Archived Tech-Notes
Published by: Larry Bloomfield & Jim Mendrala The following are our current e-mail addresses:
E-mail = hdtvguy@garlic.com or J.Mendrala@ieee.org
We have copied the original Tech-Notes below as it was sent out. Some of the information may be out of date.
**********************************************DTV Tech Notes
% Larry Bloomfield & Jim Mendrala
(541) 385-9115 or (805) 294-1049
E-mail = larrybend@aol.com
J_Mendrala@compuserve.com
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February 15, 1998
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DTV Tech Note - 014
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Sharing experiences, knowledge or anything else relating to DTV, HDTV etc. with your fellow engineers is what we are all about. For a copy of our policies and who we will make this available to, please e-mail us. Welcome to all the new subscribers. We now have over 150. We hope you will participate with question, answers and/or comments. To those of you who have been with us and wondered were we've been, read on. New things are in the works.
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24 FRAME PROGRESSIVE vs. 30 FRAME INTERLACED
From: Jim Mendrala
There are provisions for other frame rates in the Advanced Television Systems Committee (ATSC) specs. There is also talk about the 24-frame rate being used in some commercial DTV applications. Since television transmits a tremendous number of programs that originate on film, it's easy to see why this is being addressed. More than 75% of prime time is originates on film. Most TV commercials are shot on film. TV with its higher frame rate of 30 frames per second (fps) requires that the 24 frame film image be converted to 30 fps with a 3:2 pull-down. DTV however takes the 3:2 pull-down out of the incoming video, converting it back to 24 fps when it enters the MPEG compliant encoder. Since 30 fps gives you better temporal resolution with live TV, we don't seem to realize that with film there isn't anything to be gained by converting 24 fps film to 30 fps. Film has always looked "good" on TV. Part of that is because each frame, of the 24 fps, was photographed in approximately 1/48 of a second and scanned in a progressive manner and converted to interlace for the NTSC system. Telecines do not have an image that starts off as "interlace." Think about that! The film frame is progressively scanned. Today some of the new TV cameras are progressively scanned and the image output is in an interlaced mode. When a video tape of this type of image is displayed on an interlaced display in the "still" or "pause" mode, the image, because of no interlace when captured, looks twice as sharp even if the subjects were moving.
With today's new DTV standard it is obvious to me that 24-frame progressive should be of great interest to the broadcaster. Then images would be free of aliasing artifacts. If used in the NTSC format, however, each field read out as interlace may contain small area flicker due to only half the samples in the vertical direction being displayed per field. The human eye has to integrate out aliasing effects per field, to see the full resolution of the picture. This aliasing is the interline twitter we have all seen. The eye has to try and filter out the 30-cycle flicker between the two fields. In order to save bandwidth, it was decided to refresh alternately the odd lines and the even lines. This means that for a given signal bandwidth the number of pixels in an interlaced standard will be twice the number of pixels of a progressively scanned system, resulting in good static resolution but poorer temporal resolution. The flicker perception of the human eye demands a refresh rate of at least 60 times per second to prevent flicker.
But interlace scanning creates some well-known artifacts, especially with moving pictures. In a television image (not from film), it is impossible to combine two fields to make one frame or picture for moving objects, as each field comes from a different moment in time. But with progressive scanning, the image has all of the pixels captured at one time. There is no difference between that of film or the image acquired by the progressive CCD.
Compression of interlaced signals is a lot more complex and performs worse than compression of progressively scanned signals, where the entire image is captured at one time, akin to a film camera with its approximate 1/48-second-exposure time.
Those that think that the flicker would be intolerable have to bear in mind that 24 frame progressive transmitted as NTSC would be read out as interlaced and 3:2 formatting put in upon transmission at 30 fps. This would produce the 12-cycle jitter that we have become accustomed to in film transfers to TV. In the DTV standard, the decoder in the receiver would display the image similar to film image but at 72 times per second to the display, thus eliminating any perceptible flicker. (In a motion picture theater, the film running at 24 frames per second is displayed twice for each frame. The eye sees 48 frames per second on the screen. This was done very early in the film world, more than 95 years ago, to reduce flicker.)
