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 Some of the information may be out of date.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
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December 07, 1998
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DTV Tech Note - 023
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Some DTV notes from Larry's desk.
NDS has just announced that they have digitally compressed four NTSC 480i
signals into a standard ATSC 19.39 Mbps channel. In a separate test, NDS also
successfully compressed one ATSC 1080i and one NTSC 480i signal into a signal
19.39 Mbps channel. In both instances they overlaid these tests with the
basic Program Specific Information Protocol as spelled out in the ATSC specs.
See more in Broadcast Engineering on this subject.
Another tasty bit of info. EchoStar got a really good deal. 2 more birds by
Loral to go up in 1999 on News Corporations dime, the A-Sky-B facility in
Gilbert, Ariz., a leg up on half a million set top IRDs, a three year deal to
rebroadcast FOX net's O & O local-into-local plus the Fox News Channel on the
Dish Network and an EchoStar/MCI telephone bundle News deal. More to come.
*******************************************
A DISCUSSION OF 24 FRAME AND THE NEW 48s FRAME FORMAT
By: Jim Mendrala
The new ATSC standard permits the transmission of basically 4 video
formats. 2 SDTV formats (4x3 and 16x9) and 2 HDTV formats (1080x1920 and
760x1280), along with their respective variations of field per second
(Fps) and/or frames per second (fps). This seems to be creating an
impossible scenario for the production and post production industry. In
the past, the production and post production industries have delivered
their products in either NTSC, PAL, or component at either 30i fps or
25i fps. With the introduction of DTV, the networks and independent
television groups are specifying different formats. Some want 1080x1920
at 30i fps, some want 720x1280 at 60p fps, and others want 480x640 at
30i fps, and so on. This is creating an impossible financial and
operational burden on the production and post production industries.
A solution to the problem has surfaced and a 24p fps mastering format in
1080x1920 has been proposed to simplify the process of converting to the
various standards which the networks and broadcasters worldwide will
want and need in as short a time as is possible. It doesn't make sense
to transfer a film, for example, in HDTV then change the telecine to
output NTSC, then change the telecine again to output PAL, etc. Since
the new HDTV and data scan Telecines are very expensive, a way to bring
everything down to a simpler format is required.
So on December 3, 1998, a SMPTE meeting was held at Sony Studios'
Tri-Star building to discuss what SMPTE can do to put some sense and
order into the matter.
In addition to the 24p fps recommendation, format conversions and
display devices were also discussed. The proposed documents introduced
at the meeting were finished in the shortest possible time ever for a
SMPTE project and feedback from the production and post production
communities is drastically needed. SMPTE would like to have several
recommended practice documents in place by April of 1999, in time for
NAB (National Association of Broadcasters), said Engineering Vice
President William C. Miller.
Equipment manufacturers say they will be delivering 24p type of
equipment after NAB '99, so producers will be able to start turning out
high end product by the start of the fall 1999 season.
What has been proposed is (1) a straight forward 24p fps video standard,
and (2) a 24p fps 48sF standard referred to as 48sF. The second idea has
come about because of the need for a shortened time that some
manufacturers think they need in order to have product in time for the
fall 1999 season. Since a lot of equipment is now dealing with interlace
or fields, a modification to convert the operation to a slower field
rate would be easy to do. But the desire is to have progressively
scanned images. Progressively scanned images when run through an MPEG
encoder will cut the bit rate down anywhere from 25 to 35%. With
adjacent picture elements, motion vectors are easier to encode, and
since the frame rate is lower, there is more time per frame to do the
computations.
Since Telecines today all progressively scan film at 24 fps, the image
can be output as 1080x1920 at 24p fps. Recorders to do this, while
possible, have not been "invented" yet and will not be ready for the
fall 1999 season.
So to make the 24p more compatible with today's interlaced equipment.
the second recommendation has been put forward, namely 48sF. In 48sF,
the 24p fps image would be converted to two segmented images, with the
first segmented image composed of all the even lines of the
progressively scanned frame, and the second segment all the odd lines of
that same frame. The images would be captured at 24p fps but would be
converted to 48 segmented frames.
The argument presented by two of the equipment manufacturers is that 24p
equipment does not exist yet so why not use 48sF to be able to get
product out ASAP, with minor or slight modifications to existing
equipment. So it is hoped that SMPTE can provide the common grounds and
interface of ideas to come up with a working solution.
The following is how a typical telecine transfer of a one hour
film-to-video is handled in the Hollywood area.
In a typical telecine suite, an average one hour movie requires
approximately 40 hours of actual telecine machine time. Due to reel
changes and color correction considerations, a week is usually dedicated
entirely to the production of just one Telecine Transfer Color Master.
