In the process of trying to figure out how I can interface video with the Megascreen 3 video expansion card in my Macintosh SE, I’ve done a bunch of research on video signal standards. So, here it is.
First of all, information on the ECL video standard, which was used on Sun and Apollo workstations. It’s almost the same as the Megascreen 3 DE-9 connector, but not quite. The logic levels match, 5 V TTL, but pin assignments don’t quite match.
20191024/ECL video signals
20191024/https://allpinouts.org/pinouts/connectors/computer_video/ecl-video/
20191024/https://web.archive.org/web/20100103055725/http://www.monitorworld.com/Cables/video_standards.html
This looks like a good ECL to VGA adapter. It appears to use active circuitry, so it can handle odd-ball signals. Then again, it is designed specifically for the Atari TT, so I’m not sure if it is sufficiently general for other systems.
20191024/DuckDuckGo ecl video adapter
20191024/https://shop.inventronik.de/index.php?id_product=28&controller=product&id_lang=2
Interestingly, there was an Atari workstation built that also used ECL video.
20191024/https://en.wikipedia.org/wiki/Atari_TT030
This is a useful page on video signal adaption between TTL and VGA. VGA is 0.7 V peak to peak, so not compatible with TTL, except for the sync pulses which are 5 V.
20191024/Google ecl video adapter
20191024/http://www.epanorama.net/faq/vga2rgb/interfacing.html
20191024/http://www.repairfaq.org/REPAIR/F_vidconv.html#VIDCONV_023
20191024/DuckDuckGo vga 0.7V
20191024/http://microvga.com/faq/electrical/what-are-vga-voltage-levels
Composite video and component video is 1 V. Technically, the standard polarity is reversed so it is -1 V. Both for RCA jacks and for BNC connectors. So if you want to connect TTL video to VGA, you need to step down the voltage, the easiest way is using 450/470 ohm resistors. The proper voltage conversion relies on the assumption of an internal 75 ohm resistance inside the monitor… yeah, like 75 ohm BNC connectors. Otherwise, you provide an external termination resistor.
20191024/DuckDuckGo composite video voltage
20191024/http://www.broadcaststore.com/pdf/model/793698/TT148%20-%204053.pdf
20191024/http://www.bradsprojects.com/generating-video-signals-with-a-microcontroller/
20191024/DuckDuckGo composite video voltage ntsc
20191024/DuckDuckGo bnc video voltage
20191024/https://en.wikipedia.org/wiki/BNC_connector
20191024/https://electronics.stackexchange.com/questions/149511/50-ohm-terminator-voltage-divider
20191024/DuckDuckGo bnc component video voltage
How do NTSC composite video signals work? Here is some great information.
20191024/DuckDuckGo composite video voltage
20191024/http://www.broadcaststore.com/pdf/model/793698/TT148%20-%204053.pdf
My biggest question was how the horizontal sync and vertical sync signals work in NTSC composite video. First of all, the video signal is “inverted”: a full value means black and zero means white. And actually, black is not the full value. The horizontal sync signal is the full value and corresponds to “blacker than black.” It is held high for the full time to do a horizontal return. So, that makes it real easy, essentially it’s exactly the same as VGA horizontal sync, except for voltage differences.
The vertical retrace signal is the trickiest signal to interface compatibly with a VGA output signal. In abstract, you combine the vertical retrace signal and horizontal retrace signal using an XOR gate. However, at the vertical retrace signal region, the horizontal retrace actually need to be spaced a half-line apart rather than a full line apart. The time span of this half-line coding must span 9 fields. This is so that the television circuitry can detect this is an NTSC vertical. Also, keep in mind the requirement for logical “one” and “zero” must be contained within the voltage logic levels of the black and blacker-than-black region.
The color burst explanation also explains why my cheap “VGA box” has the flickering colors problem when displaying a grayscale signal. It fails to detect that there is no color burst signal and switch to grayscale mode accordingly.