VGA on Compact CRT? — #1

Post #1 by Bunsen

Source Forum	68kMLA
Category	Hardware
Post Date	Thu, 26 Nov 2009 - 06:10
Original URL	https://68kmla.org/bb/threads/vga-on-compact-crt.19126/

Post

Preliminary research and thoughts:

Inspired by the recent spate of video hacking on compact Macs, EvilTim's MPxPlayer, and of course the Poor Man's Greyscale Project, I'm wondering how to go about converting the CRT and analog board of a compact to a VGA monitor.

Ideally this would be plug and play with later Macs and PCs without requiring software hacks (as used in the MPxPlayer). And unlike the PMGS, I am looking at using the existing a/b and yoke, rather than cannibalizing another monitor.

Yes, I realise this would require extensive modifications, and may turn out to be impractical. I just want to investigate possible approaches to see whether it is remotely feasible.

This would be a step on the road to an LC630 (or similar) in SE hack.

Resolution........................................ 512 x 342 Horizontal scan rate.......................... 22.3KHZ, (4µs on, 40µs off)

Vertical scan rate ............................. 60HZ (180µs on, 16.4ms off)

the video signal is 15.6672 MHz

The VGA specifications are as follows:
* Selectable 25.175 MHz or 28.322 MHz master clock

* Maximum of 800 horizontal pixels x 600 lines

* Refresh rates at up to 70 Hz

* Vertical blank interrupt

* 0.7 V peak-to-peak

* 75 ohm double-terminated impedance (18.7mA - 13 mW)

Signal

For most common VGA mode 640x480 "60 Hz" non-interlaced the horizontal timings are:

Clock frequency 25.175 MHz

Horizontal pixels 640

Horizontal sync polarity Negative

Total time for each line 31.77 µs

Front porch (A) 0.94 µs

Sync pulse length ( B) 3.77 µs

Back porch © 1.89 µs

Active video (D) 25.17 µs

The vertical timings are:

Vertical lines 480

Vertical sync polarity Negative

Vertical frequency 59.94 Hz

Front porch 0.35 ms

Sync pulse length 0.06 ms

Back porch 1.02 ms

Active video 15.25 ms

640 x 400 @ 70 Hz is video mode used for booting most x86 personal computers.

640 x 480 @ 60 Hz is the default MS-Windows graphics mode with 16 colours.

So at a minimum, I'm looking at the following conversions:

Code:

Spec             Mac       VGA     Unit     x
=================================================
Clock frequency  15.6672   25.175  MHz      1.61
Resolution:  H  512       640      pixels   1.25
         :  V  342       480      pixels   1.40
Scan Rate :  H   22.3      31.476  kHz      1.41
         :  V   60        59.94   Hz       1
=================================================