Transforming film to video for DVD
As we've highlighted hardware technologies at DVDFILE, we've
touched upon progressive scan DVD players and line doublers
that convert interlaced video to progressive video. (You may
have read about the Faroudja line doubler installed in my
system when you read my Mr. Blandings piece; Faroudja has
the original patent for the reverse 3:2 pulldown process,
a general concept I'll soon describe.)Some home theater components
that produce progressive scan video have the commendable ability
to correct for most of the unavoidable visual distortions
caused by the 3:2 pulldown process. Or tech editor was so
impressed with the Toshiba progressive scan DVD player he
reviewed that it became his reference unit. So let's take
a look at the 3:2 pulldown process, discuss why it's necessary,
describe the artifacts, and consider how we might get rid
of its visual distortions.
Video
versus Film
You wouldn't have to read this article if film and video
weren't so different. It's due to those differences that the
3:2 pulldown process becomes necessary. So to begin, we should
examine the nature of video and film.
NTSC (National Television Standards Committee) Video is composed
of 525 horizontal scan lines. In our wonderful world of DVD,
480 of those scan lines are available to contain picture information.
NTSC video is interlaced. In other words, even though the
video is shown at 29.97 frames (pictures) per second, each
video frame contains two video fields. One field is composed
of all the odd horizontal scan lines; the other contains all
the even horizontal scan lines. So despite the reality that
NTSC video displays 29.97 frames or pictures each second,
it's actually created as 59.94 fields per second. Consequently,
any four sequential video frames (A, B, C, D) are drawn on
the video display as A1, A2, B1, B2, C1, C2, D1, D2, where
the 1 or 2 represents the field number within the frame. This
is what the vast majority of video displays - including, most
likely, yours - expects from any video signal source.
Conventional 35 mm and 70 mm film is shot at 24 frames per
second. On the motion picture screen, visible flicker is minimized
by projecting the film at 48 frames per second. To maintain
proper speed onscreen, the projector repeats each frame. So
any four film frames would be projected as A, A, B, B, C,
C, D, D. Since the frame rates of film and NTSC video are
quite different (24 film frames per second as opposed to 29.97
video frames per second), when we transfer film to video or
try to display film from video, we have a bit of a problem.
Simply transferring each film frame onto each video frame
would result in a film running about 24.9% faster than intended;
29.97 film frames would be shown during each second rather
than the correct 24. The clever solution to this problem is
to repeat film frames periodically in a very straightforward
but mathematically significant way; the resultant redundancy
prevents the apparent speedup of the film when shown at the
conventional video frame rate. This is how it's done.
Telecine
to NTSC Video
The telecine machine used to transfer film to video for composite
D2 masters (which may be used for VHS cassettes, laserdiscs,
and broadcast) projects film onto a video imager at 59.94
frames per second (identical to and synchronized with the
video field rate) and repeats film frames in a recurring 3:2
pattern. In other words, the film frame sequence is A, A,
A, B, B, C, C, C, D, D, and so on:
The
Telecine 3:2 Pulldown Process for NTSC Video
The first film frame, A, is repeated three times and is recorded
as field 1 and field 2 of the first video frame, and field
1 of the second video frame. The second film frame, B, is
repeated twice and is recorded as field 2 of the second video
frame and field 1 of the third video frame. The third film
frame, C, is repeated three times and is recorded as field
2 of the third video frame and fields 1 and 2 of the fourth
video frame. The fourth film frame, D, is repeated twice and
is recorded as field 1 and field 2 of the fifth video frame.
See the pattern?Repeat this sequence six times and 24 frames
of film become 30 frames of video.
MPEG-2
and DVD
So the basis of this technique is to restore proper timing
by generating redundant image information from four film frames
within every five NTSC video frames. But wouldn't it be silly
to waste 20% of the storage space on every DVD with duplicate
picture data?Fortunately the MPEG-2 standard nicely avoids
this inefficiency. When a film source is encoded for presentation
on DVD, it is stored at 24 frames per second; each video frame
contains all the picture information from each film frame.
There is no redundancy or duplication. Such a transfer is
written to DVD as 720-pixel wide by 480-pixel high interlaced
frames (where each frame contains two 720 by 240 fields),
and there are only 24 frames for each second of film. This
is known as 480i24. On each DVD encoded from a film source,
a flag is inserted within the MPEG-2 data stream that instructs
the player to repeat certain fields to reconstruct the 29.97
frame per second interlaced video. The player obliges by performing
the 3:2 pulldown in real-time, continually creating interlaced
frame sequences just like the one shown in the above figure,
"The Telecine 3:2 Pulldown Process for NTSC Video."This
capability enables the player to produce video compatible
with conventional displays that were designed based on the
NTSC video standard.(As we shall see later, progressive scan
DVD players take a different approach.)
The
Downside
While the 3:2 pulldown process restores the proper speed
of the film on video, it generates some unpleasant problems.
Two sequential video frames within every five video frame
sequence contain images from different film frames. If there
is movement of the images on film, 40% of the video frames
will contain visually distorted information. Let me steal
a figure from my Anamorphic
Widescreen piece to demonstrate.
