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 Some of the information may be out of date.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
(408) 778-3412 or (805) 294-1049
E-mail = larrybend@aol.com or J_Mendrala@compuserve.com
February 19, 1999 DTV Tech Note - 027
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Published when any of us have something to share and it is sharing our experiences, knowledge or anything else relating to DTV, HDTV, etc., with fellow engineers: That's what we are all about. We will send this directly to anyone asking, just E-mail us. There is no charge for this Newsletter and no one gets paid (sigh!). Welcome to all new subscribers. We're now over 260. If you are receiving this newsletter and want to get off the list, just e-mail us that request. There are no “majordomo” automatic servers, we administer this manually. We hope everyone will participate in all ways with comments, experiences, questions and/or answers. This is YOUR forum! Past issues are available at: WWW.SCRI.COM
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Subj: DTV Express and DTV Utah
From: Ed Williams - PBS
The DTV Express was in Salt Lake City the last week of February. DTV Utah folks had just been to see the FCC re their local plan and it was well received. They take an interesting approach: Eight stations on non adjacent channels alternately on two antennas with four to each antenna. Cost for each station is well less than if each station did
their own DTV installation. KUED had made arrangements with Harris to have an experimental DTV station on channel 40 for demonstrations during our visits. Folks in several TV retail stores were watching.... and calling us when we were making adjustments to the system which caused some temporary outages. KLS took their field measurement truck around town to check on signals. Nice demo. They sure have a problem with LPTV and translators tho. Took some of the best channels for their main DTV signals and now must move many of the secondary users. I expect DTV translators will be easy and cheap to implement. Lower in power and able
to be put on first adjacent channels in some cases through same antenna and line, the costs should be acceptable. More on this later.
(Ed. Note: Ed Williams asked if we’d do a hyperlink to the DTVExpress (dtvexpress.org) and have a list of where their tour will be. No problem, we’ll run the list, but some or our subscribers e-mail service can not accommodate attachments or hyperlinks, so we don’t do that. The ball is in Ed’s court.)
Ed Williams, PBS -- Tel: 703-739-5172 -- Fax: 703-739-8114 -- email: ewilliams@pbs.org
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In response to Anonymous in Burbank (Tech Notes # 25 & #26 )
Is the Peacock Changing Colors?
The networks aren't the only game in town anymore. Most of the money is being spent on programming like ER which cost's "too much money per episode". As far as technical quality, if the masses are not annoyed by a picture that has been compressed down to nothing, why not do it? A cable network operates on a budget that is a fraction of what NBC spends and as far as the viewer is concerned the picture looks the same. The bottom line is there isn't lots of money to be spent anymore. A reasonable profit would be acceptable, but the other networks are losing money and we are not far behind. As far as NBC being sold, Bob Wright himself said that he wouldn't know how to go about selling just the network and then who would buy it? A merger is more likely. Television networks will never be what they once were there is just too much to do out there technologically these days and they had better find their new niche fast or face extinction.
Anonymous
NBC, New York
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(Note: This is a good simplified piece that can help engineers explain to non-engineering types what’s happening. There’s also a lot of engineers who could stand the refresher.)
Subj: What is HDTV and what does it look like?
By: Jim Mendrala
We all know that High Definition Television (HDTV) is television with a lot of detail and an image in a wide aspect ratio. Let's break this down.
1) What the camera is required to do.
2) What the broadcaster is supposed to do.
3) What the digital television (DTV) set does.
The HDTV Camera
The camera's lens focuses an image on to three CCD arrays. One for the red, one for the green and one for the blue. We all know this because that's the way it works in standard SDTV television. The major difference between a SDTV and a HDTV camera is that the aspect ratio in a SDTV is 4x3 (1.33:1) whereas in a HDTV camera it is 16x9 (1.78:1). Because of the desire to have photographic quality pictures, the HDTV camera has to have a lot more resolution than the SDTV. Lets compare.
