FPGA NTSC signal with 2 resistors and PWM

Hi

I am evaluating the possibility to use 2 resitor video output from
FPGA similar to:

http://www.excamera.com/articles/15/ntsc.html

to generate color NTSC output, so far my results show less quality
then the test picture in the link above
I use 300/600 ohm with Xilinx Spartan3A starterkit

I am not yet sure if the difference in visible output is from the
different receiver USB video grabber vs TV receiver, but the video as
grabbed with cheap 25EUR grabber is very noisy, almost like the 80MHz
would be pass the input filtering so that the 16 shaders are almost
not recognizeable, also there is too much noisy flickering at the top
the screen. I will be testing with real TV set later, so I can compare
better.

so just asking if anyone has any compare results or suggestions for
ultra low cost and simple direct video output with FPGA, there is one
design with 9 resistors and 2 bipolar transistors, or then R-2R type
networking, both are too complex, if a PWM based solution could work
with some little neat tricks

Antti

On Sep 28, 10:34 am, Antti <Antti.Luk...@googlemail.com> wrote:
> Hi
>
> I am evaluating the possibility to use 2 resitor video output from
> FPGA similar to:
>
> http://www.excamera.com/articles/15/ntsc.html
>
> to generate color NTSC output, so far my results show less quality
> then the test picture in the link above
> I use 300/600 ohm with Xilinx Spartan3A starterkit
>
> I am not yet sure if the difference in visible output is from the
> different receiver USB video grabber vs TV receiver, but the video as
> grabbed with cheap 25EUR grabber is very noisy, almost like the 80MHz
> would be pass the input filtering so that the 16 shaders are almost
> not recognizeable, also there is too much noisy flickering at the top
> the screen. I will be testing with real TV set later, so I can compare
> better.
>
> so just asking if anyone has any compare results or suggestions for
> ultra low cost and simple direct video output with FPGA, there is one
> design with 9 resistors and 2 bipolar transistors, or then R-2R type
> networking, both are too complex, if a PWM based solution could work
> with some little neat tricks
>
> Antti


A television will have a low-pass filter on the video and may look
better.  If you didn't add at least a capacitor (1 pole LPF) after
your PWM, the digital grabber will no doubt show the effects of
sampling the 80 MHz PWM signal.

Noise at the top of the screen may indicate some problem with
sync levels.  My guess is that a proper LPF might fix this, too.

Good luck,
Gabor

On 28 Sep., 17:39, Gabor <ga...@alacron.com> wrote:
> On Sep 28, 10:34 am, Antti <Antti.Luk...@googlemail.com> wrote:
>
>
>
>
>
> > Hi
>
> > I am evaluating the possibility to use 2 resitor video output from
> > FPGA similar to:
>
> >http://www.excamera.com/articles/15/ntsc.html
>
> > to generate color NTSC output, so far my results show less quality
> > then the test picture in the link above
> > I use 300/600 ohm with Xilinx Spartan3A starterkit
>
> > I am not yet sure if the difference in visible output is from the
> > different receiver USB video grabber vs TV receiver, but the video as
> > grabbed with cheap 25EUR grabber is very noisy, almost like the 80MHz
> > would be pass the input filtering so that the 16 shaders are almost
> > not recognizeable, also there is too much noisy flickering at the top
> > the screen. I will be testing with real TV set later, so I can compare
> > better.
>
> > so just asking if anyone has any compare results or suggestions for
> > ultra low cost and simple direct video output with FPGA, there is one
> > design with 9 resistors and 2 bipolar transistors, or then R-2R type
> > networking, both are too complex, if a PWM based solution could work
> > with some little neat tricks
>
> > Antti
>
> A television will have a low-pass filter on the video and may look
> better.  If you didn't add at least a capacitor (1 pole LPF) after
> your PWM, the digital grabber will no doubt show the effects of
> sampling the 80 MHz PWM signal.
>
> Noise at the top of the screen may indicate some problem with
> sync levels.  My guess is that a proper LPF might fix this, too.
>
> Good luck,
> Gabor- Zitierten Text ausblenden -
>
> - Zitierten Text anzeigen -

ha, eh the PWM is clocked with 80MHz and for 16 grey levels the PWM
frew is 5 MHz :(
yes i already tried some cap for filter, it makes it a very little
better only

for color signal i guess at least the R-2R thing is needed
I will try without by using 500MHz clocked delta sigma, that should be
much better then 80mhz clocked PWM but not sure if even that is
useable for color.

