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.

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
...
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.

"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 31 May, 13:27, PeterD wrote:
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.


No, I can manage potentiometers, just worried about damaging other
equipment should failure occur (shorting through the wiper while at
full scale for example) - hence my original suggestion of a seperate
signal amp which would be sacrificial in that instance. It was,
however, ~1am while i was writing it and it didnt come out quite as I
had planned it in my head so I can see why you thought that

On 31 May 2007 06:13:07 -0700, CoyoteBoy
wrote:

On 31 May, 13:27, PeterD wrote:
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.


No, I can manage potentiometers, just worried about damaging other
equipment should failure occur (shorting through the wiper while at
full scale for example) - hence my original suggestion of a seperate
signal amp which would be sacrificial in that instance. It was,
however, ~1am while i was writing it and it didnt come out quite as I
had planned it in my head so I can see why you thought that

I can't imagine how it would fail... High end of pot to the video
card, low end to ground, wiper to monitor/projector. Any failure there
would be rather unusual, to say the least.

That said, I've not seen any VGA outputs that won't survive some abuse
such as shorts. They are not high current devices... I don't think an
additional amplifier is worth the effort, afterall it could be the
failure point!

I can't imagine how it would fail... High end of pot to the video
card, low end to ground, wiper to monitor/projector. Any failure there
would be rather unusual, to say the least.

I was thinking if you had it faded "up" and then accidentally shorted the
wiper you could blow the card, im unaware of the protection on VGA cards and
overly aware of the lack of care/knowledge of students when handling
hardware so I like to be extra specially sure it cant cause damage,
especially as it may end up being used with something other than a basic PC
etc, or even have a bored idiot pushing the end of a biro into it. I just
like to cover my back!

That said, I've not seen any VGA outputs that won't survive some abuse
such as shorts. They are not high current devices... I don't think an
additional amplifier is worth the effort, afterall it could be the
failure point!

Thats the kind of info I needed really, I know they arent high current
devices but I've blown a few low current devices in my time with stupid
mistakes and accidental shorts - the input device *could* be a custom-built
high-end machine and I dont want to be liable for replacing the card, but as
you say its not overly likely to fail. I'll knock one up at the weekend and
let you know how it goes on.

Cheers
J

On Fri, 1 Jun 2007 00:42:32 +0100, "Coyoteboy"
wrote:

I can't imagine how it would fail... High end of pot to the video
card, low end to ground, wiper to monitor/projector. Any failure there
would be rather unusual, to say the least.

I was thinking if you had it faded "up" and then accidentally shorted the
wiper you could blow the card,

Just as likely as shorting any vga cable, right?

im unaware of the protection on VGA cards and
overly aware of the lack of care/knowledge of students when handling
hardware so I like to be extra specially sure it cant cause damage,
especially as it may end up being used with something other than a basic PC
etc, or even have a bored idiot pushing the end of a biro into it. I just
like to cover my back!

That said, I've not seen any VGA outputs that won't survive some abuse
such as shorts. They are not high current devices... I don't think an
additional amplifier is worth the effort, afterall it could be the
failure point!

Thats the kind of info I needed really, I know they arent high current
devices but I've blown a few low current devices in my time with stupid
mistakes and accidental shorts - the input device *could* be a custom-built
high-end machine and I dont want to be liable for replacing the card, but as
you say its not overly likely to fail. I'll knock one up at the weekend and
let you know how it goes on.

Cheers
J

Just as likely as shorting any vga cable, right?

True, of course you're right. Only if there is another box between the
short and the dead item, the blame (by the unknowing) may lie with the
box, not the short. It seems the general public assume that each box
will be a protective buffer for the item behind it. I dont like the
idea of being the person who has to argue it. If it were my own
hardware for use in my house I'd not even have questioned it!

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?!?!