From: SFPhysics@aol.com
Date: Fri Dec 06 2002 - 11:50:39 PST
From: SFPhysics@aol.com Message-ID: <50.157cd726.2b22598f@aol.com> Date: Fri, 6 Dec 2002 14:50:39 EST Subject: Television Lines
> One of my students asked how it is possible for a television set (CRT) to
> move the electron beam at such a high rate and with such accuracy that we
do
> not begin to perceive the "lines" that are being "drawn". Anyone got a
good
> answer for the lad?
> Charlie Bissell
> Hillsdale High School
>>
Greetings Charlie:
I would like to address the "how" of moving the beam. Behind the screen
where the electrons hit the phosphors and excite them to glow, there is an
electron gun that accelerates them to impart energy. This is done in a high
vacuum so the electrons won't hit any gas molecules on their way to the
screen. The thin accelerated beam would normally go for the center of the
screen and all we would see is a dot. In order to move the dot, and move it
fast, we use rapidly fluctuating magnetic fields. Electrons will take a
curved path in a magnetic field. So on the back of the CRT, cathode ray
tube, we put two electromagnet structures. One moves the beam up and down
while the other one moves the beam left and right. Control circuits feed
just the right amount of current to each assembly for an accurate placement
of the dot. These are kept in proper time with each other by what is called
a sync circuit. The "sync" is short for synchronization. As a single frame
of the picture starts, the beam is in the upper left corner where the
electromagnets have placed it. The frame is then "painted" downward line by
line with each line being across the horizontal before for the next line down
is started. The sync circuit keeps the dot precisely where it should be for
the entire picture.
Dr. Paul talked about the persistence of vision making one picture out of
this one little flying dot but also the phosphors are formulated to have a
persistence of luminance of their own for a fraction of a second and this
adds to our own for what looks to be a complete picture. The luminance can
be seen sometimes when we enter a dark room where our eyes have adjusted to
the dark and a TV has just been turned off. The screen will still have a
soft glow.
We can still see the lines on many TVs with a magnifying glass near the
screen face, especially on an old black and white TV. Computer monitors are
different again. They have more lines and faster sweeps so that the lines
cannot be made out. This means the circuits have to be more accurate and
complex so that is why they charge more for a monitor than a regular TV.
Hope my comments have helped,
Al Sefl
Retired but still blabbing on...
This archive was generated by hypermail 2.1.3 : Mon Aug 04 2003 - 16:18:09 PDT