fully resolve the horizontal bandwidth of 1920 lines across the full width of the screen.

Horizontal resolution would be specified in the order of 1500 lines.

Light output capability would be placed at 10 foot-Lamberts for a 72-inch wide screen with no visible blooming in the image.

Display quality for color uniformity would parallel the EDTV set. Luminance uniformity would be within 30 percent.

If you thought the specifications for geometry and convergence were tight for the EDTV category just wait until you see what we come up with for this category.


HDTV Plus:

We mentioned this in the article. We don’t now have many display devices that can truly show off the capability of a 1080p signal, let alone the 2K by 2K or 4K by 3K images that are now being created in the graphics industry. It is our feeling that a 12-inch electromagnetically focused CRT projector would be required for this task. While we’ve seen such projectors, none of them that we’ve played with were designed for high-resolution video display. A requirement for this type of display is here now because the signals are being generated all of the time.