SDTV
Number of scan lines: 525
Frame Rate: 30
Number of active scan lines: 480
Horizontal line time: 63.5 usec
Active pixels: 480 x 850
Resolution: 360 x 480
Bandwidth: 4.2 MHz
HDTV
Number of scan lines: 1125
Frame Rate: 30
Number of active scan lines: 1080
Horizontal line time: 29.6 usec
Active pixels: 1080 x 1920
Resolution: 720 x 1280
Bandwidth: 30 MHz
Frame rate in both cases is 30 fps. If we lower the frame rate then the same amount of spatial picture information will lower the bandwidth. If we increase the frame rate, we will need an increase in spatial bandwidth. For example, HDTV at 30 fps requires about 30 MHz of bandwidth. 24 fps however only requires 24 MHz of bandwidth to get the same picture quality per frame.
Most cameras scan the image in an interlaced mode, that is the even lines first and the odd lines second creating two fields of information. Since most cameras have no shutter the images come out as field one (the odd lines) and field two (the even lines). Any movement during each field is faithfully recorded with half the number of active scan lines. In the case of SDTV 240 x 850 with a resolution of 180 x 480 and for the HDTV 540 x 1920 and a resolution of 360 x 1280. The rows of the CCD sensors in the camera are usually to produce an image free from aliasing artifacts between fields. In the new 24p, 30p and 60p cameras, an optical low pass filter (O-LPF) rids the image of aliasing artifacts.
In the new 24p cameras, just like a Telecine, the image is progressively scanned one frame at a time in 1/24th of a second. Generally an electronic shutter with a exposure time of 1/48th of a second or less is needed so that the image doesn't look to soft from motion blur on moving objects. If the shutter time is too short than strobing of the image will become apparent like in animated cartoons.
Incidentally, film on a Telecine is always progressively scanned. The image was photographed on the film in typically 1/48th of a second 24 times a second. The shutter closes and for the next 1/48th of a second the film mechanism moves the next unexposed film frame into position and readies it for the next 1/48 second exposure. The image then can be progressively scanned and output as two fields. Combining these two fields in a frame buffer puts the progressive frame back together. This is known as 48sF or 48 sequential frames. This is what the new 24p recorders do internally. They record two fields derived from the progressively scanned image that was captured in 1/48th of a second.
DTV
We all know the cameras used in the digital domain have to convert the analog images on their sensors to a digital signal. This is done by an Analog to digital (A/D) converter. The output than is either a serial or parallel bit stream. This combined with audio and data is what the broadcaster transmits. The digitized pictures and audio are compressed and are put into digital packets. The packets also contain data and header information to tell the DTV receiver what type of signal it is, and how it might be handled. Display of these packets is left up to the DTV receiver. Panasonic's DTV receiver, for example will output any picture at 480p only, 1080i only or any of the formats in Table 3 of the ATSC.
In order to transmit all of this information within the 6 MHz channel the Moving Pictures Expert Group (MPEG) invented a compression scheme MPEG-2. It systematically allows the massive amount of information to be honed down to fit into the 6 MHz available to the broadcaster and is transmitted as an 8 vestigial sideband RF signal (which is an analog signal, by the way).
Because the information to be transmitted is digital it is converted into packets of video, audio, data etc. Therefore it doesn't matter what the frame rate or what the aspect ratio is.
In NTSC the image must be an analog signal, must have a 30 fps frame rate and must be 2:1 interlaced as that is what the NTSC receiver can only handle. (Because of the sound carrier being 4.5 MHz above the visual carrier. To eliminate moir+d+k-n the picture, the frame rate was lowered from 30 fps to 29.97 fps (frame rate x 1000/1001).)
In DTV, it doesn't matter what the frame rate is or what the numbers of scan lines are as long as you can compress the information into the 6 MHz channel. Since SDTV has a much narrower bandwidth requirement, several SDTV channels can be transmitted on one DTV channel. The limit to how many is determined by the amount of degradation to each sub-channel that is acceptable. An HDTV signal has a much higher bandwidth requirement so only one HDTV or possibly one HDTV and one SDTV channel can be transmitted on one channel at the same time.