Now, if you take this concept up to HDTV at 1080 progressive at 24 fps, you have a lot less to compress than at 30 fps with a 2:1 interlace. The resultant image would look much better displayed at 72 fps, because of the absence of flicker, than the way film is transferred with a 3:2 pull-down putting in a 12 cycle jitter pattern. HDTV at 1080i 1125/30/2:1 just would not hold up to 1080p 1125/24/1:1 for transmission of film transferred to video. (The same would hold true for those that want to transfer film at a 25
fps as in PAL countries.)
Compression systems of today extract the 3:2 pull-down and work only on the 24 frames per second. This allows more bits to make a better picture than a live TV broadcast where the encoder has to be pushed to the wall because of the differences in the interlaced field images.
It is still true that certain things will have artifacts, such as the wagon wheels in a western will probably still rotate backwards. But, hey, we've lived with that for an awful long time and it doesn't really detract from the story. NTSC images from film have always had that problem. It's true that if you film at 30 frames per second or higher these artifacts tend to disappear the higher you go but at the expense of a lot more data to go along with it.
My conclusion is that since prime time will mostly come from film in the early days of DTV, then it makes a lot of sense to work at a 24 frame per second capture rate. There would be more bits available for the MPEG compliant compression encoder, even at the same variable or fixed bit rate to improve the pictured. The progressive images at 24 frames per second would be flicker free and would be displayed at the rate of 72 times a second, just like a computer monitor, instead of interlace at 30 times per second for each field. This would also require less bandwidth for the same quality of picture. So for the NTSC transmitters, the quality would be about the same as what we now accept as standard. But for the new DTV, a virtually flicker free image would be displayed on the new DTV receivers, even though the image was captured at 24 fps. When DTV in the future wants to go to MPEG's HP@HL (High Profile @ High Level) or more commonly known as the HDTV standard it will not be necessary to go to 60 fps at the transmitter. The receiver will take care of that.
Any comments or suggestions?
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OBSERVATIONS - Living with Change
By Larry Bloomfield
We've seen several major changes and additions in the development of television over the years. Change should not be anything new to any of us. Probably the first significant change was when the decision was made to adopt an all-electronic scan system, replacing the mechanical disc.
Frequency and channel allocation changes are not uncommon to our industry either. In the early days when Television Productions, Inc., W6XYZ (Paramount Pictures excursion into early television) first signed on the air in Los Angeles, they were on Channel 4. With FCC changes, their commercial successor, KTLA, operates to this day on Channel 5. The same action that made that channel change, also did away with Channel 1. The John Pool experimental UHF station on Signal Hill, just north of Long Beach, California, first operated on a frequency somewhere close to the modern day successors, KWHY, Channel 22.
The next significant change happened when an all-electronic color system was settled on in the late 50's. It may not have been the best system, but we've been living with it for over 40 years. We've gone from image dissectors and iconoscopes, which required hundreds of foot candles of light, to the CCDs, and other devices, which will produce a very clean and clear picture in almost total darkness. And let's not forget the addition of Stereo, SAP and Pro audio. I'm probably leaving some things out that you may think are important, but you get my point.
Smoke, Mirrors and Engineering Know How: Our industry is an industry of "buzz words." This may not be true in the smaller markets, but in the bigger markets if you don't have a switcher made by the XYZ company or an audio console with TQV's name on it, you may very well loose business. The sad part is that most of the better know manufacturers, and some of the smaller ones, make fine versions of equipment that do the same job. That's probably an arguable point, but in essence, true. It's the "buzz word" identification that counts and in many instances not the technical quality of the either the equipment or the installation. In smaller markets, cost usually plays a bigger roll, in what equipment they'll have, than quality or reliability. It is unfortunate when non-technical or inexperienced people decide what equipment their station will buy and use. With all the new technology we will be required to buy, install and use over the next several years, a trend back to relying on good experienced engineering input is most essential to avoid major disaster.
