Black Letters?

[joey]

Hi,

I’m working on an emergency communication project that involves video. The VE board is great and gets me the bare essentials of what I need to do.

I’m trying to do more and am not quite sure how to accomplish it because of a problem: the white lettering is getting washed out on daytime images. I’m having to increase to the max font size just to make it visible but then it doesn’t allow me to put enough data down.

What I really want is jet black lettering but I don’t see any way to do that. So I was thinking that I could invert the overlay lines but it comes out really fuzzy and you loose the image behind the overlay.

I need to be able to display crisp, readable data on the very top line and the very bottom line. It contains station ID, GPS coords, Time, Date, and Temperature.

Thanks,

Joey

[Michael]

Joey, I have been working on a solution for that. I have accomplished white letters on a black bar at the bottom of the screen. Would a black bar at the bottom and top of the screen solve your problem? The bar is not entirely black (dark gray) but I’m waiting for a new part that may help me make it darker. The readability is still much better than the current overlay.

The solution involves an n-channel MOSFET. Do you have any handy? If so, I can help you get started.

[joey]

@Michael wrote:

I have accomplished white letters on a black bar at the bottom of the screen. Would a black bar at the bottom and top of the screen solve your problem?

That will work too!

If you can tell me what I need to solder, and where, the board is simple enough that I should be able to do it without a problem.

Joey

[Michael]

Ok, here’s what I did. I used an IRF510 MOSFET but any n-channel enhancement-type MOSFET should behave the same. I would just breadboard this first to see if it works.

1) Connect the Video Experimenter INPUT pin (on the 6-pin breakout on the right of the board) to the MOSFET drain pin.
2) Connect the OUTPUT pin to the MOSFET source pin.
3) Connect Arduino digital pin 10 to the MOSFET gate pin.
4) Add pinMode(10, OUTPUT) to your sketch setup.
5) Remove the SYNC SELECT jumper from the Video Experimenter board.
6) Set the OUTPUT SELECT switch to SYNC ONLY.
7) Add these lines to the beginning and end of the function render_line5c() in the TVout library video_gen.cpp:

void render_line5c() {

// For the lines at the bottom of the screen, turn off the video feed by

// setting D10 low.

if ((display.scanLine > 216) && (display.scanLine < 231)) {

PORTB &= ~(_BV(2));  // video off for lower lines

}



...



// turn on video signal again by setting D10 high

PORTB |= _BV(2);

} // end of render_line5c

That gave me a dark gray bar at the bottom of the screen where I could overlay white text. You can add more dark lines at the top of your screen as needed. I did notice that my dark bar has a vertical line of white pixels at the far right — not sure why.

I want to achieve true black, but my MOSFET is not turning off the current completely. I have a 4066 analog switch on the way which I will try. You may also try different types of MOSFETs (like a p-channel depletion-type) if you have them. The logic might be opposite for different MOSFETS (gate HIGH to turn off current, LOW to turn on). You may know more about MOSFETs than me…

[acton]

If a semi transparent background (just dimmed) would be OK, then I can’t see why you couldn’t simply connect the output pin (Pin 10 in the example)to video via a resistor (say 100ohm).

Chris

[Michael]

A resistor won’t dim/blacken selective areas of the image being overlaid. I tried n-channel MOSFETs p-channel MOSFETs and the 4066 analog switch ic and found that the 4066 does the best job. The background is gray but looks good.

[variador]

How about just changing the letters to black without using the invert=2 or the white = 1 anywhere on the screen?
Haven’t gotten as far as to know what you were talking about in previous messages but it sounds interesting. Another passage to take someday.

Don

[tim]

Michael,

I did some experiments with the black bars.

First I had to add pinMode(10, OUTPUT); to my sketch as pin 10 was kind of high impedance before.

Remove jumper and set to sync.

1.2K resistor between D10 and output on 6 way connector

33ohm resistor between output and input.

Seems to work well

[attachment=0:w6uqguss]black bar.jpg[/attachment:w6uqguss]

[tim]

Hi Michael, Following on from your idea to use a MOSFET, I experimented with using a BS170 N-Channel MOSFET since I think the IRF510 is maybe a
little slow and has to higher capacitance. Drain to video input – Gate to Pin 10 – Source to pin video output.

I know its not completely black but its fine for my purpose and pretty simple. I have included some pictures of the board with MOSFET, looking at a normal scene and
look straight into a bright lamp. The text can be easily read.

Thanks again for the MOSFET idea.

Tim

[rtek1000]

Hello,

The Arduino output pin is low power, but if you use the driver, maybe the HCPL3150, the MOSFET will switch very fast!

http://pdf.datasheetcatalog.com/datasheet/hp/HCPL-3150060.pdf

Good luck!

[Author]

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