Newsgroups: rec.video

In rec.video, jeff@jlk.b11.ingr.com (Jeff Kilpatrick) writes:

>Now that I've been looking around at PTV and CRT sets, I have a few questions
>about some terms.  1)  What is a comb filter?  2)  What is a 'black' screen (I've
>seen Pioneer rant and rave about theirs and I also see the term in CRT sets) 3) 
>Why do you need a 'flat' tube?  4) I can't think of the term, but it allows you
>to view movies as they were intended; the picture is blank against the top and
>bottom of the screen and supposedly you see the entire scene.  What is this
>called and is it on every set and are there any advantages/disadvantages?

1.  A comb filter is a method of extracting better resolution from the
transmitted color video signal.  It turns out that most of the video information
is contained at harmonics of the horizontal scan frequency of 15,734.26 Hz.
In order to shoehorn in the color information onto what what originally just
a black-and-white signal, the color infomation was put on a subcarrier frequency
of 3579545 Hz.  The color information is also contained on harmonics of the
horizontal scan frequency.  Now, the black-and-white (luminance) harmonics
are found as multiplies of 15,734.26 Hz, or n*15,734.26 Hz.  The color
information (chromanance) will be found at frequencies of
3579545 +/- n*15,734.26 Hz (in both cases, n = 1,2,3,...).  If you do the
arithmetic, you will find the the harmonics of the luminance and harmonics
of the chromanance interleave in frequency.  Visually, this minimizes the
observed interference between them, though it doesn't prevent it totally.

Now, some more detail, so we can get on to comb filters.  The bandwidth of
the chromanance signal is limited, such that n < 63 or so.  This means that
the chromance signal can extend down to about 2.5 MHz (3579545 - 63*15734.26).
On order to not have visible interferance from the chroma signal, many TVs
are designed to cut off the composite video (luminance and chromanance)
at about 2.5 MHz.  Note that this signal will be used just for the luminance,
the chromanance having been extracted earlier.  Unfornately, this also cuts
off the higher harmonics of the luminance as well.  These higher harmonics
carrier the information for high details (sharp edges).  To get this extra
detail, but not make the interference from the chroma signal visible, takes
what is know as a comb line filter.  This filter places a whole series of
notches in the frequency response, right at the positions of the harmonics
of the chromanance signal, but allows harmonics related to the luminance 
signal to pass.  Note that a comb filter can also be used in the chroma
path to filter out interference form the luminance.

2.  The whole idea of a black screen is to enhance the contrast of the
picture.  Note that contrast can be reduced by room light being reflected
off the CRT, and also by bleeding of light from illuminated portions of
of the picture into dark portions.  There are two points of reflection
of room light.  The first is right off the front of the face plate.  This
can be made somewhat less annoying by making the faceplate flat, since then
only a small section of the room (which we hope is dark) is reflected.
Second, some surface treatments can be done to reduce reflections a bit.
There is yet another reflection off the back of the faceplate, inside the
tube.  This can be reduced by making the glass itself dark, since the
room reflection needs to pass though the glass twice, while the light for
the viewed picture needs to pass though only once, thus the room light
is absorbed twice.  This does necessitate more "horsepower" to get a
bright picture on the screen.  Secondly, a black surround can be used
around the phophor stripes.  The stripes themselves are lighter in color and
thus reflect light, but there is no need to have the periferery, which
does not produce light, light color as well.  This black surround also
prevents light from bleeding over into dark areas of the screen.

3.  I mentioned the flat tube above.  The other advantage is that straight
lines will stay staight, no matter what the viewing angle.  Just take a
baloon and draw a straigh line on it.  Then rotate the baloon around.  You
will see the line curve as you do this.  Same thing with a curved CRT.

4.  What you are talking about here is lack of overscan.  Some people find
black edges inside the CRT annoying.  I guess they feel that the picture
should fill the entire screen that they paid for.  To make sure this is so,
the set is designed/adjusted to that the scanned picture is slightly larger
than the display area of the CRT.  Of course, some of the picture at the
edges gets lost this way.  Many videophiles feel otherwise, and so buy
sets with little overscan, or have theirs adjusted for minimum overscan.
Note that overscan also hides scan non-linearities which are the worst at
the edges.


Donald Borowski    WA6OMI    Hewlett-Packard, Spokane Division
"Angels are able to fly because they take themselves so lightly."
                                       -G.K. Chesterton