Classic Mac Video Signals Demystified Designing a Mac-to-VGA Adapter with LM1881 | Hacker News Reader

Classic Mac Video Signals Demystified Designing a Mac-to-VGA Adapter with LM1881

bigmessowires.com

74 pointszdw12 comments

Short of using microcontrollers or just discrete parts, the same function can also be obtained by less specialized chips such as the 74LS123. Here's an application schematic from Elektor.

https://www.worldradiohistory.com/hd2/IDX-UK/Technology/Tech...

The hero we really need, nice work.

Screenshots include cameo of much-loved (by me anyway) network tank game, Bolo.

https://en.wikipedia.org/wiki/Bolo_(1987_video_game)

Ha nice. Programming 'Bolo Brains' or whatever they were was a cool challenge at the time.

Can someone smart in EE explain why the author wants to avoid using a microcontroller? Cost per unit isn't an issue for this type of project, it makes things easier, allows to employ more advanced logic like sensing the monitor type to decide what monitor ID to send to the computer, you can have LED's and so on. It already requires 5V power for the chip, what would be the downside apart from added complexity?

Someone was able to output 1080p(ish) with an RP2040, so yeah, it'd be doable.

https://hackaday.com/2023/03/31/could-1080p-video-output-fro...

The monitor signal is analog which means you'ld need a three-channel ADC to get it captured. might be able to do it with 4 or 6 bits per channel on a single rp2040, but you're probably getting into small FPGA (ice40?) territory there.

Maybe an pi5 could handle all three channels in 8-bit, depending on how the RP1 actually works. I'm hoping that it can handle a 40msps 10-bit ADC - a Domesday Duplicator hat would be wonderful.

The other option would be to find a vga->hdmi convertor that likes the signal coming out from this board, then you could plug that into anything modern.

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When I first saw this I thought it was about an actual 9" classic mac, which I guess would be useful but IMO those are really designed around the CRT too much to make a replacement 'fit' well.

My approach would just be to sample one channel, like green, with the ADC and generate the sync signal, avoiding digital processing of the video itself. Just pass it through with any necessary level shifting in analog.

That'd definitely work.

RGBtoHDMI will convert the classic mac video to HDMI https://tinkerdifferent.com/threads/my-little-guide-on-rgbto...

It is also useful for several other kinds of machines such as the Amiga

Interesting read, albeit the comments seem to suggest it's clickbait and a thought exercise, rather than a PoC or working solution. I was thinking more about an MCU that supplements the LM1881, not replaces it.

The part where he talks about avoiding a micro is in regards to detecting activity on the various sync signals to turn on LEDs which is really simple and easy to do without having to resort to a micro

The microcontroller can't really help deal with the dip-switch mess because it would have no way of sensing whether the monitor supports Composite sync/Sync-on-green etc

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