The DTV receiver
In the DTV receiver the incoming packets of information are received and based on how the packets are packaged, the DTV receiver can then display that image on its native display at whatever frame rate works. The native display could be, for example, a Panasonic 480p DTV set and converts everything to 480p at 60 fps progressive or with 48 fields on an interlaced native display. A Sony HDTV Trinitron DTV receiver would display everything at 1080i at 30 fps with 60 field per second.
To do this with 24-fps material, the receiver would convert to 30 or 60 by adding the 2:3 pull-down sequence to the video information or in other words display each frame 3 times at 72 fps. This is great news for we no longer are tied to the display device, specific frame rates or aspect ratios.
Remember the 16 x 9 (1.78:1) aspect ratio results in the smallest amount of “letter-boxing” for all active aspect ratios from 1.33 (4 x 3) on up to 2.35:1 (scope).
Unfortunately, the broadcaster still wants to transmit the same images to his NTSC transmitter. The NTSC transmitter can only accommodate a 30 fps/60 fields 1.33:1 aspect ratio and without any additional compression a maximum of 4.2 MHz of picture information. Interlacing of the image and the reduced bandwidths of the chroma channels are two forms of compression.
What Does HDTV look like?
So far, all of the DTV receivers of today are deficient in the resolution domain. Yes most have 1080 scan lines and a greater than 20 MHz video bandwidth but there is more to the resolution issue. The most critical is the electron beam's spot diameter. These are the beams that are magnetically focused, deflected and are used to excite the three phosphors, red, green and blue.
Another issue is dot pitch or the aperture grill. In an 800 x 600 14-inch diagonal computer display the dot pitch has to be less than .02 inches or .51 mm. Now I'm referring to the native display of the CRT tube not the number of scan lines that are on it. The number of scan lines for this display has to be at least 600 to give it about 400 lines of resolution.
If you are looking at a 34-inch diagonal HDTV DTV set then that screen is going to be approximately 17 x 30 inches. If you display the scan lines so that they just touch each other, the spot diameter would have to be 1/1000th of 17 inches or 0.03 inches. Most 35-inch CRTs, with a 1.33 aspect ratio, today have a spot diameter of about 0.04 inches. This yields a maximum resolution of just about 350 lines. That's very different from the 720 lines of resolution with 1080 scan lines. The picture then tends to look soft compared to a 480-scan line image with about the same resolution. Overlap of the scan lines reduces resolution in the vertical. Bigger spot diameter does the same thing in the horizontal, reduces resolution and smoothes over detail. Remember the spot has a bell shape to it. It is not a hard edge spot. It is brighter in the middle than at its edges. With electron beam overlap, the display brightness tends to increase up to a point. With space between the scan lines, the scan lines themselves become visible and the picture looks apparently sharper.
The eye can see more detail in the horizontal then in the vertical. To prove this to yourself look at you're present TV from a distance where the scan lines just blend in. Now tilt your head 90 degrees and you will again see the scan lines. This is because of our stereoscopic vision. We need to detect the vertical edges (horizontal scan) for the brain to give us the sense of depth perception. The eye is constantly vibrating back and forth a minute amount to help distinguish the vertical edges of what's in front of us.
When some of the new display technologies start showing up the displays will take on a much better look. Texas Instrument's (TI) Digital Micromirror Device (DMD), as an example, with 600 x 800 pixels would give a resolution of about 400 x 500. If TI ever came out with a DMD of 1080 x 1920 pixels the resolution of that display device would be about 720 x 1280. As a point of reference that's about 40% of what a 35 mm academy aperture film resolves.
When looking at the various HDTV sets, keep these things in mind. The salesman will tell you the marketing propaganda with little technical specs. As you can see line doubling or quadrupling yes makes a smoother (soft) picture at the sacrifice of resolution unless the electron beam is smaller and doesn't overlap. If interlaced then to eliminate interline twitter the repetition rate must be at least 60 or better especially if the display is very bright.
Remember too that "You ain't seen nothing yet" when watching a HDTV DTV broadcast. Display devices of today won't let you see all the detail. When display technology improves the screen size in the home most likely will grow to 4.5 ft. x 8 ft. That's not only a big screen by today's standards but also a wide screen. You will be able to view the screen from the same distance you now view your 27-inch TV set.
