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Video Formats:
WHY BOTHER?
You’ve no doubt noticed that most DVD
players have an “S-video” output, as well
as the familiar “composite” video output.
And many newer models also provide
outputs for “component” video . So what’s
the reason for this extra complexity when it
comes to video signal connections?
N
OT SO LONG AGO, video was
just video – or that’s the way
it seemed. Video monitors and
TV sets had single RCA sockets for
their video inputs, as did VCRs for
their video inputs and outputs. With
this format, you simply fed the video
signal from one unit to another via
a single RCA-to-RCA coaxial cable,
with other cables only needed for
the audio.
When Laser disc players came
along, most of them used exactly the
same arrangement (although they gave
much clearer pictures than VCRs).
However, when DVD players arrived,
even the early models had an extra
video output socket (usually a 4-pin
mini DIN socket) which was marked
“S-video”. At the same time, TV sets
also started to appear with an S-video
input socket – this in addition to the
more familiar RCA-type video input,
which was now being called the “composite video” socket.
So you now had a choice when it
came to connecting a DVD player to
the TV – use either a single RCA-RCA
cable or one of the new 4-pin DIN to
4-pin DIN “S-video” cables. And the
word soon spread that using an S-video
cable gave better picture quality.
Then things got a little more complicated again. Some of the higher-end
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DVD players started to appear with a
third kind of video output known as
“component video”. This was usually
made available via three more RCA
sockets marked Y, Cb (or Pb or B-Y) and
Cr (or Pr or R-Y). Naturally, component
video inputs also began appearing on
TV sets and video projectors at about
this time, giving the consumer yet
another choice when it came to connecting video signals.
As before, word soon spread that
using component video cables gave
the best possible picture quality – even
better than S-video. And it wasn’t too
long before component video outputs
appeared on even low-end DVD players.
So what’s it all about? Why have
video connections become so complicated and do the fancy, newer formats
really deliver better picture quality
than good, old composite video? Let’s
find out!
About composite video
First of all, let’s talk about composite
video. As the name suggests, this really
isn’t just one signal but is a “composite” or a collection of a number of
signals (or components).
First, there’s the black-and-white
or “luminance” (Y) video component,
which conveys the basic picture detail
by JIM ROWE
and contrast information. Then there’s
the “chrominance” (C) video component, which conveys the picture’s
colour information (the chrominance
component is itself actually two components, not one, but we’ll go further
into this shortly).
Finally, there are the synchronising pulses and the colour subcarrier
burst pulses, which collectively form
a third component in the composite
video signal.
Although these components are
all lumped together and sent along
a single coaxial cable, they really are
different video signal components
with distinctly different functions. But
why were they originally all lumped
together to produce composite video
signals in the first place?
The answer to this is that when TV
broadcasting began, engineers needed
to pack all of the video components
into a single video signal to be modulated onto the TV station’s radio carrier (along with the sound signals, of
course). This also meant that when the
TV signals were demodulated again in
the TV set, they reappeared initially as
the same composite video signal.
In the TV set itself, the composite
video signal then had to be split up
into its various components before
the pictures could be displayed on the
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screen. First, the luminance information had to be extracted so that it could
be used to vary the three picture tube
beam currents together (ie, from the
three “guns”), to recreate the picture
contrast and details.
Second, the chrominance information had to be extracted so that it could
be used to control the beam currents
individually, to recreate the picture
colours. And third, the synchronising
pulses and colour burst information
had to be extracted so that it could
be used to lock the picture scanning
oscillators and ensure that the colour
information was decoded correctly.
Whew!
When VCRs subsequently came
along, the easiest way to handle the
video information that they fed to a
TV set was to use this same composite
video format. That’s why domestic
video connections were originally all
made using the now-familiar single
coaxial cables with an RCA plug at
each end, usually with yellow colour
coding.
Compromises, compromises
Although composite video signals
can produce quite good picture quality,
there are a few compromises involved
in “packing” all of those video components into a single composite signal
– and then subsequently processing
them in this form. The big problem is
that it’s relatively easy for the various
signals to interact with each other, in a
way that actually degrades the ultimate
picture quality.
