Archived from groups: comp.sys.ibm.pc.hardware.video (
More info?)
"Bob Niland" <email4rjn@yahoo.com> wrote in message
news
😱psi0bkbqcft8z8r@news.individual.net...
> But there are opportunities for the signal to get
> visibly degraded if it goes to analog before it gets
> to the LCD panel lattice. In the entirely unscientific
> test I just ran, where I saw exactly what I expected to
> see, the analog happened to be running through two 2m
> lengths of HD15 cable and a KVM switch. The LCD image
> went from pixel-perfect to slightly fuzzy, and perhaps
> also reduced "contrast".
Oh, sure - but then, that's a bad thing to do to any connection.
Have you tried the corresponding experiment with a
digital interface running at its max. pixel rate? (Nope -
because passive switchboxes and the like simply don't
work with digital interfaces.) In an apples-to-apples
comparison, say a VGA vs. a DVI over the standard
2-3 meters of good quality cable in each case, the
differences you will see are due to sampling errors in the
analog case. Or in other words, the advantage of the digital
interface is that it brings its "sampling clock" along with
the data.
> Umm, if the bits in the frame buffer are going thru a
> DAC (which can introduce noise and distortion), then
> thru a cable (which <ditto>), even if the LCD is not using
> an ADC, and is using the analog signal directly, that
> extra noise and distortion may show up on screen.
Sure; the question is always going to be whether or not
that "noise and distortion" is below the level we care
about. Digital interfaces are not error-free, either; that
they are acceptable, when they are, is the result of the bit
rate being below perceivable levels. Similarly, if the analog
interface delivers a stable image with the video data to
the desired level of amplitude accuracy (in most cases here,
to an 8 bit/sample level, or an accuracy of about +/- 1,5 mV
in "analog" terms), the difference between the two interfaces
will not be distinguishable. It is ALWAYS a matter of how
good is good enough, and neither type of connection is
ever truly "perfect."
> I sorta suspected that, but in the DVI-D model, the
> signal remains digital until it hits the rows & columns, no?
Well, until it hits the column drivers, yes. On the other hand,
there HAVE been LCD panels made, notably by NEC,
which preserved the analog video signal in analog form clear
through to the pixel level.
> Does the typical analog-only LCD have a DAC? Or does it
> just sample the analog signal and route values to drivers?
It has an ADC right up front - it generally has to, especially
if it supports any sort of image scaling, which is definitely
something best done in the digital domain. Scaling does
not necessarily imply a full frame buffer; modern scalers
make do with a few lines' worth of buffering, unless
frame rate conversion is also required - in which case at
least a good deal of a frame's worth of data must be stored,
and in the best versions a full frame buffer or two of memory
is used.
> Even if the clocks align, there's also the matter of
> whether or not the analog signal has completely slewed
> to the value needed. If the DAC-cable-ADC path has
> bandwidth-limited (softened) the transitions, or
> introduced color-to-color skews, that will show up.
> I see it, or something like it, doing analog on my LCD.
Sure - but you can't really lay the blame for having a BAD
analog interface on analog connections in general. The
point is that a very good interface is still most definitely possible
in the analog domain, and is in fact achieved quite often. There
are also analog systems which take advantage of the rather
forgiving nature of analog to enable truly cheap and nasty
cables, connectors, etc., at the expense of performance.
Digital, as noted, either works or it doesn't - which is a big
part of the reason that digital interfaces are not as inexpensive
as the cheapest (and lowest quality!) of the analog types.
You simply HAVE to meet a certain minimum level of
performance with digital, or you don't get to play AT ALL.
> > ... the New Analog Video Interface standard, or simply
> > NAVI. ... It's not clear yet how well NAVI will be
> > accepted in the industry, but it IS available if
> > anyone chooses to use it.
>
> I suspect it's irrelevant at this point. Analog is
> the "economy" graphics connect now, and what we have
> is sufficient for the market.
Possibly; we'll see how it plays out. While digital
interfaces are becoming a lot more popular, analog
connections still account for well over 80% of the
video actually being used in the desktop monitor
market, even though LCDs took over from CRTs
as the unit volume leader this past year. As you know,
a gargantuan installed base has certain advantages
(or problems, which is often a different word for the
same thing!
🙂).
Bob M.
Facebook
Twitter
Instagram