720 pixels are sampled to allow for little deviation from the
ideal timing values for blanking and active line lenght in analog
signal. In practice, analog video signal - especially if coming
from a wobbly home video tape recorder - can never be that
precise in timing. It is useful to have a little headroom for
digitizing all of the signal even if it is of a bit shoddy
quality or otherwise non-standard.
720 pixels are also sampled to make it sure that the signal-to-be-digitized
has had the time to slope back to blanking level at the both ends.
(This is to avoid nasty overshooting or ringing effects,
comparable to the clicks and pops you can hear at the start and
end of an audio sample.)
Last but not least, 720 pixels are sampled because a common
sampling rate (13.5 MHz) and amount of samples per line (720)
makes it easier for the hardware manufactures to design multi-standard
digital video equipment.
It means that the sampled horizontal range of the signal is a
bit wider than the actual active image frame:
Yes, you understood correctly. 720x576 is not exactly
4:3, and neither is 720x480. The real 4:3 frame (as defined in
the analog video standards) is a bit narrower than the horizontal
range of signal that actually gets digitized.
Yes, it is the same for all generally available digitizing
equipment; tv tuner cards, digital video cameras and such. It is true
even for all-digital systems; otherwise they would not be
compatible with ITU-R BT.601.
I admit that the figures presented on this web site are not
very well-known facts even amongst professional videographers,
not to mention hobbyists. Aspect ratio is one of the most
misunderstood "black magic" issue in digital video.
That is precisely why I constructed the web site in the first
place - to share the knowledge.
As for my calculations; feel free to prove them wrong. For
starters, you might want to read the documents in the Related Links section.
That may very well be the sad truth. Fortunately, even if you had
used wrong methods for scaling/resampling the image, the
difference between the correct aspect ratio and a wrong aspect
ratio is often small enough to go unnoticed unless you really
start looking for it.
525/59.94 video signal has 486 active (image-carrying)
scanlines, but modern digital video equipment usually crops 6
of them off. Why? To get the height of the image down to 480 pixels,
which is neatly divisible by 16. See for yourself:
Also note that 720 / 16 equals exactly to
45 so the width of the image is divisible by 16, as well!
Modern digital video applications such as DV, DVD and digital
television (DVB, ATSC) often use MPEG-1 or MPEG-2 formats (or
their derivatives) which are all based on 16×16 pixel
macroblocks. Having the height and width of the image readily
divisible by 16 makes it easier and more efficient for an MPEG
encoder to compress video.
Correct, but the information might not have been that valuable
in the first place. Most 525/59.94 video work is already done
solely in the digital domain and in the 720×480 format, so there
is usually nothing to digitize on those scanlines anymore.
Moreover, in the good old days (when all of those 486 scanlines
were still in active use) most of the time the edges only carried
flickering VCR head noise.
The video image is masked by the overscan edges of a CRT based
television, so you would not normally see the "missing"
Think it this way:
There is also another way of thinking it:
The latter way of thinking will also lead to cropping off the side edges
of the image to get it inside a 4:3 rectangle (albeit a bit
smaller than the "real" one), but then again, if you
are restricted to using 704×480, that decision has already
pretty much been made for you.
As can be seen from the example in section 3.2.2,
the answer is no. If you simply resample from 720×576 to 720×480,
the analog active areas of the source and target formats will not
match. Fortunately, there is a bit fool-proofness built-in to the
relationship of these two frame sizes. What you will actually get
from the process is an image in which the original analog active
area (702×576 centermost pixels of 720×576) has become 702×480
in the target format's pixels. This, in turn, almost
represents a 4:3 area, albeit a bit smaller than what would be
needed for a perfect conversion.
The area that 702×480 covers is not the same as the
actual analog active image frame (which would be 710.85×486, or, in practical
terms, 711×486). It is
more like a smaller 4:3 frame inside it.