During the second week, with more film handling required for the many
and varied broadcast formats, Pan Scan, letterbox, anamorphic or
squeeze, NTSC, PAL in 4x3 and 16x9, and HDTV in 16x9, are done for
worldwide distribution.
All NTSC, PAL and HDTV distribution masters made from that Telecine
Transfer Color Master take anywhere up to 3 to 5 times the length of the
source to process.
In order to simplify the process and make it more economical in the post
production area, a simpler way has to be found.
24p fps seems to be the format of choice. It is progressive. It is at
the same frame rate as the film. It compresses more efficiently than
other formats. Up to five SDTV channels can be transmitted at the same
time in the DTV 6 MHz channel with plenty of room for other data
services.
24p fps can easily be up converted to a higher frame rate such as 72p,
60p or 30p. Interlace can be introduced as well as 3:2 pull down. Time
code is the same as the film and is non-drop frame.
24p fps can also be played back at 25 fps for those countries that
require it.
24p fps can become the universal standard of choice for all
film-originated video.
But there are some problems that rear their ugly head. For example: How
do we monitor 24p fps. If you've ever watched a movie at 24 fps with a
single=bladed shutter, the flicker would drive you nuts. 25p fps is no
better. Then how do you use 24p fps?
In the film world, the use of a two-bladed shutter to reduce the flicker
was adopted about a hundred years ago and it tends to work. All movies
today use this technique. Viewing a movie at 24 fps with a three-bladed
shutter is better but more light is required. The flicker at 72 times a
second is imperceptible.
To view a 24p fps video, the display device will display the frame in
1/24 of a second and be updated in a very small amount of time to the
next frame. This is how some of the new micro-display products, used in
some projectors, and the flat panel plasma displays work. The problem in
a CRT display though is that the phosphor doesn't stay lit and has a
decay time of x number of milliseconds. The phosphor needs to be
refreshed continuously. For a 24p fps video to have only a small amount
of flicker, the idea is to display the frame twice or three times for
each frame. In other words, the monitor would be reproducing the 24p fps
at either 48 or 72 times a second. Each CRT monitor then would require a
frame buffer that would be filled in 1/24 second but read out then
either two or three times during that 1/24 of a second. The price of the
monitor would probably be a little more expensive initially because of
the added frame buffer. To make the monitor work at other standards
would be more costly and would still only show the same image, no matter
what frame rate, interlaced or progressive.
Since in the DTV standard the display is handled in the receiver, it is
no problem in the home. In the production and post production areas, a
need to view the images before they are compressed and transmitted is
critical. Engineering and creative judgments have to be made to evaluate
these images.
A few years ago people who use surveillance cameras to record images of
bank robberies and holdups asked for a way of improving the clarity of
images still-framed on playback. A few cameras, developed by Sony and
others, captured the images with a 30p fps camera that output the image
as a 60sF image. In those countries that are at 25 fps, the camera
output 50sF from the 25p fps images. These images were easily recorded
on conventional equipment and worked quite well. As a matter of note,
all film images scanned today, on most Telecines, are scanned
progressively. The resulting image is read out as two fields. Those two
fields in reality are two segmented frames, frame one containing all
even lines and frame two containing all odd lines. 3:2 is added for the
conversion to 30 fps as there has been no requirement for 24 fps. In 25
fps countries, the image is scanned progressively and read out as two
segmented frames also. There is no need for a 3:2 conversion.
So the decisions that have to be made are: Do we come up with another
video standard--24p 48sF, or do we stay at 24p (25p) only and convert at
the display device (if it is a CRT) using 48 Hz (or 50 Hz) with flicker,
or a flicker-less display at 72 Hz (or 75 Hz) repetition rate.
Other display devices like the flat panel plasma display, Texas
Instrument's DMD (Digital Micro mirror Device), or the LCD and D-ILA
(Direct Image Light Amplifier) do not require any conversion from the
24p fps, as their images are updated almost instantly. There is no
fading of the image with time. The image is there until each pixel is
updated, unlike the decaying phosphor image.
Another matter discussed was the need for a standard format conversion
from 24p or 48sF to those frame rates that are not simple multiples of
the 24 or 25 frame rate. An example is 24 fps to 30 fps. In Hollywood,
3:2 can be a nightmare if the sequence is interrupted. Today there are
many DVDs out in the market place that do not have a continuous 3:2
sequence. Some have intermittent 3:2, 2:3, 3:2, etc. sequences, due to
careless video editing. True, one can be meticulous in keeping an eye on
the 3:2 sequence, but "Murphy's Law" says it will happen and it does. To
extract 3:2 for MPEG encoding, be it SDTV or HDTV, when the sequence is
out of order, will cause some artifacts.