There is much to be said for the engineer who has been with his or her station or network position since dirt was invented. They are truly the backbone of their operation. These are the folks the Chief Engineer turns to when he wants to know what route a particular "temporary" feed takes that was installed back in "02" from Studio A to Master Control. To compliment these "old timers" are the engineers who have been around or who have done freelance work. They bring to the staff a wealth of experience. These are the engineers who have seen and know how to do the same job several different ways. They help balance out the people whom, when asked why, say: "That's the way we've always done it." A good mix of people is essential for a good balance of know how.
Stay on Course: It is said that those who don't learn from history will be condemned to relive it. In this day and age of very rapid change, words were never truer. It is absolutely imperative to keep ourselves informed. In this new section of Broadcast Engineering, I hope to share with you not only all that is new, but to temper it as best as I can with what has happened. There is no question that most of us are setting sail on uncharted waters. If we can share with each other our trials, tribulations and solutions, we can get through all this with out reinventing the wheel at every turn. This is the time to make printed technical material available to your fellow engineers, attend SBE meetings and other seminars. Keep abreast of what is going on or you'll get left behind. Engineers from the days of Harry Lubcke at W6XAO have always shared their knowledge and solved the industries common problems. This is not the time to change any of that. You can associate, solve common problems and exchange ideas with other engineers at other stations, with out giving away your plans for the invasion of Normandy, programming or sales secrets. SBE meeting is a good place and this newsletter is another. I never knew an electron that ever gave two hoots or a holler over whose cable they were going down or what network they were helping to stay on the air. We, if we are to succeed, must be a fraternity.
It is said that those who know "how" will keep their jobs and those who know "why" will lead the rest. Remember this: If you're good at what you do and you're not having fun, you probably don't know what you're doing and are doing it wrong.
These are some of my thoughts on how to make our jobs easier and, at the same time, have fun. I'd like to hear yours.
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DTV Channel Reassignment and other notes
By Larry Bloomfield
I subscribe to the FCC's daily digest. I get relatively good information from it, but the information on the Commission taking up the issue of DTV Channel reassignment was good coming in, but got corrupted some where in the "works." They pulled it off calendar at the very last minute. Now that I'm working for Broadcast Engineering, I've been able to establish some really good, reliable contacts. We all thought it was going to happen. I don't like to report irresponsibly and for that I apologize. I'll make every effort to let you know just as soon as I find out anything further. If any of you hear anything, I'll be glad to check it out. This is a very important issue and must be address before many of us can proceed. (Boy there's an understatement!)
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If you subscribe to Broadcast Engineering (BE) and haven't seen the January 1998 issue yet, please let me know. I understand they had problems getting it out to the West Coast. It took me two weeks to see my first efforts after I had gotten phone calls from associates on the East Coast who wished to make comments. (FYI - It begins on Page 14) Sorry if I'm a little proud, but from our one hundred thirty subscribers to over 45,000, that's quite a jump. By the way, your comments and input on its contents are most welcome.
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I don't want to turn the DTV Tech Notes into a "preview of coming attractions" for BE, but I've run across some really neat and interesting things so far. Like anyone who enjoys what he does, I'd like to let you know what's up. With BE, I can go into most anything that relates to the technical side of broadcasting. Probably the most exciting to date is that Sarnoff Labs has a new CMOS chip which will turn the whole camera industry on it's heals. It's reported to be 100 time better than the very best CCD. I've done a story on that for the Feb. issue. Don't buy or commit to buy any cameras until you've read that.