Antti

"Antti" <Antti.Lukats@googlemail.com> wrote in message 
news:1190994565.617357.99810@50g2000hsm.googlegroups.com...
>
> ha, eh the PWM is clocked with 80MHz and for 16 grey levels the PWM
> frew is 5 MHz :(
> yes i already tried some cap for filter, it makes it a very little
> better only
>
> for color signal i guess at least the R-2R thing is needed
> I will try without by using 500MHz clocked delta sigma, that should be
> much better then 80mhz clocked PWM but not sure if even that is
> useable for color.
>
> Antti

At better than 100:1, I'd expect the 500 MHz delta to give good results. 
The filtering will be less of an issue for the delta scheme.

Until or unless you have the nice modulator configured, would you consider 
using some TV analog filter chips to clean up the video?  Your low-cost end 
solution might not use a part like the US$0.59 MAX9512 from maxim-ic.com, 
but it could make the initial development a little easier.

- John_H 

On 28 Sep., 18:56, "John_H" <newsgr...@johnhandwork.com> wrote:
> "Antti" <Antti.Luk...@googlemail.com> wrote in message
>
> news:1190994565.617357.99810@50g2000hsm.googlegroups.com...
>
>
>
> > ha, eh the PWM is clocked with 80MHz and for 16 grey levels the PWM
> > frew is 5 MHz :(
> > yes i already tried some cap for filter, it makes it a very little
> > better only
>
> > for color signal i guess at least the R-2R thing is needed
> > I will try without by using 500MHz clocked delta sigma, that should be
> > much better then 80mhz clocked PWM but not sure if even that is
> > useable for color.
>
> > Antti
>
> At better than 100:1, I'd expect the 500 MHz delta to give good results.
> The filtering will be less of an issue for the delta scheme.
>
> Until or unless you have the nice modulator configured, would you consider
> using some TV analog filter chips to clean up the video?  Your low-cost end
> solution might not use a part like the US$0.59 MAX9512 from maxim-ic.com,
> but it could make the initial development a little easier.
>
> - John_H

Hi John,

wau thanks for this link - I was considering that
http://www.jcwolfram.de/projekte/vhdl/fbas_enc/main.php

but I think feeding the maxim thing from FPGA would give better
results

the 0.59 may still be in the budget, well the total self cost are
below 12USD (hopefully havent got FPGA pricing yet) so it is more than
just peanuts, but not so bad

Antti

Antti wrote:
> Hi
> 
> I am evaluating the possibility to use 2 resitor video output from
> FPGA similar to:
> 
> http://www.excamera.com/articles/15/ntsc.html
> 
> to generate color NTSC output, so far my results show less quality
> then the test picture in the link above
> I use 300/600 ohm with Xilinx Spartan3A starterkit
> 
> I am not yet sure if the difference in visible output is from the
> different receiver USB video grabber vs TV receiver, but the video as
> grabbed with cheap 25EUR grabber is very noisy, almost like the 80MHz
> would be pass the input filtering so that the 16 shaders are almost
> not recognizeable, also there is too much noisy flickering at the top
> the screen. I will be testing with real TV set later, so I can compare
> better.
> 
> so just asking if anyone has any compare results or suggestions for
> ultra low cost and simple direct video output with FPGA, there is one
> design with 9 resistors and 2 bipolar transistors, or then R-2R type
> networking, both are too complex, if a PWM based solution could work
> with some little neat tricks
> 
> Antti
> 

That's interesting.  I think several of the Xilinx evaluation boards 
have VGA outputs with R-2R networks so you get a very limited set of 
colors.  I'm not sure why you wouldn't just use a video DAC, though. 
Are they too expensive?  They can be very small and low-resolution 
versions don't use too many inputs.
-Kevin