In conclusion, I believe that 24p at 1080x1920 should be the mastering
format standard, and any conversion for processing and/or display be
done internally in those devices. Otherwise there will be a
proliferation of intelligent sequencers and de-seguencers needed to keep
every thing in order. 24p is the K.I.S.S. (keep it simple stupid)
approach without playing the numbers game and giving the production and
post production industries a standard that will work. Adding another
video standard to the 18 ATSC standard is not wanted or desired.
*****************************************************************
Film, Tape or DVD?
By Larry Bloomfield
Not too long ago I was giving a presentation to a network affiliate in one of
the top 10 markets on a system of managing multichannel television program
material. They, like many other broadcasters, are faced with the option of
being able to attract more advertising revenue by simultaneously offering more
widely diverse programming using multiple channels. The reasoning is that,
"yes, there is a finite and limited number of advertising dollars in any given
market, and if we can offer viewers most everything they want, they'll not
have to go else were, and the same applies to the advertisers and their
dollars." Although I don't think that they really believe they'll get all the
advertising dollars, it is reasonable to think that if you do offer more of
what viewers want, you'll get a greater percentage of that available revenue
too.
The engineers at the presentation seemed to be impressed with the ability and
ease with which they will be able to manage the various aspects of
multichannel television, but their main concern, and one that seems to be all
too common and growing exponentially with the approach of the "turn on date"
for DTV, is how they will do local origination, giving their viewers good
quality pictures, sound, etc. and not try to make a silk purse out of a sows
ear by bumping up from NTSC.
The reason I've not mentioned who these folks are is because they've asked me
not too. I find engineers are more open if they can trust you when sharing
their problems.
One solution offered by the engineers at this meeting to the lack of available
high quality video for digital TV is to bring back the telecine chains.
Remember: academy leaders, hairs in the gate, the broken splices, etc.? I
asked if they were serious and if the lack of HDTV programming on tape was
really that scarce and the response was, without hesitation, was-yes!
I recall a few years back trying to raise the money to build a telecine
facility for the purpose of doing film to "data" transfers. I knew that it
was only a matter of time when there would be a big demand for program
material in formats other than NTSC or PAL. I still believe that if movies
could be digitally put on tape, capturing the highest number of component
pixels reasonably possible, that they could be reassembled, during playback,
in almost any format the user might need, including HDTV. I knew that this
was being done, to a limited extent, with digital videodisk (DVD), but they
were only opting for quality slightly greater than NTSC. I have never figured
out why they didn't master with HDTV in mind. You can always get rid of bits
you don't need, but the picture will look like *=&% if you try to insert
something there that was there to begin with.
Another, less acceptable, solution for quality video for DTV is the DVD. I
know of several stations that are using DVD today, with its NTSC output for
various repetitive playback situations, such as promos, Ids etc. There's no
reason, if you can get the required clearances, why you couldn't play movies
to air from DVD. There are no tape dropouts (unless you recorded them there),
it's hard to get a crease in a DVD and I've never heard of one being erased by
a magnetic field.
Although not an immediate answer to the lack of quality video one other means
of program distribution was mentioned; getting program material delivered
directly on a server. Now wait a minute. Don't laugh. Think this out. Why
not deliver an HDTV movie, at a very slow bit rate, at any time of the day,
directly into a server, via low cost telephone lines. It may take 4 or 5
times as long to get the movie onto the hard drive as it does to play it back,
but who cares. All it takes is about 12 Gbts. total storage capacity for a
2-hour movie. This approach does have some merit and it is a step toward a
tapeless station.
***************************
Two on One
By Larry Bloomfield
I don't know how it is where you're at, but one of the loudest screams you can
hear in our industry today, followed by, "You want to do what?" comes from the
local frequency coordinator when you ask for a new microwave path.
With some of the 2 GHz channels going away and the fact that those remaining
don't handle DTV, many stations are having to look at other equipment,
frequency ranges and even the possibility of abandoning their STL in favor of
replacing it with fiber to get their signals to the transmitter site. I did a
story not too long ago about a company who had developed a technique of
converting the analog NTSC signal to digital and feeding it and the ATSC
digital signal on the same digital STL link. I wondered, at that time, if
anyone would come up with a way to combine the two services on the same path.
Microwave Radio Communications (MRC) has come up with their TwinStream radio
system which they say is the first dual carrier radio system for the
transmission of uncompressed legacy NTSC plus ATSC signals in a single 25 MHz
RF channel. MRC has applied for a patent for what they call the "Gemini IF"
approach to combining analog and digital signals as offset "intermediate
frequencies" (IF) in microwave system design. It sounds like this is one
possible solution for the studio-to-transmitter link (STLs), transmitter-to-
studio link (TSLs), and satellite backhaul requirement issues.
