Digital Video
and Field Order
This article was written in 2002, so some of the details may be
a little out of date.
There is a certain amount of confusion in the world of desktop
video editing over
“ Field Order”. Video newsgroups
and bulletin boards attract a steady flow of queries about problems
that people are experiencing
with field order settings in
their video projects.
Much of this confusion is caused by conflicting terminology -
“upper-field-first”,
“ field order A”,
“ field 1”,
“odd field ” - and so on.
Hardware and software manufacturers often give vague or ambiguous
advice on this subject, leaving many users with a feeling that they
just don’t know what is really going on, and why this should all be
so complicated.
So I decided to get to the bottom of this. This involved going back
to the standards for analogue video on which most digital video
formats are derived and finding out how all of this confusion has
arisen, and the correct definitions for many of these contradictory
terms.
The standards themselves make fairly dry reading, but there are
some interesting details there which help to explain the roots of
digital video. If we understand some of this then it all becomes a
lot clearer. So, I’ve extracted some of these details and presented
them in a series of simple diagrams which help to explain, for
example, why certain resolutions are used and the concept of the
notorious rectangular pixels
used in the video world.
The PAL television system runs at 25 frames a second. Each frame is
comprised of 625 horizontal lines, and is made up of two
interlaced fields separated in
time by 1/50th of a second. If you
number the lines from the top of the frame, the top line is 1 and
the bottom line is 625. The
first field contains the
odd-numbered lines and the
second field contains the
even-numbered lines. But you’ll probably know most of this already,
so I’ll move on to the more interesting stuff.
Although in desktop video we are dealing
with digital video, the
roots of digital video are firmly planted in the
original analogue video
standards. Most readers in the UK will be interested in PAL which
has 625 horizontal lines, but we will also be describing NTSC which
has 525 lines. In fact it’s more accurate to refer to 625-line and
525-line systems rather than “PAL” and “NTSC”, so that is what we
will do in most of this article. We will start by describing the
625-line system because it has fewer complications.
Analogue and Digital Active Areas
OK, so let’s start with analogue video. “Interlaced” video is a
continuous waveform comprising a stream
of fields which are “shot” at a
rate of 50 fields per second. But
each field only contains the
alternate lines in the original picture. The
first field “ field 1”
contains the odd numbered lines (assuming that you start counting
from 1 rather than 0), and the
second field “ field 2”
contains the even numbered lines 1/50th of a second later - we will
refer to these fields as “F1” and
“F2” for brevity.
The analog waveform of
a field carries a 'signature'
that effectively identifies it as an F1 or
F2 field . The signature occurs
as a pattern of sync pulses that differs in F1 and
F2 fields - this ensures that the
lines of eitherfield appear in the correct
position on a television screen. When these
two fields are interlaced you end
up with all of the lines in the picture making a complete
“Frame”.
Each field has several lines
which do not contain any picture information. These lines occur
during the Vertical Blanking Interval (VBI) when the electron beam
in the TV tube returns from the bottom to the top of the screen.
The VBI also covers several lines which carry data such as Closed
Captions and Teletext.
The lines which contain picture information comprise the “active”
area of each field . The ITU-R
BT.470-6 recommendation shows where the active picture area starts
and ends in both fields . This is
the analogue active
area.
ITU-R BT.601-5 and 656-4 describe a digital active area. This is
used when the analogue video signal is converted to a digital
format, and it does not exactly coincide with the analogue active
area.
The diagram below shows the analogue and digital active areas for a
625-line (PAL) system.
Diagram 1 - 625-line Analogue and Digital Active Areas
You can see how the
two fields (F1 and F2) are
interlaced together to make a frame.
The frame-based line
numbers are shown on the left in black - these range from 1 to 625
from the top to the bottom of a complete frame.
However, the ITU documents number the lines
in temporal (i.e.
chronological) order as they appear in the analogue waveform, so
the lines of field 1 are numbered
1,2,3,4 and so on up to 313, and the lines of the
subsequent field 2 are numbered
314,315,316 up to 625. The numbers in brackets show a different
line numbering system used in some standards
for field 2 which is 1 to
312.
The solid lines show the analogue active area. Note that it starts
and ends with a half-line. The dotted lines show the Vertical
Blanking Interval.
The digital active area is shown between the two grey Digital
Blanking areas. The digital active area starts with line 23
of field 1 and ends with line 623
of field 2 - that's 576 lines in
total. The first half of line 23 and the second half of line 623
are part of the (analogue) VBI and do not contain any picture data,
but they are part of the digital active area all the same.
