Download Understanding DTV

Understanding Digital Television (DTV)
Tom Ohanian and Michael Phillips, Avid Technology
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The DTV story will continue to develop and change. Avid currently has the only
DNLE Editor where users are able to switch from 4:3 to 16:9 aspect ratios, thereby
allowing its use in editing some forms of DTV. As standards come to be realized and used,
Avid will continue to create solutions for this new era of television broadcast.
With the recent specifications released by the Grand Alliance at this year’s NAB, there
seems to be a lot of confusion as to what is HDTV vs. DTV etc. In order to understand
the situation we are in today, we need to get a little background on the future of television.
History
For many years, the Grand Alliance ( a term given to a consortium of broadcasters
and manufacturers (whose charter was to create a standard) struggled with the task of
bringing forth a standard for HDTV (High Definition Television). In 1995, the American
Television Standards Committee produced a table with guidelines for a strategy of
providing DTV on a scaleable basis. The Advanced Television Systems Committee
(ATSC) was formed to establish voluntary technical standards for advanced television
systems, including digital high definition television (HDTV). The ATSC is supported by its
members who are subject to certain qualification requirements. For more information on
this committee and how to become a member, readers should point their browser to
www.atsc.org.
In 1997, the Federal Communications Committee (FCC) decreed that it would “loan a
digital channel to each of the 1600 Television stations in the United States for the purpose
of creating and distributing program content using one of the HDTV standard proposals.
Instead of charging the television stations for the use of the digital channel, it is being
provided free to jump start the demand and delivery of the next generation of television
broadcast.
There tend to be some misconceptions about exactly what DTV is and is not. DTV is
merely “digital television” meaning that the signal being delivered to the home is a digital
signal and not analog as today’s cable and some satellite services. 16x9 often confused as
being synonymous with DTV, is not DTV; 16x9 refers to an aspect ratio (relationship
between width and height of the a visual image). Television has been a 4x3 aspect ratio
since the beginning. It merely imitated the 35mm film frame at the time of which the aspect
ration of the full aperture 35mm frame is 4x3. 4 is the number of units across by 3, the
number of units high. When television started becoming popular, studios feared that
audiences would no longer go the movies so they wanted to create a difference between the
film going experience and television. One of the methods was to become widescreen.
Today’s definition of widescreen is anything that is wider that 4x3. Film aspect ratios are
described as the multiple of the horizontal in relation to the height. For example, 4x3 is a
1.33 aspect ratio in film (4 divided by 3 is 1.33) This was for any given measure of the
vertical, multiply that by 1.33 and you will have the horizontal measurement for that aspect
ratio. Movie theaters started masking the gate in the projector on the top and bottom to
make the image appear as widescreen. The actual print still contains the full 1.33 image.
The American projection standard is 1.85 and the European projection standard is 1.66.
16x9 is 1.78.
There have been many widescreen methods invented over the years. One of them was
Techniscope which was to divide the current 1.33 frame in half. Instead of having an image
that was 4 perfs high (standard 1.33 has 4 perfs), the cameras were modified to shoot an
image in only 2 perfs. This not only made the picture wider, but gave the filmmaker twice
as much film for the same camera roll.
Another method was to actually change the image through lens technology to squeeze a
wider image into the 1.33 image area. The projector would then have a special lens that
would reverse this effect during projection thereby un-squeezing the image to look normal.
Many of you may have seen “The Good, the Bad, the Ugly” on late night TV, Clint
Eastwood looks even taller and thinner than he already is. This is because the transfer was
not corrected and they are showing the image as it appears on the film. This methodology is
exactly what is going on behind 16x9 aspect ratios and 16x9 cameras.
The image on the tape being shot through a 16x9 video camera is still being stored as a 4x3
pixels on the tape. This is exactly the widescreen lens method used in film. It is a trick of
the lens and the playback monitor that will squeeze and un-squeeze the pixels to look
normal to the viewer.
1.33
1.66
1.66
1.77
1.77
1.85
1.85
2.35
2.35
original film aspect ratio
television aspect ratio
graphic from “Digital Filmmaking, The Changing Art and Craft of Making Motion Pictures” by Tom
Ohanian and Michael Phillips
As the chart above shows, differing aspect rations need to either be cropped or reduced to
fit into the standard 4x3 (1.33) aspect ratio of today’s television broadcast standard.
DTV: Digital Television
HDTV: High Definition Television (could be any one of the 18 accepted formats) see the
standards page on www.atsc.org for full specifications.
