[sdiy] Multiplexing (Atari video out)

[Hallvard Tangeraas]

On 21-06-07 10:29, Dave Manley wrote:
> Hallvard Tangeraas wrote:
>> It's all about AND/OR/YES/NO type electronics based upon the status 
>> of two physical switches and a logic high/low signal.
>> I've drawn all of this as "logic boxes" in my otherwise schematic 
>> diagram:
>> http://img405.imageshack.us/img405/3559/vga19bv6.png
> A couple of questions and comments:
>
> 1. What are the input voltages and frequencies into the multiplexer?
a) Horizontal sync
    - 31.5KHz (high resolution mono video mode), 1V peak to peak
    - 15.75KHz (low/mid resolution colour mode), 1V peak to peak
 
b) Vertical sync
    - 72Hz (high resolution mono video mode), 1V peak to peak
    - 50-60Hz (low/mid resolution colour mode), 1V peak to peak

c) Composite sync
Not sure about this, but according to Wikipedia the peak to peak is 1V 
(http://en.wikipedia.org/wiki/Composite_video)
I don't know about the frequencies though, but it should be pretty 
standard. Perhaps someone else here knows more about the details 
surrounding composite video?

d) +12V

e) mono detect
    - 5V

There's some more details available here 
(http://www.howell1964.freeserve.co.uk/Atari/ST/Atari_ST_Video.htm), but 
I think I've just about covered it.

> 2. Are the various sync signals digital or analog?
I'm really no expert, but I assume we're talking plain old analog.

> 3. Generally you don't want to have the outputs of two devices 
> wire-or'd together (the blue and violet boxes)
Yeah, I was getting a little lost there.

> 4. Generally you don't want floating inputs (B input on mux, IN2 on 
> red box)
Forget about input B. It's just the remainder of an earlier drawing I made.
I assume 2 inputs will suffice.

> 5. Have you selected a multiplexer?  The one you show seems to complex 
> for the application, you just need a 2-1 mux (ie you're not using the 
> B outputs).
No, I haven't chosen which multiplexer to use. Frankly, I don't really 
know what a multiplexer is, but after a lot of searching and reading I 
came to the conclusion that an "electronically controlled solid state 
switch" is know as a multiplexer/demultiplexer... I think.


> Make a truth table, for all the input conditions, define the outputs A 
> and C:
>
> Inputs                  Outputs
> =====================   ========
> MonoDetect    S1   S2   A  C
> 0             0    0
> 0             0    1
> 0             1    0
> 0             1    1
> 1             0    0
> 1             0    1
> 1             1    0
> 1             1    1
>
>
> Once you have this table it is easy to determine the logic function 
> necessary to drive the A and C inputs. 
The closest I've come to a truth table is this diagram:
http://img405.imageshack.us/img405/5424/vgalogicblockdiagram19co6.png

The problem is designing it so that U3 and U4 don't interfere with each 
other.


Hallvard