On 28 Sep., 19:43, Kevin Neilson <kevin_neil...@removethiscomcast.net>
wrote:
> Antti wrote:
> > Hi
>
> > I am evaluating the possibility to use 2 resitor video output from
> > FPGA similar to:
>
> >http://www.excamera.com/articles/15/ntsc.html
>
> > to generate color NTSC output, so far my results show less quality
> > then the test picture in the link above
> > I use 300/600 ohm with Xilinx Spartan3A starterkit
>
> > I am not yet sure if the difference in visible output is from the
> > different receiver USB video grabber vs TV receiver, but the video as
> > grabbed with cheap 25EUR grabber is very noisy, almost like the 80MHz
> > would be pass the input filtering so that the 16 shaders are almost
> > not recognizeable, also there is too much noisy flickering at the top
> > the screen. I will be testing with real TV set later, so I can compare
> > better.
>
> > so just asking if anyone has any compare results or suggestions for
> > ultra low cost and simple direct video output with FPGA, there is one
> > design with 9 resistors and 2 bipolar transistors, or then R-2R type
> > networking, both are too complex, if a PWM based solution could work
> > with some little neat tricks
>
> > Antti
>
> That's interesting.  I think several of the Xilinx evaluation boards
> have VGA outputs with R-2R networks so you get a very limited set of
> colors.  I'm not sure why you wouldn't just use a video DAC, though.
> Are they too expensive?  They can be very small and low-resolution
> versions don't use too many inputs.
> -Kevin- Zitierten Text ausblenden -
>
> - Zitierten Text anzeigen -

VGA is much simpler then color NTSC/PAL ;)

R-2R is an option of course, but i would rather use 2 resistors  then
the R-2R network

Antti

On Sep 28, 3:03 pm, Antti wrote:
> VGA is much simpler then color NTSC/PAL ;)
>
> R-2R is an option of course, but i would rather use 2 resistors  then
> the R-2R network

Here is what my experiments with four resistors looked like (the
camera wasn't a good one - the image was nicer live):

http://www.merlintec.com:8080/hardware/uploads/14/oliver7a.jpg

-- Jecel

On 29 Sep., 03:23, Jecel <je...@merlintec.com> wrote:
> On Sep 28, 3:03 pm, Antti wrote:
>
> > VGA is much simpler then color NTSC/PAL ;)
>
> > R-2R is an option of course, but i would rather use 2 resistors  then
> > the R-2R network
>
> Here is what my experiments with four resistors looked like (the
> camera wasn't a good one - the image was nicer live):
>
> http://www.merlintec.com:8080/hardware/uploads/14/oliver7a.jpg
>
> -- Jecel

Hi

hm, i browsed your website and found interesting things :)
and hints for more interesting thing without download links :(

hm by 4 resistor you mean 4 bit 2N DA stage or something else

on the test PCB I included 5 bit R-2R network, that could be little
better then 4 resistors,
but if delta sigma with high clock gives nice results I would go with
it, or then 4 R version ;)

Antti
ah, the results of the 2 resistor PWM greyscale also look better on TV
set then on usb capture device.
the 16 grey bars are all visible nicely.

On Sep 29, 5:31 am, Antti  wrote:
> hm, i browsed your website and found interesting things :)
> and hints for more interesting thing without download links :(

I have some random stuff at http://www.merlintec.com/download/ but
unfortunately that doesn't include any HDL code yet. I will try to
find the NTSC test next week (in VHDL) and make it available.

> hm by 4 resistor you mean 4 bit 2N DA stage or something else

I considered the output to be a 75 ohm resistor to ground, so just had
4 resistors (N, 2*N, 4*N and 8*N) between the FPGA pins and the RCA
connector. Video quality was not a requirement for this project (the
video output was only used by me during development and not by the end
customers) so I didn't bother adding capacitors to filter things a
bit. The DA was calculated to go from 0 to 1.5V in 0.1V steps with the
FPGA i/o programmed for 3.3V TTL levels. For black and white going to
1V would be enough, but for color you have to add a sine and need to
go higher.

> on the test PCB I included 5 bit R-2R network, that could be little
> better then 4 resistors,

You would get twice the brightness levels and twice the sine
amplitudes (saturations) for a total of four times as many colors. You
would also reduce the noise level, which would improve the image
considerably.

> but if delta sigma with high clock gives nice results I would go with
> it, or then 4 R version ;)

It is interesting to see what the delta sigma DAC can do, but it might
introduce too much jitter for the color circuits to work properly. On
the other hand, real TV circuits have limited bandwidth and might just
ignore all the extra noise.

> ah, the results of the 2 resistor PWM greyscale also look better on TV
> set then on usb capture device.
> the 16 grey bars are all visible nicely.

That is what normally happens. A digital video capture circuit is
probably more picky about the signals it accepts.

One cute trick I came up with (but did not implement in the test
circuit I mentioned) was using Xilinx 16 bit shift register slices to
generate phase modulated sines for the chroma circuit. For each bit
you cascade two of them, where the first is always 16 bits long and
feeds back to itself (at reset it gets the correct pattern to repeat)
and the second one varies from 1 to 16 bits of delay. You can do a
four bit generator with only 8 slices.

-- Jecel