I'm looking to create a VGA fader - I have a projector that takes a VGA
signal from a PC, but I want to be able to fade that signal down to black
between projections, manually using a slider. I know there are production
items out there but they are in the region of upper hundreds and over-spec'd
for what I need. I just need to be able to fade in and out one input.

I cant find, on a brief hunt, the VGA signal spec but I would guess its got
a sync line and 3 AC signals for pixel brightness. I hear it may be ~0.7v
p-p but i dont have that confirmed. Would it be fairly simple to create a
variable gain amp to maintain 1:1 or lower gain to fade the signals to black
without losing sync with the projector (it pops up a blue screen when the
cable is unplugged). Obvioulsly one amp on each analogue line.

Any hints/pointers would be of great help!

J

"Coyoteboy" wrote in message
...
I'm looking to create a VGA fader - I have a projector that takes a VGA
signal from a PC, but I want to be able to fade that signal down to black
between projections, manually using a slider. I know there are production
items out there but they are in the region of upper hundreds and
over-spec'd for what I need. I just need to be able to fade in and out one
input.

I cant find, on a brief hunt, the VGA signal spec but I would guess its
got a sync line and 3 AC signals for pixel brightness. I hear it may be
~0.7v p-p but i dont have that confirmed. Would it be fairly simple to
create a variable gain amp to maintain 1:1 or lower gain to fade the
signals to black without losing sync with the projector (it pops up a blue
screen when the cable is unplugged). Obvioulsly one amp on each analogue
line.

I don't think you can gracefully fade a digital signal.

"Charles" wrote in message
...

I don't think you can gracefully fade a digital signal.

VGA is analog.

--

Brian Gregory. (In the UK)

To email me remove the letter vee.

"Brian Gregory [UK]" wrote in message
...
"Charles" wrote in message
...

I don't think you can gracefully fade a digital signal.

VGA is analog.

Yes it is. But how about the synch pulses? Decreasing amplitude might
reduce contrast to a point, but then synchronization will be lost.

"Charles" wrote in message
...
Yes it is. But how about the synch pulses? Decreasing amplitude might
reduce contrast to a point, but then synchronization will be lost.

Why would one even think about fading the sync pulses?
They're on separate lines in the VGA connector.

--

Brian Gregory. (In the UK)

To email me remove the letter vee.

On Tue, 29 May 2007 17:04:30 -0400, "Charles"
wrote:


"Coyoteboy" wrote in message
...
I'm looking to create a VGA fader - I have a projector that takes a VGA
signal from a PC, but I want to be able to fade that signal down to black
between projections, manually using a slider. I know there are production
items out there but they are in the region of upper hundreds and
over-spec'd for what I need. I just need to be able to fade in and out one
input.

I cant find, on a brief hunt, the VGA signal spec but I would guess its
got a sync line and 3 AC signals for pixel brightness. I hear it may be
~0.7v p-p but i dont have that confirmed. Would it be fairly simple to
create a variable gain amp to maintain 1:1 or lower gain to fade the
signals to black without losing sync with the projector (it pops up a blue
screen when the cable is unplugged). Obvioulsly one amp on each analogue
line.

I don't think you can gracefully fade a digital signal.


Standard VGA is analog, not digital.

A quick version would require a triple ganged pot, perhaps 150 ohms
each (one for each color signal). I guess three sliders with some sort
of homemade handle to gang 'em together might do just fine.

I don't think you can gracefully fade a digital signal.


Standard VGA is analog, not digital.

A quick version would require a triple ganged pot, perhaps 150 ohms
each (one for each color signal). I guess three sliders with some sort
of homemade handle to gang 'em together might do just fine.

That makes sense.

"Charles" wrote in message
...


I don't think you can gracefully fade a digital signal.


Standard VGA is analog, not digital.

A quick version would require a triple ganged pot, perhaps 150 ohms
each (one for each color signal). I guess three sliders with some sort
of homemade handle to gang 'em together might do just fine.

That makes sense.


I'm concerned about shorting the signals to ground. I'll experiment with a
cheap video card and see what is required to pull the signal low without a
full short. It'd be fairly easy to set this up with a set of digital pots
too, is my thinking, and enjoy the lack of mechanical failures (as a
version2).

"Coyoteboy" wrote in message
...

"Charles" wrote in message
...


I don't think you can gracefully fade a digital signal.


Standard VGA is analog, not digital.

A quick version would require a triple ganged pot, perhaps 150 ohms
each (one for each color signal). I guess three sliders with some sort
of homemade handle to gang 'em together might do just fine.

That makes sense.


I'm concerned about shorting the signals to ground. I'll experiment with a
cheap video card and see what is required to pull the signal low without a
full short. It'd be fairly easy to set this up with a set of digital pots
too, is my thinking, and enjoy the lack of mechanical failures (as a
version2).

http://www.maxim-ic.com/products/digi_pot/

Good luck and don't worry about shorting to ground with a potentiometer
(unless the load on the wiper arm is a short).

On Thu, 31 May 2007 01:02:40 +0100, "Coyoteboy"
wrote:


"Charles" wrote in message
...


I don't think you can gracefully fade a digital signal.


Standard VGA is analog, not digital.

A quick version would require a triple ganged pot, perhaps 150 ohms
each (one for each color signal). I guess three sliders with some sort
of homemade handle to gang 'em together might do just fine.

That makes sense.


I'm concerned about shorting the signals to ground. I'll experiment with a
cheap video card and see what is required to pull the signal low without a
full short. It'd be fairly easy to set this up with a set of digital pots
too, is my thinking, and enjoy the lack of mechanical failures (as a
version2).



That reply leads me to believe you have not a clue about what you are
doing. Best just buy the setup in this case. If you can't wire up a
set of potentiometers this is beyond your abilities.

On Tue, 29 May 2007 19:34:29 +0100, Coyoteboy wrote:

I'm looking to create a VGA fader - I have a projector that takes a VGA
signal from a PC, but I want to be able to fade that signal down to black
between projections, manually using a slider. I know there are production
items out there but they are in the region of upper hundreds and
over-spec'd for what I need. I just need to be able to fade in and out one
input.


Where are you?!?!