If it doesn't have to do with DTV or HDTV, I'll simply mention it and go on, otherwise we'll give it space here. On the other side of that coin, here's your chance to influence a national trade publication. If you know of any special products, techniques, newsworthy items etc. in any part of our industry, please let me know and I'll look into them. But as I said, we'll still keep this newsletter dedicated to the new ATSC standards as they relate to digital delivery and the 18 forms of video, the audio and data associated there with. We can get a notice or issue of this out reasonably fast -- type it up and out it goes. We've tried to put these out only when we've got something to talk about. Since many of us are in a "wait and see" mode, not much is happening. If you have a special question or issues you'd like help with, we will put it out to the subscribers for possible solutions. The Telecine Interactive Group (TIG) does that and they have as many comments and exchanges a dozen or so a day. No problem, we can and will do that here too. Use this. It's your tool.
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NOTES:
One of our subscribers is interested in the colorization of Black and White film, as it would apply to HDTV. As you know we have said that most all libraries will have to be retransfer into the new formats or they'll have to be (GOD forbid) bumped up. Any comments or input?
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RCA put out a book, many years ago, on the principals and fundamentals of NTSC color. As I remember it, it had a dark green cover. Howard W. Sams used much of it in their Color TV training book, now long out of print. The Sams book had a light blue and white-stripped cover. The Sams book also had a very good section on color picture tubes, the various types and how they were constructed. I'm sure there were other publications similar to these two. Am looking for anything along this line. Larry is asking.
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Tektronix is having a special seminar on DTV - HDTV etc. in Portland this next week. If they cover what they are advertising, it's something worth going to. Call your sales rep for more info.
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NBC put out a set of books on the principals of DTV to their affiliates, about 3 months ago. A very good set. If you work at an NBC affiliate and don't know about them, find out. I'd be curious to know if the other networks have done anything like that for their folks. Please advise.
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The Harris/PBS Digital Express now has their full compliment of people. This is the mobile training demo unit that will start at NAB and tour 45 or so markets during the next 18 or so months. The idea is to show you a "typical" DTV - HDTV situation and hold classes. We've reported on it here before. They plan to get about $200 per person to sit in on their sessions. If you plan to build your digital facility in a truck and move it around the country, then pay the money and see their show. Since our first mention, however, other manufacturers have stepped forward and said they are planning on giving away what Harris/PBS is charging for. We'll see. It appears that there are a lot of organizations stepping up to the bat and telling PBS "who is doing it." (If PBS doesn't do it, who will?)
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One of one of our readers suggested we define the specific areas of television, as it would help in addressing issues. We agree. This is what we came up with. There are 5 parts of a television system: 1. Acquisition - Film or Electronic (Includes Film to tape or data transfers), 2. Manipulation - Corrections - processing - Post etc., 3. Storage - Tape - Disk - Film (prints) etc., 4. Distribution - Electronic (terrestrial or satellite) - Terrestrial shipping. 5. Presentation - Projection - Electronic or Film.
Any comments?
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Thanks to those who answered our survey about NAB. We'll begin putting them in here starting with our next issue. We'll be contacting you for permission, as promised.
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Issues using EAS on DTV and with Direct Satellite broadcasting.
By Larry Bloomfield
I know some of this will be old hat, but you can lift it off of here and use it as needed at your facility for the unfamiliar. First a little history. Alerting the public in times of emergency, is one of the highest charges of the broadcast licensee. This was first done in our industry when President Truman signed into law the CONLRAD system. Remember when we were required to either transmit the messages on 640 kHz, 1240 kHz or go off the air. Although a step in the right direction, it didn't take into account the people who were either listening to FM radio or watching television. In 1976 the Emergency Broadcast System replaced CONLRAD and for many years, the vehicle through which emergency messages were disseminated to the public was the Emergency Broadcast System or EBS. The Emergency Broadcast System, as designed, was intended for the President to convey messages and information to the citizens of the nation in times of pending disaster. The EBS further allowed for the Governor of a state to convey the same to the citizens of a State. In recent years, Local Operational Areas have adapted the two-tone attention signal system, to alert smaller groups of the population in times of pending disaster. It is the latter use of the EBS that saw the most actual emergency duty, with the National and State EBS primarily limited to occasional testing.