SDTV: Standard Definition Television, (digital delivery of today’s broadcast programs)
16:9 Widescreen TV Aspect Ratio for DTV and ATV (advanced television)
4:3
Older, standard aspect ratio
If we examine these terms, we can see that the term Digital Television is a generic
term, and that both HDTV and SDTV are categories of service that fall within the DTV
realm. Those of you who wish to know more about the DTV standards and what is in store
for future formats can point their browser to: http://www.pcube.com/
Now that you are familiar with both the history of formats and aspect ratios,
Aspect Ratios
The 16:9 aspect ratio obviously provides for a greater degree of width for any given
image. Shown below are (representations; they are not precisely shown below) of different
aspect ratios.
16x9 original image played as 16x9
16x9 image squeezed into the 4x3 image area
16 x 9 cropped to fit the 4 x3 image area
Images from “Digital Filmmaking, The Changing Art and Craft of Making Motion Pictures” by Tom
Ohanian and Michael Phillips
Scanning Methods
In the NTSC standard a video frame consists of two fields which are combined, or
interlaced together. The odd lines are scanned to create field 1, the even lines are scanned to
create field 2, and both fields are combined to form a single frame. In television, there are
60 fields, or 30 frames, per second (fps). This process of scanning is known as interlaced
scanning.
Film, as well as computers, do not incorporate interlaced scanning methods.
Rather, the image is drawn progressively from the top to the bottom of the screen. (is this
also true for film as well as computers? Need to clarify.) When complete, the entire frame,
or image, is drawn. This is known as Progressive scanning. Typically, it requires twice
the storage of interlaced scanning.
Square vs. Non-Square Pixels (Pixel Aspect Spacing)
Computers create images which are based upon the spacing between the pixels
(aspect ratio of pixels) being square. However, video does not incorporate square pixel
spacing. The formula for determining pixel aspect for NTSC and PAL is shown below:
NTSC Video: 486 (active vertical lines) ÷ 711 (active pixels) x 4/3 (aspect ratio) = 0.91
PAL Video:
576 (active vertical lines) ÷ 702 (active pixels) x 4/3 (aspect ratio) = 1.094
Avid’s DTV Post-Production Capabilities: Today
Digital recordings done in 16:9 (for example, those on Digital Betacam) can be
loaded directly into the Avid Media Composer. The digitization process can be done in
either analog component or composite form (via ABVB) or, to keep the signal in the digital
format using the Serial Digital Top card.
There is often some confusion as to what the relationship is between an image shot
in 16:9 recording and the 4:3 aspect ratio on videotape and on the Media Composer. This
will happen every time to digitize footage shot in 16x9 into a Media Composer. For
instance, why would something shot on 16:9 be transferred to videotape and then imported
into MC as opposed to importing the 16:9 image directly into the MC, especially if it can
me done this way? To clarify, let’s take a closer look at the digitizing process.
A.
B.
An original image is shot in 16:9
The image is recorded onto videotape, but the 4:3 aspect
ratio results in an image that looks “squeezed”.
C.
D.
16:9
The image is digitized from videotape into the Media
Composer.
Now, within the Media Composer, a 16:9 interface
change is selected, and the Media Composer’s
hardware “un-squeezes” the image we saw in
Example C. to provide us with the original
aspect.
With the original 16 x 9 aspect ratio restored, all editing can proceed in the 16:9 aspect
ratio, thus preserving the essential details which framing in 16:9 facilitates. Note that there
is a simple software toggle switch in the Composer interface which allows the user to easily
switch between 4:3 and 16:9.
All effects currently available on the Media Composer (with the exception of circle and
diamond wipes and titles) on the current MC 8000 (version 6.5.1) can be accessed from
within the 16:9 interface. Entering Effects Mode while in the 16x9 interface requires
version 6.5.2 of the software and a hi-rez monitor.
Once editing is completed, output can be made to Digital Betacam, again in 4x3, and also
with a direct digital path and the monitor will display the image as 16x9 if the monitor is
16x9 capable.
Note that Ikegami’s Editcam/W (Widescreen version) Fieldpaks can be mounted
directly onto a 16:9 Media Composer. Editcam/W is available in both NTSC and PAL.
As you can see, Avid is providing digital editing solutions for all types of video, film and
audio programs. Avi d will continue toprovide solutions as the standards of television
continue to change over the next few years and a clear definition of HDTV is adopted by
the broadcasters.