Converting the analogue video waveform to digital involves sampling
the analogue waveform at regular intervals. ITU-R BT.601-5 states
that both 525-line and 625-line systems are sampled at
the same rate of 13.5 MHz
- that’s 13.5 million samples per second. This common sampling rate
is partly to keep equipment costs down.
Now, interlaced PAL has 25 frames per second, each with 625 lines.
So the number of samples on a single line must be:
13500000 / 25 / 625 = 864 samples per line
And for NTSC which has 29.97 frames per sec, each with 525
lines:
13500000 / 29.97 / 525 = 858 samples per line
However, each line in the analogue video signal contains an area
of horizontal
blanking before and after the picture data
which is used to keep the TV display synchronised. And we have
already explained that eachfield contains several
complete lines of blanking (the VBI) before and after the picture
data. The sampling process is a continuous one which covers the
whole video waveform including the horizontal and vertical blanking
regions, not just the actual picture area. So we can represent a
complete frame (of two
interlaced fields ) as an area of
samples 625 lines in height by 864 samples in width, and within
this is the window which contains the actual picture - the digital
active area.
The process of digitally capturing the analogue signal involves
storing only the samples which contain picture data - the samples
from the horizontal and vertical blanking regions are usually
ignored.
Diagram 2 - 625-line Digital Sampling Space at 13.5MHz
The ITU documents define the location of the Digital Active Area by
specifying its first and last line number and the first and last
sample number on the line. These numbers are shown on the diagram
above. The area is therefore 720 samples wide and 576 lines high.
This forms the image area which is stored in digital video files
which is of course the well-known
720x576 pixels used in DV,
DVD and other digital video formats.
OH is the
location of the line sync pulse which marks the start of an
analogue line. (For clarity on this diagram I have numbered the
samples starting at 1 from the position of OH; however the ITU start numbering samples
from 0 at the start of the Digital Active Area.)
Now, ITU-R BT.470-6 states that the active part of an analogue line
(i.e the part of the analogue line which contains the picture)
lasts for 52 microseconds. Using the 13.5 MHz sampling rate, this
means that the width of
the analogue active area
only covers 702 samples. But the digital active area is wider, at
720 samples. Why the difference? Well, the extra 9 pixels on either
side are to accommodate the growth and decay of the analogue
waveform so that there is no abrupt clipping which may cause
“ringing”. 720 also happens to be an exact multiple of 8 which
helps in MPEG and DV encoding - but we won’t go into that here.
Some analogue video capture cards only capture 704 samples per line
which is also an exact multiple of 8. However, many digital video
standards specify 720 samples/pixels width. So if you have some
video “footage” which is 704 pixels wide and you want to convert it
to a 720 pixel wide format then you really need to add 8 black
pixels to either side so that the correct proportions of objects in
the picture are accurately preserved.
Rectangular Pixels
The analogue active area represents the area of the picture which
can be displayed on a television screen. The aspect ratio of a TV
picture is 4:3, that’s 4 units wide and 3 units high (for the sake
of clarity we’ll leave out widescreen). We know that the picture
has 576 lines, so we might think that its width in
pixels should be
576 * 4 / 3 = 768 square pixels
but we know that the width of the analogue active area is just 702
samples (which are effectively pixels). Wouldn't 702 pixels make
the picture too narrow? If that is the question you are asking then
the problem is that you are thinking in terms of “square” pixels
which are used in the computer graphics world.
Square pixels take up the same amount of space horizontally and
vertically on a screen. For example, 1000
pixels across a computer
screen takes up the same space on the screen (measured in inches,
centimetres or any other spacial units) as 1000
pixels down the screen. A
500x500 pixel image will appear exactly square on a perfect
computer screen.
But in the world of digital video we
have rectangular pixels,
and the width of the pixel arises from the sampling rate. As we
said earlier, the length of the analogue active part of a line is
fixed at 52 microseconds, and if we sample it at 13500000 times a
second we end up with 702 samples covering the complete 52
microsecond time period. If instead we sampled at twice this rate,
i.e. 27000000 times a second we would end up with twice the number
of samples (1404) but they would be covering the same 52
microsecond period. We know that in 52 microseconds the electron
beam must cover a particular distance across the TV screen, so we
can see that there can be any number of samples across this
distance depending on the sampling rate we choose. A higher
sampling rate means more samples across the same screen width.
