VGA signal through breadboard?

Hello,

I own a Spartan 3E Starter Kit, which I plan to use for crowd-
entertainment purposes. Since the 3-bit VGA output is way too limiting
for my project, I am planning to add a 12-bit VGA port to my Starter
Kit. To do so, my idea was to buy the Breadboard accessory board from
DigilentInc. I don't have much knowledge of high-speed signal
propagation so my question is: Will the VGA signal be noticeably
distorted by the breadboard? The signal's highest frequency would be
25MHz. The plan is to use a projector to display the pretty colours. I
understand using wire wrap would be a better solution, but I'd like to
avoid that if possible.

Cheers!


-checo

checo wrote:

> I own a Spartan 3E Starter Kit, which I plan to use for crowd-
> entertainment purposes. Since the 3-bit VGA output is way too limiting
> for my project, I am planning to add a 12-bit VGA port to my Starter
> Kit. 

Since MikeJ must be napping... ;)

<http://home.freeuk.com/fpgaarcade/displaytest.htm>

Regards,

-- 
Mark McDougall, Engineer
Virtual Logic Pty Ltd, <http://www.vl.com.au>
21-25 King St, Rockdale, 2216
Ph: +612-9599-3255 Fax: +612-9599-3266

"Mark McDougall" <markm@vl.com.au> wrote in message 
news:465659ec$0$17960$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
> checo wrote:
>
>> I own a Spartan 3E Starter Kit, which I plan to use for crowd-
>> entertainment purposes. Since the 3-bit VGA output is way too limiting
>> for my project, I am planning to add a 12-bit VGA port to my Starter
>> Kit.
>
> Since MikeJ must be napping... ;)
>
> <http://home.freeuk.com/fpgaarcade/displaytest.htm>
>
> Regards,
>
> -- 
> Mark McDougall, Engineer

Hi Guys,
I wonder if you could improve the colour depth by just increasing the dot 
rate. So, instead of just on or off for each pixel, you could have 0, 1/2, 
1. Probably good enough for "crowd entertainment"?
Cheers, Syms. 

> Hi Guys,
> I wonder if you could improve the colour depth by just increasing the dot 
> rate. So, instead of just on or off for each pixel, you could have 0, 1/2, 
> 1. Probably good enough for "crowd entertainment"?
> Cheers, Syms.

Hi,

this could work on an old crt monitor. If you use a tft screen, the vga 
signal will be captured one time per pixel, so you will not get the avarage 
over one pixel.

G�nther 

"Symon" <symon_brewer@hotmail.com> wrote in message 
news:f367up$elm$1@aioe.org...

>>> I own a Spartan 3E Starter Kit, which I plan to use for crowd-
>>> entertainment purposes. Since the 3-bit VGA output is way too limiting
>>> for my project, I am planning to add a 12-bit VGA port to my Starter
>>> Kit.
>
> I wonder if you could improve the colour depth by just increasing the dot 
> rate. So, instead of just on or off for each pixel, you could have 0, 1/2, 
> 1. Probably good enough for "crowd entertainment"?

Haha, that's evil :-)

I'd say overclock the DAC by 4x, leaving the rest of the sync signals as 
they were, and then you have five colour levels (2.322-bit colour) at the 
same resolution. I don't think there's a VGA DAC on the market that can't 
cope with 100MHz operation.

Use some temporal dithering to very the bit-pattern that's sent, e.g. for 
half-brightness you want to alternately send 0101 and 1010; for 
three-quarters swap between 1110, 1101, 1011 and 0111, etc. etc.

I'm almost tempted to try this myself, just to prove that it can be done :o)

    -Ben- 

On May 25, 6:09 am, "Ben Jones" <ben.jo...@xilinx.com> wrote:
> "Symon" <symon_bre...@hotmail.com> wrote in message
>
> news:f367up$elm$1@aioe.org...
>
> >>> I own a Spartan 3E Starter Kit, which I plan to use for crowd-
> >>> entertainment purposes. Since the 3-bit VGA output is way too limiting
> >>> for my project, I am planning to add a 12-bit VGA port to my Starter
> >>> Kit.
>
> > I wonder if you could improve the colour depth by just increasing the dot
> > rate. So, instead of just on or off for each pixel, you could have 0, 1/2,
> > 1. Probably good enough for "crowd entertainment"?
>
> Haha, that's evil :-)
>
> I'd say overclock the DAC by 4x, leaving the rest of the sync signals as
> they were, and then you have five colour levels (2.322-bit colour) at the
> same resolution. I don't think there's a VGA DAC on the market that can't
> cope with 100MHz operation.
>
> Use some temporal dithering to very the bit-pattern that's sent, e.g. for
> half-brightness you want to alternately send 0101 and 1010; for
> three-quarters swap between 1110, 1101, 1011 and 0111, etc. etc.
>
> I'm almost tempted to try this myself, just to prove that it can be done :o)
>
>     -Ben-


I think the point is that the OP doesn't have a DAC, just three 1 or 0
outputs from the FPGA, one to each color signal, possibly through an
external buffer.  Or you could say he has "one bit" DAC's.  Dithering
will work with a low-pass filter, but if there are additional pins
available it would be better to use a resistor ladder to make
a simple DAC.  TFT VGA displays generally have a very narrow sampling
window (see the Analog Devices AD9888 datasheet for instance) which
is centered in the pixel time slot.  This system works best when there
is no low-pass filter smearing out the beautiful contrast of the
original image.  By the way, the video AC bandwidth is significantly
higher than the pixel rate in order to give the sharp edges between
pixels and flat voltage mid pixel.  Generally it's important to match
the cable impedance from the DAC output to the board connector, or
if this isn't possible to reduce the length of the mis-matched portion
as much as possible, preferably to much less than a pixel time in
terms of propagation.

