241 lines
6.0 KiB
HTML
241 lines
6.0 KiB
HTML
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>Using Interlaced Modes</TITLE
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>14. Using Interlaced Modes</H1
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><P
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>(This section is largely due to David Kastrup
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<TT
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CLASS="email"
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HREF="mailto:dak@gnu.org"
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>dak@gnu.org</A
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>></TT
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>)</P
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><P
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>At a fixed dot clock, an interlaced display is going to have
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considerably less noticable flicker than a non-interlaced display, if
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the vertical circuitry of your monitor is able to support it stably.
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It is because of this that interlaced modes were invented in the first
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place.</P
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><P
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>Interlaced modes got their bad reputation because they are inferior to
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their non-interlaced companions at the same vertical scan frequency,
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VSF (which is what is usually given in advertisements). But they are
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definitely superior at the same horizontal scan rate, and that's where
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the decisive limits of your monitor/graphics card usually lie.</P
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><P
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>At a fixed <EM
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>refresh rate</EM
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> (or half frame
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rate, or VSF) the interlaced display will flicker more: a 90Hz
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interlaced display will be inferior to a 90Hz non-interlaced
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display. It will, however, need only half the video bandwidth and half
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the horizontal scan rate. If you compared it to a non-interlaced mode
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with the same dot clock and the same scan rates, it would be vastly
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superior: 45Hz non-interlaced is intolerable. With 90Hz interlaced, I
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have worked for years with my Multisync 3D (at 1024x768) and am very
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satisfied. I'd guess you'd need at least a 70Hz non-interlaced display
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for similar comfort. </P
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><P
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>You have to watch a few points, though: use interlaced modes
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only at high resolutions, so that the alternately lighted lines are
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close together. You might want to play with sync pulse widths and
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positions to get the most stable line positions. If alternating lines
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are bright and dark, interlace will <EM
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>jump</EM
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> at
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you. I have one application that chooses such a dot pattern for a menu
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background (XCept, no other application I know does that,
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fortunately). I switch to 800x600 for using XCept because it really
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hurts my eyes otherwise.</P
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><P
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>For the same reason, use at least 100dpi fonts, or other fonts where
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horizontal beams are at least two lines thick (for high resolutions,
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nothing else will make sense anyhow).</P
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><P
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>And of course, never use an interlaced mode when your hardware would
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support a non-interlaced one with similar refresh rate.</P
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><P
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>If, however, you find that for some resolution you are pushing either
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monitor or graphics card to their upper limits, and getting
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dissatisfactorily flickery or outwashed (bandwidth exceeded) display,
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you might want to try tackling the same resolution using an
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interlaced mode. Of course this is useless if the VSF
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of your monitor is already close to its limits.</P
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><P
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>Design of interlaced modes is easy: do it like a non-interlaced
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mode. Just two more considerations are necessary: you need an odd
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total number of vertical lines (the last number in your mode line), and
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when you specify the "interlace" flag, the actual vertical frame rate
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for your monitor doubles. Your monitor needs to support a 90Hz frame
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rate if the mode you specified looks like a 45Hz mode apart from the
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"Interlace" flag.</P
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><P
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>As an example, here is my modeline for 1024x768 interlaced: my
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Multisync 3D will support up to 90Hz vertical and 38kHz horizontal.</P
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BORDER="0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> ModeLine "1024x768" 45 1024 1048 1208 1248 768 768 776 807 Interlace
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>Both limits are pretty much exhausted with this mode. Specifying the
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same mode, just without the "Interlace" flag, still is almost at the
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limit of the monitor's horizontal capacity (and strictly speaking, a
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bit under the lower limit of vertical scan rate), but produces an
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intolerably flickery display.</P
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><P
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>Basic design rules: if you have designed a mode at less than half of
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your monitor's vertical capacity, make the vertical total of lines odd
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and add the "Interlace" flag. The display's quality should vastly
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improve in most cases.</P
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><P
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>If you have a non-interlaced mode otherwise exhausting your monitor's
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specs where the vertical scan rate lies about 30% or more under the
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maximum of your monitor, hand-designing an interlaced mode (probably
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with somewhat higher resolution) could deliver superior results, but I
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won't promise it.</P
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