TTL RGB ==> PC

[Alexis]

>Eventually I'll be shipping various 8-bit machines to the US, and was
>thinking
>of using a PC with a TV card as a display (most of the machines have UK
>PAL RF
>outputs, and I could add modulators to the ones that don't).
>
>However:
>
>   - All TV card software I've seen has been utter crap, when it even works
>     at all.
>   - Typically there seems to be no scaling of the picture to fill the PC
>     screen.
>   - There's an obvious quality drop in the RF stages.
>   - I'm warned that the digital tuners in TV cards often have problems
>     locking on to the weaker signal produced by home micros.
>
>So... how about hooking straight to the TTL RGB outputs of the vintage
>machines and somehow sampling lines of data into the PC for display in a
>window (with appropriate scaling in software as/when necessary so that the
>image more or less fills the PC display)?
>
>Surely someone's homebrewed something like this already? I presume the speeds
>at which things need to work can be pretty high - but in theory it's just an
>RGB framegrabber but without all the analogue-type circuitry needed to decode
>a picture?
>
>I'm planning on shipping a few TTL RGB displays to the US, but maintaining
>them is going to be more difficult than maintaining the machines (and I
>imagine there are all sorts of pitfalls in taking a US-designed TTL RGB
>display and trying to use it with a UK-designed micro, as the frequencies
>involved won't be quite the same)
>
>thoughts welcome...
>
>Jules

Hi,

First, have you tried Linux with tvtime and a boring old analogue BT878
capture card? There will always be quality loss when transmitting over RF
so if you don't want that then it's probably best to seek something else.

Also does it have to be attached to a PC? You could try adapting the RGB
output to a PC VGA monitor directly.

>From the minimal research I've done the VGA standard appears to use 1Vp-p
RGB signals terminated to a 75 ohm load. So to convert from TTL RGB to VGA
it should be something like this (hope you use a fixed font!):

                             O +5v
                             |
                             \
                             /
                             \  300R
                             /
                             \
                             |
          |\                 |
 Luminance| \       51R      |
        O-|  >----/\/\/\/----o
          | /                |
          |/  7405           |
                             |
          |\                 |
  R/G/B   | \       27R      |
        0-|  >----/\/\/\/----o------------O VGA Out terminated
          | /                |              to 75 ohms.
          |/                 \
                             / 100R
          7405               \
          (OC non-           /
          inverting)         \
                             |
                             |
                           -----
                            ---
                             -

Whew. Good to brush up on Ohm's law. What this might produce is a voltage
of approximately 0.7V when both inputs are high, 0.5V when the Luminance
input is low, and about 0.2V when the R/G/B input is low, when terminated
into 75 ohms... maybe.

If you blow up a nice monitor I assume no responsibility. Also I have not
tested this at all :)

If you copy this circuit three times for the R, G and B outputs you should
be able to wire it up to a VGA monitor. But now we need sync. Do your
vintage computers have two lines for sync? One for horizontal and one for
vertical? Or is it some sort of combination or even sync-on-green?

Cheers,

Alexis.