If HDTV takes off then reduced resolution SDTV pictures will only be okay on the smaller screens with 34 inch or less diagonal.
I hope this helps in understanding where HDTV is today and where it will go in the future. I'd like to hear some comments and/or questions regarding the three areas discussed.
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Subj: Television and the NAB
By: Larry Bloomfield
The annual gathering of the broadcast clan, under the banner of the National Association of Broadcasters, is just around the corner. Broadcasters, vendors and an interesting assortment of other folks will descend on what has become the de facto point of confabulation for the broadcast industry, Las Vegas, Nevada. Television, the largest part of this whoopty doo, hasn't always been a part of NAB.
NAB was formed as a group for radio broadcasters in early 1922. It's kind of interesting to compare the attitudes of the "then broadcast industry" about this new "upstart" television to how many now feel about the onset of digital television today. Despite the fact that many of television's early engineers came from radio, as television emerged from the laboratories, it was a curiosity. Despite this, the NAB was dominated by AM radio interests who weren't happy about TV's encroachment.
Back in early 1940, Fred Wolcott, Technical Director of Gilfillan Brothers and Irwin Stanton of RCA discussed the desirability and need for an organization that would further the interest of television both locally and nationally. As early as the 40s there were enough people actively working in television to make it a reality. Harry Lubcke, Director of Television for the Don Lee Broadcasting System since 1931, and recognized for his pioneering efforts even in those early days, was contacted by Wolcott and Stanton to assist them in laying the ground work for the establishment of the Society of Television Engineers (STE).
STE, which first met in July 1940, had a charter membership that reads like a "Who's Who" out of a television-engineering history book. Their stated aims and purposes, which have remained unchanged through the years, are: "To advance the theory and practice and to enlarge the use of television and to promote harmony and cooperation within the television industry."
The STE played a very major roll during its early days in proposing the NTSC standards. Remember this was way before Color. STE enlisted the support of the industry and as the result of their efforts, the NTSC standards became effective on April 30, 1941. It was during this effort that they concluded that the television industry needed of a national organization, within which industry differences could be resolved so that a united and harmonious front could be presented to government agencies.In June 1943, Klaus Landsberg, Chief Engineer of Television Productions, Inc., a Paramount Pictures subsidiary, was asked to contact television and allied industries leaders, as a representative of STE to explore the possibilities for such a "national" organization. Because of the overwhelming positive response to Landsberg's efforts, the constitution for a new, national organization was unanimously adopted by STE at their August 1943 meeting forming under the name of The Television Broadcasters Association (TBA). The first organizational meeting of TBA was held at the Palmer House in Chicago, IL on Jan 17, 1944.
Content the formation of the TBA would carry on the original goals and objectives on a national level, STE withdrew from involvement.
Despite the many pressing wartime activities of the STE membership, those early members took the time to continue holding monthly meetings throughout the war years, in the interests of television.
The time for television to be a part of the NAB was still down the road. The "Television Digest and FM Reports," in their April 30, 1949 issue said that the TBA is "a small but cohesive alliance of telecasters and major TV manufacturers decided ….. to go it alone." But time changed attitudes. Within two years, in a mailing entitled "News from NAB" it was reported from Chicago, IL that, "A new autonomous television organization to operate within the structure of the 29-year old NAB was formed here today." A few days later Broadcasting-Telecasting reported the "formation of a completely autonomous NAB-TV, operating under the aegis of NAB, but to act as its own court of last resort, was unanimously voted."
Paul Raibourn of KTLA was the chairman of TAB's committee to study the feasibility of a single trade association under NAB auspices. The voting took place only after various segments of the TV industry were assured that autonomy could only be achieved under NAB if each segment was its own court of last resort. Raibourn was quoted as saying, "that having achieved the autonomy it desired, TAB will in the near future wind up its affairs. Not even two weeks later "Television Digest" reported that the NAB would become the NARTB in April 1951. It further reports that the Television Broadcasters Association (TAB) will be dissolved and its manufacturer members urged to join NARTB as associate members." On Jan 1, 1958, the NARTB changed its name back to NAB completing the circle.
