Date   : Thu, 19 Jan 2006 23:01:00 +0100
From   : "W.Scholten" <whs@...>
Subject: Re: SCART

David Hunt wrote:

> On the TV upstairs (a 1988 vintage 25" Sony) I just use it for the Master so
> the lead is nice and simple. No resistors anywhere, 3.8v to RGB and 4.1v to
> sync. You've obviously got to turn down the contrast, but the picture is
> absolutely pin sharp. As bright as a Microvitec but as sharp as a Philips.

Right, my dad made a cable for the first TV with SCART I got ca. 1986, 
and it had no resistors either. I had to turn down contrast as well 
(enormously I might add!).

Regarding the 74LS86, I'd noticed that (I've not updated the web page 
though) and I think the output was not at all designed for any TV but 
for special monitors. TVs with SCART were just getting used (anywhere 
but France) around 1985. Other earlier options seem to have been 
specially made (IIRC ads for nordmende TVs in the mags for example) so 
would take into account the micro.

I also recall the high impedance input issue. Again, this is probably an 
old TV (or SCART specification) issue (possibly another reason why an 
old SCART cable didn't work on a newer, ca. 1994, TV).

I measured the input on the sync on my TV (which is a few years old) and 
it definately has 75 Ohm impedance and it works fine with the 82 Ohm 
resistor. So my comment that the actual level of sync might not be very 
important as TVs can decode them anyway, is still valid as the 0.4 mA 
max output at '1' of a 74LS IC will just give .03V (it will be a bit 
more than .4 mA before the voltage drops that much of course).

As to the sync/composite, I guess my brain took a timeout as composite 
is not used but sync uses the same input. I guess 17 needs to go to 0V 
then also....


-- 
Wouter
---
BBC microcomputer information: http://www.xs4all.nl/~swhs/whs/