In digital video each sample is effectively a pixel, so the number
of pixels across the screen width can vary depending on the
sampling rate. So you can see that if we increase or decrease the
sampling rate we are effectively squeezing or stretching the pixels
horizontally so that in total they will occupy the same spacial
distance across the screen.
ITU-R BT.470-6 states that the sampling rate is 13.5MHz, so at this
rate the pixels turn out to be slightly wider than their height for
625-line systems. In fact the aspect ratio of this
rectangular pixel is
54:59. Unfortunately the convention for stating
the pixel aspect ratio
is height:width (or y/x)
which is the inverse of the convention for stating
the frame aspect ratio
(i.e. the aspect ratio of the picture) which is width:height (or
x/y).
These rectangular pixels are often referred to as “Rec 601” pixels
and they apply to D1, DV and DVD digital formats.
If we do a little maths on this we find that 576 lines comprising
702 of these rectangular pixels would make a picture with the frame
aspect ratio of:
702 * 59 / 54 / 576 = 1.332
which is approximately our ideal 4:3 (1.33333) frame aspect
ratio.
The diagram below compares the pixel aspect ratios of square
(computer screen) pixels and the Rec 601 rectangular pixels for
625-line systems. It is based on the width of the digital active
area which is 720 Rec 601 pixels wide. Note that 720x576 is
spacially wider than the 4:3 frame aspect ratio, and corresponds to
a width of 786.667 square pixels, i.e.:
720 * 59 / 54 = 786.667 square pixels
Diagram 3 - Pixel Aspect Ratios
The third type of pixels shown are for SVCD. SVCD stands for “Super
Video Compact Disc” and is an official extension of the CD standard
which can be used for storing interlaced digital video on standard
compact discs. At the moment this medium is quite popular among
video hobbyists but will probably become less so as the cost of
writing DVDs reduces. Compact discs are relatively small for the
use of digital video, so a much lower sampling rate is used. This
results in only 480 pixels covering the same spacial width as the
720 Rec 601 pixels. SVCD pixels are therefore much wider.
525-line (NTSC) Systems
Let’s now look at the 525-line system used in NTSC. The diagram
below shows the analogue and digital active lines for 525-line
systems:
Diagram 4 - 525-line Analogue and Digital Active Areas
OE2 is
the field sync pulse which marks
the start of field 2.
The ITU standards show the analogue active picture area starting
half way through line 282
of field 2 and ending half way
through line 263 of field F1.
However, over the years the FCC have allocated the first few of
these lines to non-picture data such as line 20 and 283 carrying
source identification data, and line 21 and 284 carrying closed
caption and program rating information. Consequently the analogue
active area is now specified by the FCC to start at line 22
of field 1, with all the
preceding lines being part of the Vertical Blanking Interval (VBI).
In addition, line 22 which is in the FCC's visible picture area has
been authorised to carry “electronic verification of television
broadcasts” to track which programmes and adverts are actually
aired.
The ITU's digital active area starts on line 20
of field F1 and ends on line 263
of field F1, which seems to give
487 digital lines, although the last line only has picture data in
its first half. However, in practice one of these lines is dropped
and only 486 lines form part of the digital active area – but which
one? Chris Pirazzi in his excellent and
oft-linked-to “Lurker’s Guide to
Video” says that SGI hardware drops line 20
of field 1. However the working
group of the ITU who are responsible for the ITU-R BT.656-4
document believe that line 263 should be omitted, perhaps because
the start of the digital blanking period
for field 2 can’t be on line 263,
because this line begins
in field 1!
Most digital formats such as DV, DVD and SVCD specify 480 lines as
this is exactly divisible by 16 for MPEG encoding. This means that
6 lines must be dropped from the ITU’s 486 - but which 6? In
practice this too seems to vary, but it would appear prudent to
drop an even number of lines above and below to preserve
the field order. Because the FCC
specify that the (analogue) picture area starts on line 22, we
could drop the four lines from line 20
of field 1 to 284
of field 2, and drop the
remaining two lines from 262
of field 1. Or we could also drop
line 22 of field 1 and 285
of field 2 and keep line 262
of field 1 and 525
of field 2. Some implementations
do not even use a full 480 lines of picture data. Confusing, isn't
it?
Nevertheless, the following diagram shows the ITU's digital
sampling space for 525-line systems.
