Re: Glass Cockpit Video Rates

From: (Ken Stoorza)
Organization: Network Computing Devices Inc., Mt. View, CA, USA
Date:         02 Dec 94 02:48:58 
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I do not have first hand knowledge of the refresh schemes and rates
used in these cockpit displays. Here are a few thoughts, however,
which will apply once we learn more specific information from others
in this newsgroup.

1.  These displays are either raster scan, vector scan or a
combination of the two. Raster scan is employed in your PC monitor;
vector scan (or "stroke" displays) create images by positioning the
electron beam at a specific location, turning it (or "them" for color
displays) on, deflecting the beam to the stop point and extinguishing
it. Such systems present smooth straight lines regardless of
rotational position. With higher density raster scan displays and
smoothing techniques, very good slightly rotated lines can now be
made. This might be a viable, modern alternative to much more
expensive vector displays.

2.  The phosphor trade off (longer persistence vs. refresh rate) has a
few disadvantages when considering slow phosphor:

- slow phosphors tend to be less efficient; more beam current is
required for equivalent light output. In a high resolution display
this is not desireable as increased beam current eventually leads to
an effectively larger spot size which compromises resolution.

- slow phosphors are not a good choice when moving information is
displayed. An undesireable characteristic of long phosphors is "tails"
associated with moving objects as the phosphor decays.

- when considering color, I do not believe that there are likely
choices for long persistence that include all three phosphors. It's
tough enough getting good reds and blues as it is without the added
constraint of long persistence. I would expect that these cockpit
displays use B22, the common monitor phosphor.

- phosphor decay is a difficult phenomenon to quantify when
considering flicker.  Some phosphors initally decay rapidly and then
very slowly, yet seem to work well when a long phosphor is desired.
The best rule of thumb is to try it and see how it actually performs. 

3. With common, regular or high effeciency phosphors, practically
everybody can easily see a 60 Hz refresh. This is aggravated by high
image intensity and when viewed in the peripheral vision areas (i.e.
corners) of the eye. Fewer people can discern 70 Hz and 75 Hz or
greater pretty much shuts out everybody. In either a stroke, raster or
stroke and raster system, refresh is defined the same way:  the time
required to re-illuminate any point on the screen. The reciprocal of
this time is the refresh rate in Hertz or cycles per second.

I left out the possibility of "penetration phosphors" as I expect that
modern glass cockpits do not use them. If I am incorrect, I hope the
real experts will tell me. Refresh rate considerations are the same,
although I would expect that these seldom used phosphors would be less
likely candidates for long refresh optimization. 

Ken Stoorza