Stationary
Camera Camera Panning
Left
The video frame on the left is fine. The circle on film was
quite still and so the odd and even scan lines paint a stable
video picture. Now let's pan left on film, causing an apparent
motion of the circle to the right. Notice that within the
video frame on the right - one of the two frames in the five
video frame sequence that contains fields from two different
film frames - the circle is in one position based on the odd
scan lines and in another position based on the even scan
lines. What a mess. Repeat this process 40% of the time and
the eye sees a loss of focus, a smearing of detail, for any
moving object. For those frames that contain a quick cut from
one scene to another, the image may become even odder:
Scene
Change
Here, the film editor has cut from our image of the black
circle to images of a green rectangle to the right and part
of a blue cone to the left. For a video frame that captures
images from these two different film frames, one before and
one after the scene change, all three objects appear on the
video display for the duration of the video frame. For that
brief snatch of time (33.37 msec), our vertical resolution
has been cut in half for that film frame.
These
are the two spatial artifacts caused by interlacing; I'll
touch upon a temporal artifact soon. Now, let's see if we can minimize these spatial
flaws as we watch our DVDs.
Line
Doublers
Let's
start with the solution that's been around the longest, reverse
the 3:2 pulldown as the video is converted within a line doubler
from interlaced video to progressive video:
Each progressive video frame is reconstructed by weaving
together the odd and even fields from images that were derived
from the same film frame. The video frames are then shown
at double the conventional NTSC video frame rate in a 3:2
repeating pattern. This effectively doubles the number of
horizontal scan lines during each second, hence the name of
the instrument that performs the work: a line doubler.(It
isn't clear whether C2 is derived from interlaced frame 3
or interlaced frame 4; I arbitrarily showed frame 4.)The technically
astute might notice that this scheme seems to require going
forward in time to reconstruct frame B. Actually, frame buffers
and double buffering techniques are used to overcome this
problem. This implies that there is a slight delay through
such complex video circuitry, but experience has shown that
synchronization with non-delayed audio is not an issue.
While lacking strict temporal consistency - every other film
image is on the screen for one and a half times longer than
the previous film image causing an apparent subtle jerking
or juddering during smooth scans - the spatial distortion
of interlacing fields from two different film frames is gone.
Since the reverse 3:2 pulldown is somewhat complex, requiring
some field analysis on the fly to get it right, it's not available
in all line doublers. You'll find this feature in the surprisingly
inexpensive DVDO line doubler and such high-end video processors
as those from Faroudja. Variations on this theme may be found
in some quadruplers, interpolators, and scalars, but that's
a topic for some other time.
Progressive
Scan DVD Players
As I mentioned earlier, film is stored on DVD as 480i at
the equivalent of 24 frames per second. When a conventional
player recognizes the appropriate MPEG-2 frame repeat flag,
it performs the 3:2 pulldown in real-time, but progressive
scan players can react to this flag in a different way. Such
a player can create progressive video in real-time .It reconstructs
each video frame by weaving together its odd and even fields,
then repeats the video frames in a recurring 3:2 pattern.
The resulting video signal will contain the same frame sequence
and the same horizontal and vertical scan rates as are produced
by the line doubler. This is a simpler process than is required
in a line doubler since the player does not have to examine
the fields to determine how to perform the weaving; no DVD
derived from film contains a video frame with images from
two film frames.
One potential advantage of performing this process within
the DVD player is that it's done entirely in the digital domain,
so no signal degradation occurs. An external line doubler
accepts a DVD's video signal in analog form, such as component
or S-video. The line doubler must digitize the video to bring
it into its digital processing circuitry. The line-doubled
digital video is then transformed to analog once again for
compatibility with the video display. With no less than an
analog buffer, an anti-aliasing filter, a sample-and-hold,
an analog-to-digital converter, a digital-to-analog converter,
another anti-aliasing filter, and another analog buffer involved
in the conversions from analog to digital to analog, there's
quit a bit of circuitry that can get in the way of a pristine
signal. Only the most expensive video processors, costing
thousands of dollars, will perform these tasks without visibly
degrading the video.
Please note that for a video display to properly present
such progressive video or line-doubled signal, it must be
capable of dealing with about 31,500 scan lines per second
- twice the normal rate. Interestingly, the vertical sync
rate remains the same as conventional NTSC video, 59.94 Hz.
The
Computer and a Possible Future
Because many computer displays are capable of broader ranges
of horizontal and vertical scan rates, it is possible to create
temporally symmetrical progressive video that runs at two
or three times the film's frame rate: 48 or, more commonly,
72 frames per second. To maintain proper timing, each frame
must be repeated two or three times, respectively, so the
sequence becomes A, A, B, B, C, C, D, D or A, A, A, B, B,
B, C, C, C, D, D, D. Each film image is shown on the video
display for precisely the same amount of time, creating our
temporal symmetry. So not only have we eliminated the spatial
distortions, juddering during smooth pans is now gone as well.
48 frames per second require a horizontal scan rate of 25,200
Hz and a vertical sync rate that extends down to 48 Hz. 72
frames per second require a horizontal scan rate of 37,800
Hz and a vertical sync rate that extends up to 72 Hz. Interestingly,
many front projectors are capable of these rates. I've received
e-mail from home theater enthusiasts who prefer to use their
computers as DVD players to take advantage of this flavor
of progressive scan on such projectors. I suspect that as
more capable video displays become readily available, we may
see standalone progressive DVD players that offer the 48 or
72 frame per second playback option.
Parting
Thoughts
It's quite remarkable how much the image quality can be improved
by eliminating 3:2 pulldown artifacts with an appropriate
reverse process while converting to progressive video. Throw
in a good anamorphic transfer with no edge enhancement and
the presentation is surprisingly film-like. Interested? You
can expect progressive scan DVD players to be introduced by
several manufacturers this year. Or you might want to investigate
a line doubler, perhaps the affordable DVDO. And as HDTV-ready
display prices come down, more and more of you will be able
to enjoy the best home theater currently has to offer.
