From: SFPhysics@aol.com
Date: Sun Oct 14 2001 - 03:58:55 PDT
From: SFPhysics@aol.com Message-ID: <f4.10bdf4dc.28fac9ef@aol.com> Date: Sun, 14 Oct 2001 06:58:55 EDT Subject: Re: Video Scan Rate Confusion/was Blinking LED on Video
> I looked in the Reference Data Book for Engineers and found that the
> scan rate for black and white TV is 60hz but for color its 59.94 hz,
> This slight difference is what causes the LED to blink.
> Jim
>>
Jim, et al.:
The frame rate of NTSC video is 1/30th of a second but only half the lines in
the video picture are put up in what is called an interlace pattern each
1/60th of a second and that is where the 60Hz/59.94Hz scan figure come from.
Only 30 frames per second are sent because of bandwidth limitations of the
old video systems until the HDTV gets going as the new system. The first
time the picture is painted by the electron beam a fraction of a millimeter
higher than the second time the CRT is painted. This gives the effect of
having a picture with more definition. The two half-shots of the same frame
make for an interesting effect when you are very very close, closer than your
mom allowed. It appears that the lines are running down the screen because
your eyes follow the alternating lines (a total of 525 counting vertically).
The same effect is seen on advertising travelers where the lights "travel" as
an effect of being turned on and off sequentially. For a better explanation
of NTSC video and how a TV picture is made from just one traveling spot of
light:
<A
HREF="http://www.adobe.com/support/techguides/premiere/prmr_interlace/main.htm
l">Interlaced and non-interlaced video</A>
Or if the hypertext won't go through the Pinhole Server:
< www.adobe.com/support/techguides/premiere/prmr_interlace/ main.html >
Have your students use the TV screen or a computer monitor as a stroboscope.
Put on a blank screen. Put a small DC motor with a propellor in front of the
screen and vary the voltage to control the RPM. At some point you can see
the single blade standing still while at other speeds you can see multiple
blades in front of the screen. Students love this and it is a good way to
encourage exploration.
TV can be educational. If the students would play with it rather than watch
it.
Regards to everyone on the list,
Al Sefl
Who once built a Heathkit color TV......... and it has been working for 31
years!
Electronics is a fun branch of Physics.
Reminder: Fall Physics teachers meeting (NCNAAPT) is coming up in Monterey
Oct. 26th & 27th at the Naval Postgraduates School. These are always great.
San Francisco Unified always screwed me over by never paying the Eisenhower
Funds to cover "professional development" but these meetings were well worth
using my own cash. That is how much I liked them!
<A HREF="http://homepage.mac.com/cbakken/ncnaapt/fall2001.html">NCNAAPT Fall
Meeting Information</A>
This archive was generated by hypermail 2.1.3 : Mon Aug 05 2002 - 09:21:36 PDT