Technology Corner: Randy Hoffner
DTV Aspect Ratio Signaling
One result of DTV is that we now frequently find ourselves dealing
with multiple aspect ratios. A multiplicity of picture aspect ratios
has always existed in film and cinema, but decades of television
engineers were relatively well insulated from this reality. This
is no longer the case.
In the United States, television had one aspect ratio – 4:3
– and the FCC regulations on vertical and horizontal blanking
made it mandatory that the screen be filled with picture. When the
wide-angle theatrical formats were shown on television, they were
always panned and scanned so that the TV screen was filled. When
the blanking regulations were removed in the 1980’s, it became
legal to display pictures on television that did not fill the entire
screen, thus making it possible to broadcast letterboxed images.
Although wide-screen theatrical material has for some time been
routinely transmitted in letterboxed format on European television,
this practice has taken some time to become established in the United
States. It is still resisted by many U.S. viewers and broadcasters.
DUAL-ING ASPECT RATIOS
Thanks to DTV, we now have two native video aspect ratios. In
addition to the traditional 4:3, 16:9 has become a fact of life.
It is the native aspect ratio of all HDTV formats and it became
established as a standard-definition aspect ratio as well, particularly
in Europe. This, like many other things, complicates DTV.
Right off the bat, DTV displays – both HD and SD – come
in both aspect ratios, even though 16:9 is the native HDTV aspect
ratio worldwide.
Further, set-top boxes that are intended to display HD and SD
DTV images on NTSC TV sets must necessarily display on a 4:3 screen.
When a 16:9 DTV picture is displayed on a 4:3 display, three possibilities
present themselves.
If the picture fills the width of the screen, there will be black
areas above and below it.
If it fills the height of the screen, portions of the right and
left edges of the image will be cropped off.
The third option is to distort (squeeze) the image so that it
fits the screen.
The first two of these options result in the images being smaller
than optimum, and the third produces "Goyaesque" people.
Similarly, when a 4:3 image is displayed on a 16:9 screen –
unless the image is cropped vertically or stretched horizontally
– black areas will exist on the left and right sides of the
screen. A term that has been frequently used to describe this is
"pillarbox".
One of the display problems that HDTV broadcasting has spawned
occurs when 4:3 material is up-converted to HD. We know that this
is done frequently, as a substantial portion of the broadcast day
of many DTV stations consists of an up-conversion of the NTSC programming.
And up-converted commercials and other non-program material are
often integrated into HDTV programs. This typically produces a pillarboxed
image in a 16:9 field that, in turn, is letterboxed onto a 4:3 display.
The result: black bars all around the image with the viewers seeing
a "postage-stamp"-sized picture afloat in a sea of black.
RATIO-NALIZATION
It would be advantageous to have a mechanism by which to signal
a DTV receiver about aspect ratio information. For example, in cases
where 4:3 material is up-converted to HD – as described above
– if the receiver knew that this material was really 4:3, it
could display it as 4:3 so as to fill the screen rather than make
a postage stamp out of it.
There are, in fact, some standards in existence that do define
ways to transmit aspect ratio signaling data to the receiver. There
is a method described in EIA 608B whereby the aspect ratio data
is carried in the extended data services packets of Line 21, but
there has been no implementation of this capability on either the
transmitting or the receiving end, and no reason to believe that
there ever will be, as the aspect ratio problem is not a big one
in NTSC.
In the DTV world, the DVB standards that are used in Europe and
other places in the world have a standardized data structure called
"active format description," or AFD. The AFD data is carried
in the user data portion of the MPEG-2 video elementary stream.
AFD provides for a 4-bit field describing the active aspect ratio
format. The formats supported by AFD include some that we would
find useful in the United States.
The formats that we would find most beneficial include:
• Active format is the same as the coded frame. No letterboxing
or pillarboxing is used.
• 4:3 (centered). 4:3 video is pillarboxed inside a 16:9
frame.
• 16:9 (centered). 16:9 video is letterboxed inside a 4:3
frame.
• 16:9 with protected 4:3 center: This could be used to
tell the receiver how to extract and display a 4:3 image from a
16:9 transmission.
Other formats include:
• 14:9 (centered). 14:9 video pillarboxed inside a 16:9
frame or letterboxed inside a 4:3 frame.
• >16:9 box (centered).
• 16:9 box and 14:9 box at the top of the screen (all vertical
black area is below the active image).
• 4:3 with protected 14:9 center.
• 16:9 with protected 14:9 center.
Most of the above "other" formats are relevant to formats
and practices that are not found in the United States. An exception
is the "greater than 16:9 box".
Wide-screen cinematic formats such as Cinemascope are sometimes
transmitted letterboxed in a 16:9 frame in the United States, but
it is difficult to imagine what a receiver with either a 4:3 or
a 16:9 display would do with such an image other than treating it
as a 16:9 image.
The 14:9 aspect ratio, a compromise that produces "equal
pain" when boxed in 4:3 or 16:9, is a development of the BBC
that has not found favor here. The practice of pushing letterboxed
pictures to the top of the screen is not done in the United States
either.
The incorporation of the DVB active format description into the
ATSC standards is currently under study. If it is standardized and
implemented in DTV transmissions and receivers, it would facilitate
the optimal use of the display screen for any transmitted aspect
ratio.
Randy Hoffner is manager of technology and strategic planning
at ABC, New York, N.Y. The views expressed in his column are his
own, and not necessarily those of ABC. Write to him c/o TV Technology
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