Monitor refresh rate query
jnc at mercury.lcs.mit.edu
Tue Jun 21 14:40:32 CDT 2016
> From: Chuck Guzis
> It's not the refresh rate that will kill things, but the horizontal
> frequency. The high voltage in most CRT monitors (and TVs) is developed
> from the scanning signal via a high-voltage "flyboack" transformer
> Ultimately, if taken too far, the voltage in the FBT secondary exceeds
> the ratings of the winding insulation; an arc develops between windings
> and the FBT self-destructs
So, with Chuck's explanation (above) in hand, eventually my brain turned on,
and I was able to work out what the deal is, and whether my monitor is safe;
it also explains the somewhat contradictory CRT monitor manual:
HP M50 manual says "Setting the screen resolution/refresh rate combination
higher than 1024x768 at 60 Hz can damage the display." Even though the same
document lists the vertical frequency range as "50-100 Hz"!
They mention both the resolution and "refresh rate" since those two together
control the horizontal flyback frequency, which Chuck pointed out as the key.
(Well, the line count - 768 - is involved there, not the line length in
pixels - although the latter will influence the maximum video bandwith or
"dot rate" that needs to be supported - 65 MHz for this particular monitor.)
The horizontal retrace frequency is simply the vertical retrace frequency,
times the number of scan lines per vertical retrace plus a small slop factor
for the actual retrace duration.
So my monitor was running 1024x768 - but interlaced, so only 364 lines per
screen scan (alternating odd and even lines in successive scans). I was
seeing a refresh frequency of 44 Hz - but for full scan of all lines; the
actual vertical retrace is being produced at 87 Hz. So the horizontal retrace
frequency is about 87 * 364 = ~32 KHz - well within what the monitor can
handle (30-49 Khz for the horizontal retrace, per the spec). So the monitor
Probably by the time this monitor came out, the interlaced XGA format had
fallen into disuse, and so they didn't need to clarify that
"resolution/refresh rate combination higher than 1024x768 at 60 Hz can damage
the display" refers to 'progressive' displays, not interlaced.
And of course, as previously pointed out, the interlace explains why no LCD
displays will work. So I'll have to carefully hoard my remaining video
Thanks to everyone who helped me work this out...
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