In other words, the result is that the active 4:3 image frame
in the source format has shrunk a bit in the conversion: it has
lost six (target) scanlines in vertical direction and the same
relative amount of width. However, for all practical purposes, it
has still retained its original aspect ratio. The easiest way to
see this is converting 702×480 (in 13.5 MHz 525-line ITU-R BT.601
format) to "true" square pixels: 639 + 4419/4739 square
pixels by 480 scanlines is a close enough match to 640×480,
which is 4:3. Wonderful coincidence, isn't it? :)
The same peculiar relationship applies to all 525/625 "sister
resolutions" derived from 13.5 MHz:
This holds true on two conditions:
As direct resampling involves shrinkage (or when going in
another direction, enlargement), I cannot really recommend this
method for any real standards conversion work. It is more like a
quick hack, suitable for use e.g. if the software does not allow
proper resizing and cropping.
Note: Many people use
direct resampling for all the wrong reasons: 1) They think that a
720×480 frame directly equals to a 720×576 frame. 2) They also
think that both aforementioned frame sizes represent exactly the
active 4:3 (or 16:9) picture area, edge to edge. As you already
know from Section 2.1, both of these
assumptions are wrong. The fact that direct resampling
works at all is mostly a quirky coincidence
The problem with this resolution is that while you think
you are editing in a format that is both 1) 4:3 square pixels and
2) easily convertable to a standard video resolution (either 720×576
or 720×480) just by vertical resampling, you are not. See the
table. There is no real world video format that would use full
720 pixel horizontal range as the width of the active 4:3 frame.
In order to get to a standard video format from this one, you
need to take in account the actual form of the sampling matrices.
The 4:3 area in 625-line formats is 702×576, not 720×576. In
525-line formats it is 711×486, not 720×480. Resizing a 720
pixels wide 4:3 format directly to 720×576 or 720×480 simply
won't work. You will either have to resample in both
directions (unlike you originally thought, you do not
get to keep the image width neatly as 720 pixels at all times),
or to crop some top and bottom lines off.
If you need to construct an intermediary square-pixel
resolution that is a) exactly 720 pixels wide and b) covers
exactly the same area as 720x576 or 720x480 (thus only having to
resample in vertical direction for conversions), you will end up
with two separate resolutions, one for each video
Fortunately, the numbers will nicely round up to 720×527
for both standards.
Note that the original interlaced field structure (if any) will go
haywire as you mess around scaling in the vertical direction.
"Square pixels", as digitized by a TV tuner or an M-JPEG
card, are not exactly square. The "industry standard"
sampling rates used in square-pixel video equipment actually give
out pixels that are almost square, but not exactly. As
you can see for yourself in the table, the difference is very
small - for all practical purposes meaningless - but it is
still useful to know that sampled "video" square-pixels
differ a bit from ideal "computer" square pixels.
Converting "computer" square pixels to "video"
square pixels is usually a futile effort. You will not see the
difference, anyway, and probably only lose some quality in the
But that's just the way video is. Fortunately, the conversions
are not really that complicated once you practice them a
Feel free to process your video just the way you like it. But
there are still many people who would like to get as close to the
ideal aspect ratio correctness as possible, instead of only using
rough "ballpark figures" in their video work.
You may be correct. The professional video gear is very strict about
conforming to the ITU-R BT.601 standard, and you can also generally trust DV
camcorders and DVD players/recorders using the correct sampling rates and pixel
clocks. However, the PC hardware market is different: cheap mass-marketed tv
tuner cards and "tv out" cards ofter seem to have these design flaws
and inaccuracies in their drivers: sometimes they are using the common,
industry-standard frame formats
(such as 720×480) with sampling rates that are just plain wrong or sufficiently
off the mark to create problems.
It is usually not the hardware that is the culprit here – the chips on the
card may be perfectly capable of producing images (or digitizing them) using exactly
the correct sampling rates and pixel clocks, but the programmer who designed the
driver that controls the hardware may have taken some special liberties and
shortcuts, leading to inaccuracies. (Possibly the drivers for these problematic
devices were designed by someone who has not studied the relevant video
Fortunately, you can check out your devices and, if necessary, calibrate your
capture workflow by following
these instructions. (The only way you can find out these flaws for
sure is comparing test images as detailed in the above link, or using a test
card generator and an oscilloscope.)
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Edited: Dec 27, 2015 by
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