Diagram 5 - 525-line Digital Sampling Space at 13.5MHz
As shown earlier, the 525-line 29.94 frames per sec system sampled
at 13.5 MHz gives 858 samples (Rec-601 rectangular pixels) per
line. In this case the pixel aspect ratio is 11:10 which is
narrower than the 625-line system's 54:59, and also narrower than
square pixels. Consequently the pixel aspect ratio of Rec-601
rectangular pixels for 525 and 625 line systems
are not the same.
The ITU standards specify that the analogue active line length for
M/NTSC (which is used in the US) is approximately 52.66
microseconds. At the 13.5MHz sampling rate this gives a width of
the analogue active area of almost 711 samples.
So, 486 lines comprising 711 of these rectangular pixels would make
a picture with the frame aspect ratio of:
711 * 10 / 11 / 486 = 1.330
which is roughly our ideal 4:3 (1.33333) frame ratio.
The diagram below compares the pixel aspect ratios of square
(computer screen) pixels and the Rec-601 13.5MHz rectangular pixels
for 525-line systems. It is based on the width of the digital
active area which is 720 Rec-601 pixels. Note that 720x480 is
spacially wider than the 4:3 frame aspect ratio, and corresponds to
a width of 654.545 square pixels, i.e.:
720 * 10 / 11 = 654.545 square pixels
Diagram 6 - Pixel Aspect Ratios
Field Order
When two digital fields are
interleaved we end up with a complete 576 line frame. The
convention for referring to these
two fields is the “upper” or
“top” field (which contains the
top line of the frame) and the “lower” or
“bottom” field . Fields are
1/50th of a second apart, so if the
upper field is temporally earlier
than the lower field then we say
that the field order is
“upper field first”. If the
lower field is temporally earlier
than the upper field then
the field order is
“lower field first”.
In diagram 1 the top line (and alternate lines) of the digital
active area is in F1. We know that F2 follows F1 so we might
conclude that 625 line systems are
“upper field first” - but this is
not necessarily the case. It is not the video standard that defines
the field order, it is the
operation of the hardware used to capture and store the analogue
video waveform.
Let’s assume that we have an analogue video capture card which
complies with the ITU standards. It must therefore capture lines in
the range 23 to 310 of field 1,
and lines 336 to 623 of field 2.
When these are interlaced to make a frame then line 23 is the top
line. This means that the capture card (and its drivers) must slot
F1 into the upper field and F2
into the lower field of the
frame. But this still does not tell you whether
the field order of the resulting
frames is upper or
lower field first.
Remember that the video waveform is a continuous stream of
alternating
F1-F2-F1-F2-F1 fields . If the
capture card starts from an
F1 field , then the
F1 field is fed into the
upper field , and the subsequent
F2 field is fed into the
lower field of the frame. In this
case the field order is
indeed upper field first.
However, if the card starts from an
F2 field , the
F2 field is fed into the
lower field and the subsequent
F1 field is fed into the
upper field of the frame. We now
find that the field order
is lower field first,
because within the interlaced frame it is the
lower field which is temporally
earlier than the
upper field .
The diagram below illustrates an analogue video waveform being
digitised by a video capture card. It is of course vastly
simplified. To make it easier to see what is going on, the picture
carried by the waveform starts off completely white, and then fades
quickly to black in just 5/50ths of a second. So the
waveform begins with several
“white” fields , and then
5 fields getting progressively
darker, and then several
“black” fields . The diagram
shows an instant where this fade-out is being digitised. Each pair
of fields are interlaced and
stored in an AVI file as frames.
To make it easier to see what is happening only the top 8 lines of
each interlaced frame are shown. If you bear in mind that lighter
greys occur “earlier” than darker greys, then you can easily see
the field order of the interlaced
frames.
Diagram 7 - Field Order
Both of the capture cards shown are ITU compliant so they produce
interlaced frames whose top line is line 23
of field F1. However, the first
card processes field F1 first and
then field F2; the second card
processesfield F2 first and
then field F1. You can therefore
see the results of digitising an F1 or an
F2 field first.
However it is not uncommon for some hardware to start capturing on
another line, for example the line 336
of field F2. In this case it is
an F2 field which is fed into the
upper field and not an
F1 field . So now if the capture
card starts from an F1 field ,
the F1 field is fed into the
lower field , then the subsequent
F2 field is fed into the
upper field of the frame, and
the field order is therefore
lower field first - the opposite
of the previous case.
