On 29 Sep 2007 13:10:06 GMT, phil-news-nospam@ipal.net
wrote:

>On Sun, 02 Sep 2007 22:04:11 -0400 kony <spam@spam.com> wrote:
>| On 2 Sep 2007 21:53:26 GMT, phil-news-nospam@ipal.net wrote:
>|
>|>I was looking at specs for an LCD monitor and noticed it included scanning
>|>frequency and listed the vertical frequency at 56-75 Hz. Now I could see
>|>there being an upper limit perhaps even so low as 75 Hz. But a lower limit
>|>of 56 Hz?? That doesn't make a whole lot of sense to me. Why can't any
>|>LCD handle significantly slower? Isn't it supposed to be all electrical?
>|
>|
>| It is more economical to design it to accept within a
>| certain range, plus designing for sub-56Hz would be
>| unnecessary since no video cards go lower than this. The
>| reason it's not just an electrical issue is that this is a
>| data signal which must meet certain timing parameters. It's
>| not just amplified and projected onto a medium, it is
>| digitized and matrixed onto the LCD arrays at the rate the
>| controller is compatible with.
>
>Well, you're wrong about "no video cards go lower than this" because I can
>get frame rates down to 24 Hz from mine, and I suspect even lower is possible.
>I'll have to get an oscilloscope to see just how low I can set it and keep
>the signal coming out correctly.

Of course lower is technically possible but not a practical
design goal for a PC video card. Thus, video cards really
don't go lower because you'd have to change the default
behavior including the driver settings, which nobody in
their right mind would do to output from a PC to an LCD.



>
>FYI, high definition TV signals range from 60 Hz at the highest down to
>a low of just above 23.976 Hz. All LCDs should be required to go down
>to as low as 23.976 Hz.

You're talking about frames per second, yes? That's not
what is being referred to previously by 56Hz, it does not
only signal at the rate of frame change and no PC use LCDs
need to support below 60Hz to display HD.




>
>
>| Your concern will not effect your use. Generally the system
>| is set to use 60Hz and all LCDs (AFAIK) accept this
>| frequency. Unlike with CRT, there is no need to change this
>| in an attempt to reduce flicker because LCD is a different
>| technology not having flicker based on too low a refresh
>| rate.
>
>This is why a lower frame rate is now usable. On CRT it will flicker so
>it is not a usable option. On LCD is will not, so it is possible to use
>a lower frame rate.
>
>Why use a lower frame rate?
>
>With larger and larger LCD displays, you have higher and higher native
>geometry. Video cards can often handle these higher geometries without
>having to be redesigned or replaced to get a higher clock rate. The
>clock rate is the first barrier I hit on my video card when trying to
>raise the geometry. By using a lower frame rate, the geometry can be
>achieved.

No, all you need is a data link tech that allows enough
bandwidth. ~ 24FPS looks like flickery crap on an LCD not
because of the flickering seen on a CRT but because many
people can perceive too much difference between successive
frames in motion scenes. You want the FPS higher and the
signaling rate higher still.




>My CRT actually works (down to about 40 Hz), but the flicker
>is terrible. My LCD just says the video is out of range.




>
>Higher rates require better circuits inside the LCD for everything. But
>a lower rate only requires a few things like ranging the analog sample
>clock to a lower rate (not hard to do considering that it is a synthesized
>oscillator).
>
>I've seen an LCD display offered with 2560x1600 native. But it requires
>DUAL DVI connections because the pixel clock rate is too high. But with
>a lower frame rate that could have been done with a single DVI connection.

It doesn't mean the rate is "too high" to the extend that
something should be done to lower it, the goal is to retain
image quality, not degrade it just to make it possible with
a single DVI cable.