"Gabor" <gabor@alacron.com> wrote in message 
news:1180097090.950740.115520@k79g2000hse.googlegroups.com...
>
> I think the point is that the OP doesn't have a DAC, just three 1 or 0
> outputs from the FPGA, one to each color signal, possibly through an
> external buffer.

Ah, OK. That would admittedly make more sense than having a 3-channel 8-bit 
video DAC and just wiring all the input pins together :-)

> TFT VGA displays generally have a very narrow sampling window (see
> the Analog Devices AD9888 datasheet for instance) which
> is centered in the pixel time slot.

True; as someone else pointed out this is not true of CRT displays. Not sure 
about overhead projection technology, which is probably what the OP has. 
Entertaining crowds using LCD panels is probably going to blow the budget... 
:)

Cheers,

       -Ben- 

On 24 mayo, 22:37, Mark McDougall <m...@vl.com.au> wrote:
> checo wrote:
> > I own a Spartan 3E Starter Kit, which I plan to use for crowd-
> > entertainment purposes. Since the 3-bit VGA output is way too limiting
> > for my project, I am planning to add a 12-bit VGA port to my Starter
> > Kit.
>
> Since MikeJ must be napping... ;)
>
> <http://home.freeuk.com/fpgaarcade/displaytest.htm>
>
> Regards,
>
> --
> Mark McDougall, Engineer
> Virtual Logic Pty Ltd, <http://www.vl.com.au>
> 21-25 King St, Rockdale, 2216
> Ph: +612-9599-3255 Fax: +612-9599-3266

Thanks for the heads up! The problem is I don't trust my soldering
skills enough to do it myself :P, but I'll consider the option. I just
don't want to risk ruining the board since having a replacement
shipped over here to Mexico is not easy nor cheap nor fast (for a poor
student like myself).

checo

On May 25, 10:13 am, checo <chec...@gmail.com> wrote:

> Thanks for the heads up! The problem is I don't trust my soldering
> skills enough to do it myself :P, but I'll consider the option. I just
> don't want to risk ruining the board since having a replacement
> shipped over here to Mexico is not easy nor cheap nor fast (for a poor
> student like myself).

Just wire up your accessories onto a connector that _plugs in_ to the
FPGA board.  Either solder directly to the back of the connector pins,
or use a little piece of pre-etched PC board like you can (I think
even still) get from the likes of radio shack (if you have them down
there).

This is no more risky for the board than connecting it to a
breaboard... in fact it's less risky, as the almost certainty of wires
coming out of the breaboard and at some point falling against a power
supply voltage is reduced - by soldering to a connector, you only have
one chance to mess up, and if you get past that it should stay
connected the right way.

Or you could just stop owrrying and be practical... I've been known to
put resistors directly into the pin sockets on the spartan 3 kit to
connect to various things.

On 25 mayo, 07:44, Gabor <g...@alacron.com> wrote:
> On May 25, 6:09 am, "Ben Jones" <ben.jo...@xilinx.com> wrote:
>
> I think the point is that the OP doesn't have a DAC, just three 1 or 0
> outputs from the FPGA, one to each color signal, possibly through an
> external buffer.  Or you could say he has "one bit" DAC's.  Dithering
> will work with a low-pass filter, but if there are additional pins
> available it would be better to use a resistor ladder to make
> a simple DAC.  TFT VGA displays generally have a very narrow sampling
> window (see the Analog Devices AD9888 datasheet for instance) which
> is centered in the pixel time slot.  This system works best when there
> is no low-pass filter smearing out the beautiful contrast of the
> original image.  By the way, the video AC bandwidth is significantly
> higher than the pixel rate in order to give the sharp edges between
> pixels and flat voltage mid pixel.  Generally it's important to match
> the cable impedance from the DAC output to the board connector, or
> if this isn't possible to reduce the length of the mis-matched portion
> as much as possible, preferably to much less than a pixel time in
> terms of propagation.

Thank you, Gabor!

My cheap multimeter says the resistance of both, the wires and the PCB
traces on the board, is exactly zero. They do match! :)

Now seriously, according to my calculations the "pixel length" would
be 8 meters, so I guess I'm safe on that front. And yes, I understand
that the spectrum of a digital signal contains very high frequency
components. I was just mentioning the 25MHz number to give a (very)
rough description of the signal.

As for the alternate methods of displaying colors, I have already
tried them with unsatisfactory results. As Gh=FCnter mentioned, changing
the color value mid-pixel works only in CRT displays; and not even in
all CRTs, as some would display nasty Moir=E9 patterns. Even more, I
want to be able to easily convert the VGA signal to NTSC. I think this
solution would make that somehow difficult.

The other method I tried was using spatial and temporal dithering. I
grouped pixels in 2x2 squares. Half the pixels in the group would be
one color, the other half another color. If the viewer was far enough
from the screen the illusion would work. To enhance this solution, I
made de pixels swap positions every time the screen refreshed. For
example to display "orange" the 4-pixel group would toggle between:
(R is red, Y is yellow)

RY
YR

And:

YR
RY

With this method I raised the number of colors from 8 to 27 (a lot of
combinations looked gray), which still wasn't enough. Also, it halved
my resolution.

So, back to the breadboard, does anyone have a prediction? Will it
work? The fact that I am not being laughed at is strangely
encouraging. :)

-checo