So you can see that
the field order of the frames in
the captured AVI file depends on the line number where capturing
starts, and which field F1 or F2
is digitised first.
As a PC user, both of these things are usually hidden from you. If
you are lucky the documentation for the capture card will tell you
whether the card is upper or
lower field first - i.e.
the field order of the digital
video files it creates. If not then you are completely in the dark
and you must examine the files yourself to find out what
their field order is - but you
will only have to do this once as the card will always produce
files with same field order
(unless it has buggy drivers!).
Many video file formats do not carry information which tells
programs the field -order of the
interlaced video material they contain. Consequently you need to
know the field -order of your
video clips and tell the video applications what it is, although
some programs will have a stab at trying to work out
the field -order by analysing the
picture content - but they can get it wrong!
The problem really arises when you have a video editing project on
your PC which contains clips which are from multiple sources which
have differing field orders, or
you want to use another program perhaps to convert your video to
another file format.
Changing the Field Order
Suppose your capture card generates AVI files which are
upper field first and you cut to
an AVI file which is
lower field first. Now when this
is output from your capture card to a TV monitor, the first clip
will play fine as the fields will
be output upper field first
(which is the same order as
the fields were captured); but
the second clip will also be output
upper field first which is
the opposite order to
which its fields were captured.
The result is that when the playback reaches the second clip motion
will appear jerky because
the fields are being displayed in
the wrong order, effectively jig-zagging back and forth through
time.
The solution is to change
the field order of the second
clip. This effectively shifts the frame boundary by
1 field . In this example, the
new first frame will contain the
upper field of the old first
frame interlaced with the
lower field of the old second
frame; the new second frame will contain the
upper field of the old second
frame interlaced with the
lower field of the old third
frame, and so on. This would result in the old
first field and old
last field of the clip being
discarded, but it would make the clip
2 fields shorter in duration. An
alternative would be to discard only the
first field and duplicate the
last lower field which would then
be paired with the final
upper field - this would cause a
temporal discontinuity in the final frame of the clip but would
maintain its duration.
For example, the field order can
be changed in Premiere by selecting the clip in the timeline and
changing its video properties by ticking both
“reverse field dominance” and
“interlace consecutive frames”. This uses the second solution
above. Note that you must
click both of these
options.
“Reverse field dominance” just
swaps the odd and even lines within each frame which is completely
wrong as it effectively causes a vertical spacial “scrambling” of
alternate lines. “Interlace consecutive frames” swaps the
lower fields between each pair of
frames, so that the new frame 1 contains the
upper field of the old frame 1
interlaced with the lowerfield of the old frame 2,
and the new frame 2 contains the
upper field of the old frame 2
interlaced with the
lower field of the old frame 1.
The combination of these two options properly changes
the field order of the clip.
An alternative (and simpler) solution is simply to
shift all of the lines in
the frame up or down by 1 line. This effectively shifts all of the
lines that are in the
upper field to the
lower field , and vice versa, but
without causing any spacial scrambling. It does however mean that
you lose 1 line of picture detail from the top or bottom of the
frame.
Upper, Top, Odd, 1 or A ?!
And so we come to the source of so much confusion in the world of
digital video – the naming conventions
for fields and field ordering.
As we now know, an interlaced frame consists of
two fields .
One field contains the top line
of the frame and all of the alternate lines beneath, and the
other field contains the
line-below-the-top-line and the alternate lines beneath. Notice
that I have avoided using numbers in this description (such as
line 1 or first line),
and there is a reason for this which will become clear.
We need to give these
two fields different names so
that we can easily distinguish between them. There are several
naming conventions used, but almost all of them are open to
misinterpretation, and should therefore be avoided if at all
possible.
The only names which seem to be unequivocal are
“upper field ” and
“lower field ” (or
“top field ” and
“bottom field ”). The
upper field contains the top line
of the frame and the
lower field contains the
line-below-the-top line of the frame. The meaning of the terms
“upper” and “lower” become obvious if you just imagine the top two
lines of a frame.
You can now also express
the field order as
“upper field first” or
“lower field first” without fear
of confusion. Consequently we will use the terms
“upper field ” and
“lower field ” as our naming
convention.
Here are some of the other terms often used and why I think they
should be avoided:
Odd Field and
Even Field - If you look at
the frame-based line numbers you can see that
one field occupies the
odd-numbered lines and the
other field occupies all the even
numbered lines. This seems like an ideal system for naming
the fields , except that some
specifications start numbering the lines from 0 instead of 1. So
the “odd” field could be either
the upper or
lower field depending on how you
start numbering the lines. Consequently
the field order
“odd field first” is not very
helpful.