If you're ever in Los Angeles on the 3rd Thursday of the Month, except during July & August, the STE still meets. Just ask most any Television Chief Engineer in the LA area and he'll tell you the time and place. There's still interesting folks and a good program and tell 'em, Larry sent you.
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A report on the SMPTE - Hollywood Section Meeting
By Jim Mendrala
On March 17th, St. Patrick's Day, a lot of people in the TV and film world showed up at Gene Autrie's Western Heritage Museum. The meeting itself was a panel discussion on "The Digital Transition - Report from the Trenches. Among the speakers were Bill Bouroghs, Director of
Engineering, KCET, Ed Gordon, Director of Engineering, E! Entertainment Television, George Hamilton, Manager, NBC High Definition Studios, James A. Smith, Director of Engineering, KMEX and Dan Sullivan, Manager of CBS Video Tape Technical Operations. Horace Scott, VP of Engineering Operations, KCET was the moderator.
Prior to the meeting in the Mary Pickford Auditorium there was a demonstration of HDTV DTV. Panasonic supplied two DTV ready receivers, one a m/n CT 32XF55 480p only 30" DTV ready set and a 35" m/n DTM 3050 HDTV DTV ready set. Both were fed from a Panasonic DTV Receiver. Sony supplied one of their Wega Flat Screen 34" HDTV's and a DTV receiver to
feed it. In addition the HDTV signals were displayed on a computer monitor model CDM W900 in a 16 x 9 format. The signal was received off the air on an ordinary Radio Shack Antenna and split with a Winegard four way splitter.
A lot of people were impressed with the fact that the HDTV being broadcast in 16x9 format was also being shown simultaneously on the 4x3 480p only Panasonic DTV set.
KABC-DT would have transmitted some HDTV at 720p but they had some technical difficulties and were off the air at the time of the demo that was between 6:oo p.m. and 8:00 p.m. PST.
KCBS-DT was transmitting a locally produced HDTV travel log program on the sights and sounds in and around Los Angeles, CA, narrated by the local anchor person Anne Martin. The equipment used a Panasonic HD D-5 for playback with it's signal fed to a Utah Scientific HDTV switcher then to a Mitsubishi HDTV MPEG-2 encoder and uplinked to Mt. Wilson using a
Harris Twinstream Digital STL. From the digital receiver to the Harris transmitter and then to a Dialectric CP antenna.
KNBC-DT was transmitting upconverted regular programming from its regular NTSC feed. A Snell & Wilcox MDD2000 "Golden Gate" Decoder up converting to 1080i with a Snell & Wilcox HD5050 Up-Converter and encoding with a Tiernan THE1 Encoder at 19.4 Mbs the signal was linked to Mt. Wilson by a California Microwave "Twinstream" Digital STL and fed to a Comark "Advantage" Xmtr with a Dielectric CP antenna.
KTLA-DT used the Sony HDCAM playback of the "Rose Parade" using NDS encoding equipment and uplinked to the Comark IOT DTV transmitter using a STL by Microwave Associates.
In Horace Scott's opening statement he said "What you saw next door was nowhere near as good as its going to be when display devices are developed out to the 1920 x 1080 spec. and the beam sizes reduced to light a single pixel. These pictures will be much, much better than what we are looking at today."
Mr. Scott thanked Steve Bloom, KCBS and Frank Geraty, KTLA for the broadcasting of HDTV material during the SMPTE demonstration.
It was interesting listening from all the panel members. One of the panel members, James A. Smith, KMEX, said management wants KMEX-DT to go digital with HDTV and be the #1 TV station in the LA area. He elaborated on the problems up on Mt. Wilson and with the US Forrest
Service who want to clear up some of the clutter up there. KMEX-DT will erect a candelabra on Mt. Wilson and share it with other broadcasters. Other members of the panel had interesting "war stories" on their experiences with DTV.
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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 (408) 778-3412, (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, but usually not.
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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