Re: 1084 woes

[mjmahon@aol.com (Michael J. Mahon)]

Linards Ticmanis wrote:

>I guess either the quartz or the PAL chip TCA650 simply doesn't like to 
>be switched on and off very quickly. I'll try inserting a 220uF 
>capacitor between the transistor base and ground - combined with the 
>base current limiting 4.7kOhm resistor that should delay the activation 
>of the transistor for about 0.1s (if I calculated correctly), quite 
>enough to get the quartz across those 4 lines of text alive.

No crystal oscillator can be switched on quickly.  That is the nature
of a high Q circuit--it takes a while to settle down.

Instead, the output of the constntly-running oscillator can be gated
(ANDed with a control signal) very quickly, and this is the usual
means of switching a clock signal on or off.

Even if you can stabilize the color reference, it must b e supplied
on each line--including during vertical blanking--so that the color
monitor can maintain correct phase lock.  This is another "high Q"
circuit which must be kept supplied with a stable phase reference
or color rendition will suffer--usually after just a few lines without
a burst.

This is why the Apple does not switch the color burst off during
the text part of a mixed display--to do so would cause unspecified
behavior of the the "color killer" circuit in the monitor, and poor
rendition of the color for many lines after the burst was restored.

-michael

 Email:  mjmahon@aol.com
 Home page:  http://members.aol.com/MJMahon/