MRC's twin stream system permits DTV studios and transmitters to coexist with
the legacy analog infrastructure. Since the FCC has not given any additional
RF spectrum up to accommodate the new DTV requirements, finding an STL
solution has been a challenge. The twin stream approach will eliminate the
additional cost of digitizing and compressing NTSC programming to transport
across an STL in concert with the DTV signal. One other issue is that this
technique avoids the latency problems that are inherent to all current video
compression techniques.
For additional information see MRC's web page at http://www.calmike.com
***************************************
Similar but different - Two Inputs, One Output
By Larry Bloomfield
There isn't a Chief Engineer or General Manager a live that isn't looking for
a way to find the extra money to make the transition to digital television and
then the bucks to keep it running. Any savings in equipment or operating
costs should be an answer to a prayer. If you are one of the 320 stations who
have been assigned your DTV channel next to your NTSC channel, there may be a
cost savings answer from Acrodyne.
The idea of putting ten pounds in a five-pound bag has captured the
imagination of many. Well, may be it isn't quite as impossible as it may
seem. We've been transmitting two different signals, on two different
frequencies, all going out into the same antenna (in most cases) since the
early days of television; vestigial sideband (visual/picture) and frequency
modulation (aural/sound) and right next to each other frequency wise. Until
the early 1980's the combining of these two signals/transmitters was done at
their outputs and at relatively high power levels. Dr. Tim Hulick, Vice
President of Engineering at Acrodyne told me in a telephone interview, that
his company, Acrodyne, pioneered low level combining and subsequent
amplification of two different RF signals.
It stands to reason, with Acrodyne's experience in combining different types
of low level, RF signals; they would probably be looking into the NTSC/DTV
issue. Why not combine the output of an NTSC exciter through a hybrid with
the output of an adjacent channel DTV exciter? If that were possible and your
antenna was broadbanded enough, it should work. Well, Dr. Hulick told me,
it's not quite that simple.
Those of you who have kept up with all this DTV stuff know that the DTV
transmitter has to be ultra linear and there is a lot of electronic correction
"hand shaking" going on between the amplifiers and the exciter to make it all
compliant
. Considering the adjacent channel situation, there are two scenarios: one,
where the DTV channel is below (n-1) the NTSC and two, where the DTV channel
is above (n+1). In the first instance (n-1), albeit sufficient guard band is
present to design and build a channel combining filter system enabling the
user to pipe both signals to a common antenna. In the (n+1) case, the
solution isn't so simple. There is virtually no guard band between the
channels making the filter combiner solution impractical.
According to Dr. Hulick, the real answer to both situations (n-1 & n+1) may
lie in using a common high power amplifier for the NTSC and DTV signals. If
this could be successfully done, a combiner wouldn't be needed, but it
requires a very linear amplifier; one that is capable of meaningful peak
envelop power.
Since the tubes of choice today, for high power at UHF frequencies are the
tetrode, klystrode/IOT and Diacrode, the best of the types had to selected not
only for power bandwidth, but also linearity. The TH-680 Diacrode cavity
tuned for a one dB bandwidth of 14.6 MHz puts it in the ballpark. Tuned this
wide and flat, put it sufficiently far enough away from the channel edges so
that group delay would not be a problem. The Diacrode is the highest power
UHF amplifying device suitable for TV broadcasting. It is capable of at least
104 kW of unsaturated peak envelope power giving it a rating of 60 kW peak of
sync along with simultaneous provision of 6 kW of aural power.
Acrodyne certainly has made a strong argument for this type of transmitter
under these circumstances. They've developed a transmitter that will do all
this, both N-1 and n+1. They call it their Adjacent Channel Technology or
ACT, for short, transmitter.
Apparently John Long, VP of Engineering at the PBS station in the nation's
31st ranked market, Kansas City, MO, KCPT-TV believes it will do the job for
him. Acrodyne will install the transmitter at KCPT-TV, to be on the air by
November 1, 1998. KCPT presently broadcasts on channel 19 and has been
assigned channel 18 for KCPT-DT. The Diacrode equipped UHF ACT transmitter
will deliver 60 kW NTSC and 3 kW DTV to meet FCC ERP requirements. They're
doing all this in just 4 equipment cabinets.
Long is a pretty busy man these days with that kind of a deadline. In a
cellular telephone interview, he told me that he feels comfortable with, and
is committed to taking advantage of, the new technology: "We look forward to
switch-on November, 1st." Nothing was said about local origination or network
feeds. Does leave one to wonder.
******************************************************************************
**
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 (and yes Larry's e-mail is till the same) 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
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END
