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DVD Technical Notes
Video
Of course, the choice of film or video ultimately depends on
individual production budgets and artistic intent. And important
variations occur. A growing number of theatrical releases are
shot on video. And Sony’s latest HDCAM
®
high definition video
equipment can capture images at 24 frames per second
progressive scan – 24P.
We’ve seen that much of television broadcasting starts out as
movie film at 24 frames per second. This means not only
has it been converted from film to video, it’s been converted
from 24P to 60i. A machine called a telecine performs both
conversions. Simple arithmetic says that 60÷24 = 2.5. This
means that each film frame must convert to an average of
2.5 video fields. A process called 3-2 pulldown performs
this conversion. The first film frame is converted to three
video fields. The next film frame is converted to two video
fields. The next film frame is converted to three video
fields, and so on. We get a pattern of 3-2-3-2-3-2 etc, from
which 3-2 pulldown gets its name. This pattern averages out
to 2.5 video fields for every film frame. The telecine
converts a film frame to three video fields by repeating the
first field. For example, the first video field may consist of
odd scanning lines, the second field consists of even
scanning lines and the third field consists of the same odd
scanning lines as the first.
In order to fit a feature-length film onto a CD-sized disc, the
DVD format employs MPEG-2 digital compression. And
one important trick of this compression is to make an
important distinction between footage originally shot on
video and footage originally shot on film. As you would
expect, DVD stores video footage in its native 60i form.
But you might be surprised to learn that most DVDs shot
on film store the images at film’s native rate of 24 frames
per second!
Like material shot on video, the typical DVD shot on film is
encoded from 60i videotape. But in the DVD authoring
process, logic circuits in the majority of high-quality MPEG
encoders detect the telltale pattern of 3-2-3-2 in the incoming
video fields, the so-called 3-2 cadence. Since repeated fields
would waste precious disc space, the DVD eliminates them and
replaces them with First Field Repeat Flags (FFRFs) to tell
the player which fields to repeat. The remaining fields are
reassembled back into their original frames and encoded onto
the DVD in progressive scan at 525/24P. This system is 20%
more space-efficient than 60i. It’s an important advantage
because it enables DVDs to hold films that are 20% longer. Or
DVDs can encode each frame with a 20% more bits, for even
better picture quality.
The 24P encoding of film-originated DVDs means that 3-2
pulldown must be performed in the DVD player before the
picture can be displayed on a conventional television. The
exact pattern of 3-2 pulldown can have a subtle effect on the
rendering of motion. So it’s important that the DVD reproduce
the 3-2 pulldown cadence of the original master videotape.
That’s where the FFRFs come in. They identify each field to be
repeated as part of a “3.”
The 525/24P encoding of film-originated material has a special
property. In conventional 525/60i video, each “B” field repre-
sents a slice of time 1/60th second after the corresponding “A”
field. To the extent that objects in the frame are moving, the two
fields won’t match and aren’t well-suited for direct output in
progressive scan.
In contrast, 525/24P film-originated DVD is inherently progres-
sive and is perfectly suited to progressive scan display. Ironically,
today’s MPEG decoder chips automatically convert the 525/24P
progressive DVD into 525/60i interlaced video. There’s no way
to “tap into” the chips and extract the progressive signal.
Additional processing is required to convert the 525/60i interlaced
signal into a 525/60P progressive signal for output to a compa-
tible television. The required process is called 3-2 reverse
conversion. Because the process operates on a digital signal in
the digital domain, it can result in a super high-quality video
source that promises to be the ideal complement to high-end, big-
screen televisions with 525P inputs.
Unfortunately, not every DVD player with 525P outputs fully
delivers on the promise. Concerns such as flicker, motion
artifacts and 3-2 cadence glitches can visibly degrade the
viewing experience. As later sections will show, the Sony DVP-
S9000ES represents a thorough engineering solution — one that
realizes the full potential of progressive scanning.
Progressive scan 525P outputs have been promoted as a
must-have feature in high-end DVD players. But not all
progressive-scan outputs are created equal. Sony, a leader
in progressive scanning equipment for broadcasting and movie
production, understands the limitations of conventional
designs. And Sony engineers were determined to overcome
those limitations. The result is Sony’s exclusive Precision
Cinema Detection — the key to even higher performance in
525P reproduction.
Sony’s Fast and Pure Cinema Detection.
A thorough solution to the engineering challenges of
525P output, Sony’s Fast and Pure Cinema Detection
incorporates four significant advances:
Film-to-Video Transfer and 3-2 Pulldown
Film and Video on DVD
3-2 Reverse Conversion
Realizing the Potential of 525P