Probably the most serious type of
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Fig.1: as shown here (top), the luminance and chrominance signals share
the frequency spectrum between 3.2MHz and 5.5MHz. The magnified view
shows how the two sets of information exist in evenly spaced “clumps”,
with the colour clumps neatly slotting between the luminance clumps.
interaction that occurs is “cross modulation” between the luminance (Y) and
chrominance (C) information. This
can happen fairly easily with composite video, because of the way the
chrominance information is conveyed
as modulation on a separate colour
subcarrier, which has a frequency of
4.433MHz for PAL video or 3.58MHz
for NTSC. Although the colour subcarrier itself is suppressed in the video
signal (and recreated in the receiver
using the burst information), the actual
colour information “sidebands” share
some of the same frequency spectrum
as the luminance information and are
actually interleaved with it. This is
shown in a slightly simplified form
in Fig.1.
As you can see, the luminance
and chrominance signals actually
share the frequencies between about
3.2MHz and 5.5MHz. The magnified
close-up view shows how the two
sets of information exist in evenly
spaced “clumps”, with the colour
clumps neatly slotting between the
luminance clumps. This interleaving
was done deliberately, in an effort to
minimise the interaction between the
two components.
However, it doesn’t entirely prevent
interaction, which is why you tend to
see shimmering “cross-colour” bands
on a picture area where there are
finely spaced lines, such as a finely
checked shirt.
This type of visible Y-C interaction
was much more pronounced with
early colour TV receivers, because they
had to use fairly traditional analog filters to separate out the luminance (Y)
and chrominance (C) information. The
problem here was that the low-pass
filter used to extract the Y information had to have a cutoff frequency no
August 2004 9
and R-Y).
DVD player outputs
Fig.2: this diagram shows the different signal processing paths involved for
component video, S-video and composite video. Component video has the
least amount of processing (and the best picture quality), while composite
video has the most processing (and the worst picture quality).
higher than about 3.2MHz, in order to
filter out all the colour information.
Similarly, even if the high-pass or
bandpass filter used to extract the C information had a lower cutoff frequency
very close to 3.2MHz, there was still
be quite a bit of Y information present
in the chrominance signal.
By the way, notice that with this
analog filtering method of Y-C separation, all luminance information
above about 3.2MHz must be “thrown
away”, to avoid getting colour information mixed in with the luminance.
So the picture resolution is degraded
as well.
To get around this problem, later
colour TV receivers (as well as recent
monitors and video projectors) use a
more sophisticated technique to separate out the Y and C information. This
technique is known as “comb filtering” and it involves the use of digital
techniques to produce filters which
have responses shaped like combs
– with “teeth” that can separate the
two sets of information clumps. One
filter extracts all of the Y information
clumps, while the other extracts all of
the C information clumps.
That way, the Y and C video information can be separated properly,
without sacrificing the bandwidth of
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either. The result is clearer and sharper
pictures, with a minimum of crosscolour interaction.
DVD recording format
Of course, the only way to completely ensure that there’s no interaction
between the luminance and chrominance is to keep them separate in the
first place. And that’s why when the
standards were being developed for
DVD video discs, it was decided that
the video would actually be recorded
in “separate component” format – with
the Y information kept completely
separate from the C information.
What’s more, even the C information would be split into two separate
components, to keep the colour
“cleaner”.
As you may know, both the video
and audio are recorded on DVDs in
digitally compressed form (MPEG-2),
to allow everything to be squeezed into
a maximum bit rate of 9.8Mb/s (megabits per second). But the video is still
kept as three separated components,
even when it’s digitally compressed.
So when a DVD is played back, the
initial output from the player’s MPEG
decoder section is in component video
form: the luminance (Y) signal plus
two “colour difference” signals (B-Y
When the first DVD players came
out, most TV sets were only provided
with a composite video input (that’s
if they had a video input at all). So, to
ensure that people would be able to
watch DVDs on their existing sets, the
manufacturers fitted their players with
additional video processing circuitry,
to combine the decoded component
video signals into composite video.
That was fine but it meant that the
video signals had to be passed through
extra processing circuitry in the player
to produce the composite video. It then
had to go through a full Y/C separation
and colour separation process in the
TV set again, to produce the three component video signals needed for the TV
set or projector to display the pictures.
These steps are shown in Fig.2.
As you can see, this way of playing
DVDs via the composite video path
involves quite a bit of processing, not
only in the player but in the TV set
(or projector) as well. The component
video signals have to be combined in
the player and then separated again in
the TV set or projector – all so we can
connect the two pieces of equipment
using a single video cable!
All of this extra video processing
inevitably causes signal degradation.
