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  Some of the information may be out of date.
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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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      Sharing experiences, knowledge or anything else relating to DTV, HDTV,

etc., with your fellow engineers: That's what we are all about. We will send

this to anyone asking, just E-mail us. There is no charge for this Newsletter.

Welcome to all new subscribers.  If you are receiving this newsletter and want

to get off the list, just e-mail us that request. We hope everyone, new and

long time subscribers, will participate in all ways with comment, experiences,

questions and/or answers. This is YOUR forum!  And we're doing this on "a day

that will live in infamy"

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Again thanks to the many subscribers who have e-mailed and called to say:

"Welcome back."  Oh and, just in case, Happy Holidays.  Our mailing list has

become a bit untidy.  If you have missed any issues, please advise and we'll

take care of that.  Thanks.  We're over 200 now.

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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. 

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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.

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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. 

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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

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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. 

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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 (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

unaltered and credit is given to the DTV Tech Notes and the originating

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 END