Re: AppleColor Composite Monitors discussion

[ferdimh@gmx.de]

On 4 Mrz., 17:15, MdntTrain <j...@cimmeri.com> wrote:
> Related to this luminance bandwidth topic, we determined awhile back
> that in Apple II 40 col text and HGR, the "dot clock" was 7mhz..
> meaning that dots are being sent out at that rate.   For 80 col text
> or DHGR, the dot clock would be 14mhz.   Further, you discovered that
> for IIgs 320 mode, the rate was 8mhz, and for 640, 16mhz.
>
> Two questions about this.  1) How would someone with an oscilloscope
> go about verifying these dot clock rates, say for starters just on a
> IIe?   Obviously, the scope gets hooked up to the video out and the
> timings set to see one horizontal line.. but what would we need to
> have on the screen and then look for in the waveform to count this dot
> clock?
>

One can test it with a "worst case" test pattern that has alternating
black and white dots all over the screen.
If you monitor the video signal with an oscilloscope, you'll see a
square wave of half the dot clock, because there is one pixel every
dot clock which alternates the signal. You can use your oscilloscope
to find out the frequency, don't use a frequency counter, because the
square wave is not there during retrace,


> 2) How does dot clock relate to needed bandwidth?   If a 14mhz dot
> clock is used for 80 columns, how does the Applecolor or IIgs RGB
> monitor handle that with an 8mhz bandwidth?    Is it because of half
> cycles, such that 14mhz requires a 7mhz analog bandwidth?
>
I don't know what you mean by "half cycles" but your conclusion is
right. Because the highest possible frequency is half the dot clock
(see above) the monitor only needs to handle that. But the square wave
then becomes a sine wave which doesn't produce a sharp display. But
this problem is probably masked by the low-quality CRT. It's not of
great concern for white text on black background anyway. The other way
round text becomes unreadable more early.

I found out that (at least in Germany) many TV sets switch the luma
channel to high bandwith if there is no color signal. This seems to be
true for multisystem units or units with multisystem decoder chips
that are limited to PAL by other means.
I've got a 14" Lifetec and a 12" Samsung with nearly the same circuit
board. Both have multisystem decoders where the 3,58 MHz crystal
needed for NTSC mode is missing. In the 14" Lifetec I've added a
3,58MHz crystal and got a nice monitor for my Apple //e. They have a
bandwith way above 7MHz.

And a friend of mine has a Panasonic 21" multisystem TV which provides
an image which is as good as on a green-screen monitor. I don't know
which picture tube is used but it doesn't seem to be a real TV tube.
One can actually count the scanlines!

I've seen some more, e.g. one Grundig 26" set, which provides readable
80-col text.


---
Ferdinand