Simple circuit to generate VGA/Composite video? - Page 1
Forum section: electronics / projects

author: BlueApple
author_rank: Regular Contributor
timestamp: September 22, 2022, 01:45:37 pm
content: I need a simple circuit to generate VGA or composite video for a project I'm working on. I've found some online, but they all use some kind of microcontroller, which I'd like to avoid.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 22, 2022, 02:28:23 pm
content: this is about as simple as you are going to get:https://geoffg.net/picomitevga.htmla RPi PICO, with 3 diodes, 3 resistors, and a VGA socket:you just need to write a few lines of basic code to read commands from a serial, I2C or SPI port to control what ends up on the screen.cheers,rob :-)

author: BlueApple
author_rank: Regular Contributor
timestamp: September 22, 2022, 02:33:26 pm
content: That looks cool! I may try it.

author: BlueApple
author_rank: Regular Contributor
timestamp: September 22, 2022, 03:30:09 pm
content: Would it be possible to use a DAC?

author: MikeK
author_rank: Super Contributor
timestamp: September 22, 2022, 10:20:01 pm
content: If you're looking to avoid using a microcontroller then the circuit is not going to be simple. The Ben Eater videos posted above are good; he builds a simple graphic card for a simple 8-bit computer. If you want to learn more about video generation I recommend "TV Typewriter Cookbook" by Don Lancaster.

author: MikeK
author_rank: Super Contributor
timestamp: September 22, 2022, 11:36:22 pm
content: Saw this on Amazon:https://www.amazon.com/Signal-Generator-Tester-Output-Battery/dp/B01LW77ZAU

author: BlueApple
author_rank: Regular Contributor
timestamp: September 23, 2022, 11:45:52 am
content: Quote from: MikeK on September 22, 2022, 10:20:01 pmOkay, thanks.

author: Martian Tech
author_rank: Regular Contributor
timestamp: September 23, 2022, 04:02:29 pm
content: What is your starting point? What are you trying to display (text, graphics, monochrome, color) and where does it come from?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 23, 2022, 04:45:25 pm
content: An overclocked TV Typewriter?https://en.wikipedia.org/wiki/TV_Typewriter

author: BlueApple
author_rank: Regular Contributor
timestamp: September 23, 2022, 08:48:39 pm
content: Quote from: Martian Tech on September 23, 2022, 04:02:29 pmJust monochrome text.

author: MikeK
author_rank: Super Contributor
timestamp: September 23, 2022, 10:26:58 pm
content: Monochrome text without a microcontroller? Umm, why?How are you going to generate the text without a microcontroller? You have a character ROM?

author: Martian Tech
author_rank: Regular Contributor
timestamp: September 24, 2022, 12:41:34 am
content: Quote from: BlueApple on September 23, 2022, 08:48:39 pmInsufficient data.In general, you're going to need to generate timing signals (horizontal and vertical sync signals, dot clock and character row clock). There are video controller ICs that can help with this, but being general purpose devices they have internal registers which must be set. So to use one of there you'd pretty much need a processor of some sort to drive it. You haven't given any indication whether this *thing* you're trying to build already has a processor in it or not, so it's impossible to provide any kind of useful suggestions. It is possible to generate all the timing signals without using a CRTC chip (with discrete circuitry or by using a microcontroller) but without more detail nobody's going to be able to point you at a solution.The image is from the bookCRT Controller Handbookby Gerry Kane. Studying this book (as well as the previously mentionedTV Typewriter Cookbook) may give you a better understanding of what's required. A PDF can be found here:https://archive.org/download/crt_controller_handbook/crt_controller_handbook.pdf

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 24, 2022, 04:11:19 am
content: Quote from: MikeK on September 22, 2022, 10:20:01 pmThe OP didn't say what they want displayed. Unless I missed it.So it could be very simple if all they want is a solid background or a simple pattern.For the former, just a set of resistors to get the wanted color on the R, G, B signals, and for HSYNC and VSYNC, pretty simple logic could be used.

author: james_s
author_rank: Super Contributor
timestamp: September 24, 2022, 04:35:34 am
content: Quote from: BlueApple on September 22, 2022, 03:30:09 pmSure but what are you going to drive the DAC with? The DAC translates the stream of digital data into analog RGB levels, you still need to get that digital stream from somewhere. What are you trying to display?

author: BlueApple
author_rank: Regular Contributor
timestamp: September 26, 2022, 04:14:59 pm
content: I think I'm going to use a microcontroller.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 26, 2022, 04:21:21 pm
content: Quote from: BlueApple on September 26, 2022, 04:14:59 pmThey're essential, and once you put in the work on the front end you'll find many things are just easier than with discrete slapped-together stuff.

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2022, 05:21:11 pm
content: If you're interested in generating video, an obvious place to start is early video games. Space War, Pong, Breakout and several others from the 1970s did it without any microprocessor at all. Slightly later ones like Super Breakout, Sprint 2, Fire Truck, Super Bug and many others used a microprocessor to run the game but the video generation was still in hardware and all the microprocessor had to do was write values to a small RAM to display the appropriate character on the screen. If all you want is text, you can leave out the motion object circuitry and just implement the background video. Atari Dominoes is essentially the Sprint 2 design without the motion object stuff. Schematics for many of these can be downloaded.Or you can just do everything in a modern microcontroller.

author: CountChocula
author_rank: Unknown
timestamp: September 26, 2022, 05:28:34 pm
content: Here's a design using just discrete logic:https://hackaday.com/2021/04/30/vga-graphics-card-in-74xx-logic/Separately, tooting my own horn a little bit:https://github.com/mtabini/G76-MiniThis uses a CPLD, which istechnicallynot a microprocessorThe Verilog source code might be of interest, though, even just as a starting point, since it's portable to just about any logic device. I have this running as part of my SBC, and it's pretty good from a point of view of capabilities—discussionhere—you will find a bunch more designs on the forums at 6502.org.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: September 26, 2022, 07:15:28 pm
content: i got one of them text generator tubes stocked !! they where used like a video tube, but the "detector" screen is a thin metal sheet with ultra tiny holesthat makes up the entire alfabeth, scan this in sync with a CRT and you can display any text you like by ofsetting the scan arround each letter.now that would be a cool old vintage type of challengeno CPU needed,

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2022, 07:18:53 pm
content: Quote from: oz2cpu on September 26, 2022, 07:15:28 pmThose are neat. There are tubes used to generate test patterns too, I forget what they're called but they work very much like a camera tube except with a coated metal target etched with the desired image instead of a photosensitive surface.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: September 26, 2022, 07:21:52 pm
content: you are right, just never seen one with my own hands, maybe one day i power up this text generator tubeand perform some scanns on my scope, would be a super topic for one of my youtube videos :-)EDIT : just found mine, it is BRIMAR XR1000/07where to find datasheet :https://frank.pocnet.net/other/Brimar/Brimar_IndustrialCathodeRayTubes_DataSection_Issue3.pdfsee the last pages it is called MONOSCOPE TUBEalso see thishttp://tubetime.us/index.php/2018/06/04/a-vacuum-tube-rom/

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2022, 09:48:52 pm
content: Monoscope, yeah that's what the ones I was thinking of are called. I've never seen one in the flesh either but I've seen pictures of them, and some of the test patterns they were used to generate are well known. I'm just old enough to remember when some TV stations went off the air during the night, they would change to broadcasting a color bar test image with an audio tone. I remember waiting in the morning for the station to come on the air with morning cartoons when I was maybe 3 or 4 years old.