As technology advanced, the EBS did not. The lengthy two-tone attention signal became a tune out factor, as stations were required to transmit it during main broadcast hours once per week. Also because all EBS broadcasts had to take place on main broadcast channels, it forced broadcasters to become selective in their retransmission of an emergency message, especially if that transmission pertained to a narrow section of the population outside the broadcaster's coverage. Finally, because the EBS was a serial-relay system of main-channel broadcasters, the system became vulnerable to those broadcasters choosing not to retransmit all messages; the relay chain became broken.
Out of a request for input on changing the length of the two-tone EBS attention signal grew a total revamp of the entire method of emergency broadcasting to what is now upon us; the new Emergency Alert System (EAS). In the FCC rules adopted a few years ago, a digital protocol was adopted to allow for specific area alerting. In addition, the FCC opened up the path of emergency message transmission to include broadcasters, and any other means available then or in the future. As this system develops, this flexibility of transmitting emergency messages should eliminate the vulnerability of the old EBS serial-relay system, but there are some glitches!
Since the FCC addressed mostly national and state issues in their rules, scarcely mentioning how this would be accomplished locally, the local broadcasters have been left on their own to develop this system to meet our local needs. Broadcasters and local agencies have been struggling for the past year just to get their required monthly tests aired. The Sacramento area stations during floods last year first used the EAS. Reports have it that it was a success. Others have had success in its implementation, yet others have suffered considerable embarrassment with gross failures.
As we all know, EAS has been around, in the terrestrial broadcast world, now for just over a year. Cable companies will be coming on line by the end of this year. The reasoning was to fill the gaps where viewers not watching over-the-air television could be alerted to impending disasters. But there is a growing gap that isn't being filled. The direct satellite broadcast market. DirecTV has over 3« million subscribers with the number growing everyday.
There are four players in the direct satellite delivery business (DBS) that I'm aware of: PrimeStar, USSB, DSS and EchoStar. The only one of these four that has mentioned any thing about local-to-local delivery is EchoStar. The rest say they are a "National Service." Much of Middle America has gotten its signal via one or more translators. Unless the translator is own or maintained by the serving station, many of the associations and local government have been pulling the plug on their translators for any one of a bushel full of reasons. By reason of this attrition, many have become DBS subscribers if they want to watch any TV at all.
There are two DBS services in Japan, which I am aware of. The Japanese government requires them both to carry tsunami warnings and other emergency information and, as I understand it, there is no distinction between local and national. Having been to Japan, it's a big place.
If the DBS people can address each and every receiver: turn it on or off and deliver detailed programming information about each channel, they can warn viewers of disasters. The technology is there. If the philosophy stated in the first paragraph of this story is true for one, it should be true for all. If that farmer's, or other rural view's, sole source of information is DBS dish then that provider has a serious responsibility to keep him informed of any impending disaster and the detail will just have to be worked out.
That's my opinion. What's yours? Larry
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The next step:
I'd be interested in hearing what any of you might think the next step television will be taking after DTV is giving us our various kinds of signals. 3D-TV maybe? Larry
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The DTV Tech Notes are published for broadcast professionals who are interested in DTV, HDTV etc. by Larry Bloomfield and Jim Mendrala. We can be reached by either e-mail or land lines (541) 385-9115, (805) 294-1049 or fax at (805) 294-0705. News items, comments, opinions etc. are always welcome from our readers; letters may be edited for brevity.
>>> larrybend@aol.com --------- J_Mendrala@compuserve.com <<<
DTV Tech Note articles may be reproduced in any form provided they are unaltered and credit is given to the DTV Tech Notes and the originating authors, when named.
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