And because we deliberately force the
various video components through a
composite video “tunnel” (ie, the cable
at the bottom of Fig.2), it also tends to
introduce some Y-C interaction. That’s
a pity, because the signals actually
coming off the DVD video disc already
match the component video format
that’s ultimately required inside the
TV set to display them.
This is also illustrated in Fig.2,
which shows that much less processing is involved for component video
signals. Obviously, it’s far better not to
combine the component video signals
at all but to send them to directly to
the TV or projector in their “native”
form, to drive the display circuitry.
S-video input
The first big step forward was when
TV and projector makers started providing their sets with S-video inputs,
which could at least cope with separated luminance (Y) and chrominance
(C). This had already started by the
time the first DVD players appeared,
because S-VHS camcorder makers
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had got the ball rolling by fitting their
products with S-video outputs. This
was done so that consumers could
take advantage of the improved picture
quality possible with S-VHS.
By providing their first-generation
DVD players with S-video outputs (as
well as composite outputs), the DVD
makers made it possible for consumers
to take advantage of the better picture
quality offered by DVDs. As you can
see from Fig.2, an S-video link at least
bypasses the Y/C combining circuitry
in the DVD player, as well as the Y/C
separation circuitry in the TV or
projector. This removes two signal
processing steps and also means that
the Y and C components are never
combined at all – not even briefly.
As a result, Y-C interaction is avoided completely.
Users soon found that S-video was
well worth the extra hassle of having
to use a different video cable. However,
the picture quality would be even
better again if the pristine component
video that came direct from the DVD
player’s MPEG decoder could be piped
directly to the display circuitry of the
TV set or projector.
Of course, this couldn’t be done
until TV and projector makers started
providing their sets with component
video inputs. Once such sets began appearing, DVD players with component
video outputs began appearing as well.
As a result, consumers could finally
feed fully separated component video
signals from DVD players directly into
their TVs and video projectors.
Get the idea? Although S-video
and component video connections
might seem to be more complicated
and messier than composite video,
they’re actually less complicated for
the video signals. That’s because the
components are kept separate and go
through much less processing. And
that means they’re degraded less and
so you get clearer pictures.
What about RGB?
Some TV sets of European origin are
provided with inputs for component
video in yet another format known as
“RGB”, where the three primary colour
signals are already separated. This
type of component video outputis also
provided by some pay-TV and digital
set-top boxes.
In theory. RGB should offer slightly
better picture quality again than Y/BY/R-Y component video, because
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the display drive circuits in a TV or
projector do ultimately need the video
signals in this very form. However, in
practice, the picture quality is often
much the same, because even if your
set has direct RGB inputs, the signals
still have to be converted into this form
(from Y/B-Y/R-Y) in the DVD player
or set-top box.
The proof is in the picture
Perhaps you still don’t quite believe
that S-video and component video
really deliver better picture quality.
Well, the best way to be convinced is
to compare them with your own eyes.
But since you may not find it easy to
do this, we’ve taken close-up shots of
part of a standard test pattern image,
as reproduced from a PAL DVD test
disc on a video projector.
The first picture was obtained using
a composite video link, the second using an S-video link and the third using
component video links. These pictures
will give you at least some idea of the
improvements that can be achieved.
Notice in the composite video image
that there are bright multi-coloured
fringes in the circular “Fresnel Zone
Plate” pattern at centre left. These all
consist of “fake colour”, caused by
high-frequency Y information getting
into the colour information (ie, crosscolour interaction).
There are also weak bands of fake
colour in the two frequency band
squares at top centre of this image.
As you can see, the luminance
response does extend all the way to
5.5MHz, as shown by the tapering
lines on the right of the image. This
is presumably because the projector
used to display these images uses
comb filters to perform the Y/C separation from the composite video, so the
upper luminance frequencies are not
being “thrown away”. Still, those fake
colour artefacts do result in noticeable
picture degradation.
If you compare the S-video and
component video images with this first
image, you’ll see that there is much
less colour fringing using the S-video
signals and virtually none at all using
component video. There’s no doubt
that the S-video link gives significantly
clearer pictures than composite video,
while component video gives the cleanest and sharpest pictures of all.
Note: Sanity currently stock the
disc at www.sanity.com.au or phone
1300 722 121.
SC
(1). Composite video
(2). S-video
(3). Component video
Fig.3: these three pictures clearly
illustrate the improved picture quality
delivered by S-video and component
video signals. These’s much less colour
fringing using S-video compared to
composite video, while component
video gives the best picture of all.
August 2004 11
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