Macintosh External Video Kit

[Mac128]

Anybody ever heard of the Macintosh External Video Kit designed by Apple for the original Macintosh? News to me ... The designer had this to say about it (sadly he does not have the schematics) :

It was composite video with a BNC output jack. As I recall it could be connected directly to the video input of any video projector or video monitor that could handle a wide range of video scanning rates, but ran at the Macintosh horizontal and vertical scanning rates which is different than standard TV, so it couldn't dirve a standard TV set directly.

 

[LCGuy]

How....awesome. I can honestly say I've never heard of that before...wonder if any have stood the test of time?

 

[Charlieman]

Referring to previous discussions about how difficult it might be to hook up a compact Mac to an external display, note the number of components on that board. Given 1984 technology, there is no way that you could put all of that in a simple adapter such as the Power R.

However, it is comforting to note that Apple made their own Hackintosh graphics boards.

 

[Bunsen]

Did you notice his ADB Device Simulator?

 

[Bolle]

I already heard of it and once saw a mac on ebay i think that had it built in. dunno for how much it went in the end tho.

 

[wally]

...sadly he does not have the schematics..

I just noticed that Larry Pina does, at least for something very similar (separate, not composite video). See Macintosh Repair and Upgrade Secrets, First Edition 1990 page 169 for the 128K to Mac Plus and page 297 for the SE and SE/30. The circuit probably can be further improved by non-inverting buffering the vertical sync also to give some ESD protection to the external monitor connection. The scheme is just a hex schmitt trigger inverter buffer (74LS14) that inverts and buffers specific video and horizontal sync signals, passes thru a vertical sync signal and leaves it up to the external monitor to do what it will with the pixel timing. I suspect the very older manually tunable monitors could cope with these scan rates but the newer multisync ones probably are optimized narrowly around specific standard modern timing specs.
Additional info: If you happen to have a widely tunable monitor or projector that accepts only true composite video on a single line input, see the following for some combiner ideas.

Here is a way to go from TTL levels of sync to a composite sync

http://www.tkk.fi/Misc/Electronics/circuits/vga2rgbs.html

Here is a crude sync and video combiner:

http://www.repairfaq.org/sam/vidconv.htm#nvcvgtc1

This combiner could be buffered with a video opamp configured as a signal adder to drive longer and terminated video cables to your monitor of choice.

None of these address the pixel rate and line scan timing mismatch problems that cause distorted and non-centered renderings.