My new 15' extension cable makes my Samsung SyncMaster 912n look less
sharp than the shorter cable it replaced. I'm going to return it, but
don't know how to ensure the replacement will satisfy me. I have some
questions.
1) The unsatisfactory product
(
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=245598&CatId=425)
labels itself as a VGA cable, where as some other cables I've seen
describe themselves as SVGA. I overlooked this detail when ordering.
Since I'm running 1280x1024 at 60Hz using true color I'll definitely
choose a SVGA cable next time, but how are the cables different? 15
pins should be connected all the way through, right?
2) Is it reasonable to expect a 15' video cable to perform well?
3) Does this one look reasonable?
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1104144&CatId=425
I couldn't find a good answer for you, so I'll just give you the best
pages I turned up so far:
"cable lengths & degradation"
http://episteme.arstechnica.com/eve/ubb.x?a=tpc&s=50009562&f=67909965&m=409005253631
Basically, one kind of coax has 3 times the loss in DB of another kind
of coax, at 100MHz. The tradeoff could well be diameter of the cable assemnly,
and a cheap low loss coax in the cable would likely have a larger diameter.
(It is possible to get small diameter low loss coax, but my generalization
here, has to do with what is available as a commodity.)
One post in the ARS thread, suggests looking for a bandwidth of about four
times the fundamental of the signal. This page lists basic bandwidth
requirements
for various display resolutions (the lower resolutions, so this web page
isn't going
to help someone driving a 30" display with analog interface, 100 feet from the
computer).
http://www.ecs.ru.acad.bg/kp/less/dispalys/VD-site/specs/bw2.html
res Pixels/screen Bandwidth Hsync Rate
1280x1024x60Hz 1310720 78.6Mhz 61.4Khz
1280x1024x66Hz 1310720 86.5Mhz 67.6Khz
1280x1024x72Hz 1310720 94.4Mhz 73.7Khz
Multiplying bandwidth by four gives 320MHz minimum, and bandwidth is
3db down at 100ft.
This is an example of what I found by searching for "low loss VGA".
It is 4dB attenuation per 100ft at 100Mhz, versus the 10dB per 100ft
of regular VGA cables. This cable is missing the DDC signals (so the
computer cannot query the monitor using SCLK and SDAT, a serial bus
protocol).
http://www.avcable.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=SVGA&Category_Code=VGA_CABLE
There is some basic parameters on various kinds of coax here. The
coax is the material inside the VGA cable, that carries the R, G, B
color signals, and perhaps the Horizontal and Vertical sync signals.
They are supposed to be maintained in a 75 ohm impedance environment.
The other signals in the cable are just ordinary unshielded wire:
http://www.timesmicrowave.com/content/pdf/tl14/40.pdf
Two kinds of cables here...
http://store.haveinc.com/Ebus30/Parts/Default.asp?Commodity=CAV095
"Performance" cables - Nominal Loss: 11.3 dB/100 ft at 150 MHz
"Premium" cables - Nominal Loss: 3.7 dB/100 ft at 135 MHz
More differentiation of cable types used here. Note that by their
own definition, and the observable diameter of the cables, the
stuff they are selling must be the worst of the three kinds shown
at the top of the page.
http://www.montest.com/vga-cable.html
My best guess at this point, is there are two classes of cables, and
with the "good stuff", you don't really know whether all pins will be
wired to allow DDC to work or not.
I still haven't figured out enough of this stuff, to tell you what is
actually required of the cable, to pass with 3dB attenuation, a
320MHz signal.
This will help explain why you need about 4x the pixel rate bandwidth
on the cable. A pixel is a "square wave" or more properly, a trapezoid,
with a limited rise and fall time. Being able to carry the third or
fifth harmonic of the fundamental pixel rate, gives the pixel a good
shape. A good looking square wave can be made with the fourier sum
of the first, third, and fifth harmonic sine waves:
http://www.tek.com/Measurement/App_Notes/Technical_Briefs/bw_rt/55_18024_0.pdf
Post back how the new cable works
And what you paid for it.
I really don't think $9.99 is going to do it.
Paul