Field A
and Field B -
There is disagreement over
whether Field A refers to the
upper field or the
lower field . And some programs
such as TMPGEnc take
“ Field Order A” to mean
upper field first, while others
such as Ulead’s products take it to mean
lower field first.
Field 1
and Field 2 -
The analogue video waveform contains two types
of field which have different
sequences of sync pulses. These
two fields are called in the ITU
specs “ field 1” and
“ field 2” and the video waveform
alternates between the two which produces the interlaced picture on
a TV screen. However as we have seen it does not necessarily follow
that field 1 is the
upper field . Depending on how
the waveform is captured, field 2
may be the upper field . This may
be further confused by some people simply assuming
that field 1 must always mean the
upper field or the earlier of the
two fields in a frame.
Field Dominance
Field Dominance is not the same
as Field Order, although it is
often used as such. It has nothing to do with whether the upper or
lower field is temporally earlier
within a frame.
In the studio, analogue video is stored on tape as a series of
alternating
F1-F2-F1-F2-F1 fields - there are
no frames as such. When this material is edited, separate clips
must be joined together, but we must be careful to preserve the
alternating F1-F2-F1-F2 sequence
of fields across the “joins”. We
must therefore decide whether a new scene begins on an F1 or
F2 field , and provided we stick
to that rule then the sequence
of fields will be correct for the
complete project. This “rule” is
the Field Dominance.
If our scenes (i.e. our edits) start
on field F1, then our editing
system is F1 Dominant; and if the scenes start
on field F2 then our system is F2
Dominant.
However, when this video is transferred to our PC we no longer know
which field was F1 or F2. All we
see is our AVI file containing an upper and
lower field , and we usually
don’t know which was an F1 or
F2 field in the original video
waveform.
Similarly, computer-based video editing programs use the frame
boundary for their editing “joins”. A cut from one scene to another
occurs on a frame boundary. Because we know that these frames are
either upper or lower field first
then we could say that our
video material is
“upper field dominant” or
“lower field dominant”. And some
people do use these terms, although dominance is really meant to be
applied to the F1 or F2
analogue fields .
Furthermore, if we are capturing some material which has the
opposite field dominance to that
of our capture card, we could end up with a clip which has scene
changes in mid-frame. In this case we could have a clip
with field order
“upper field first” but with
“lower field dominance”!
Some Digital File Formats
Lets conclude by taking a quick look at a couple of the common
pro-sumer digital video formats and
their field order.
Most video formats stored on computer do not contain any
information about the field order
of the video material they contain. This means that we usually have
to study the file contents to work this out ourselves.
DV & DVCAM
DV & DVCAM is used in consumer and semi-professional
camcorders. In 625-line systems it has a resolution of 720x576, and
in 525-line systems is 720x480.
DV uses intra-frame compression, which means that each frame is
compressed separately, so it is very easy to edit. It also makes it
easy to start recording over material on the camcorder at any
particular frame.
The DV standards say that the digital active area of DV for
625-line systems is from line 335
of field 2 to line 310
of field 1. If you take a look at
diagram 1 you’ll notice that this range is offset from the ITU
specs by 1 line.
Similarly the digital active area for 525-line systems is from line
285 of field 2 to line 262
of field 1.
Consequently in both
systems field 1 is the
“lower” field . Field 1
dominance effectively means that
the field order of DV material is
“lower field first” for both PAL
and NTSC, which is nice!
DVD
DVD uses MPEG1 or MPEG2 compression, but only MPEG2 can store
interlaced video.
MPEG uses inter-frame compression, which means that most frames are
encoded by storing only the features in the image which have
changed compared to previous (or later) frames. This makes it more
difficult to edit because you usually have to build up a complete
frame by accumulating the changes over several previous frames.
The MPEG specs say that interlaced material is encoded as a series
of separate fields with
each field identified as a “top”
or “bottom” field . Alternatively
it may be encoded as a series of frames containing two
interlaced fields , and each
frame has a flag which states whether it is
“top field first”.
In either case the field order is
effectively contained within the MPEG file. A DVD player would have
to output the fields in the
specified order.
References
Analogue and Digital TV:
Rec. ITU-R BT.470-6
Rec. ITU-R BT.601-5
Rec. ITU-R BT.656-4
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