[comp.sys.acorn] multi-sync monitors

yorkr@p4.cs.man.ac.uk (Richard York) (03/01/91)

	I am about to buy an A540 + multisync monitor to replace my rather aged
model B. I have heard many conflicting stories about multi-sync
monitors, mainly concerning the limited number of modes that are usable
with them. I have been told that this is because the lower screen modes
of the arc have too low a vertical frequency rate, which the
multi-syncs can't cope with.
	The monitor I am specifically interested in is the Taxan 795, the one
with the sony trinitron tube. Apart from the improvement in picture
area that this gives does this have any other advantages over the 775,
770+ etc, as well as some of the NEC ones, eq the one featured several
times in the A540 literature.

	This is my first posting to usenet so lets hope it works ok!
	thanks,
		Richard York.
-- 
:----------------------------------------------------------------------:
: Richard York		         :   E-mail : yorkr@p4.cs.man.ac.uk    :
: 1st year Computer Engineering  :   Janet  : yorkr@uk.ac.man.cs.p4    :
: Manchester University		 :				       :

bdb@cl.cam.ac.uk (Brian Brunswick) (03/01/91)

In article <yorkr.667760222@p4.cs.man.ac.uk> yorkr@p4.cs.man.ac.uk (Richard York) writes:
>	I am about to buy an A540 + multisync monitor ...
>	The monitor I am specifically interested in is the Taxan 795, ...

Ok, I'll jump to this one, since I posted about the Taxan a while back.

There are basically two parameters to a mode that affect whether it
can be displayed by a given monitor - the horizontal sync rate and the
fram rate. For a normal TV, the hrate is 15KHz, and the frame rate
50Hz - so thats 300 lines (These figures are approximate) The standard
multisync modes like 20 double the hrate to 30KHz, and get 600 lines.

The trouble with the 795 (which is otherwise very nice) is that it can
only handle horiz rates of 30-57KHz, so any standard mode won't work,
though the multisync ones will.  However, it is possible to modify the
format of existing modes to bring them into this range. This is done
by loading a patch module - So if its a game that runs by some boot
sequence that you can't get at, you're well Zogged, as certain people
in Cambridge might say.

There are two other issues here as well. Firstly, a standard Archi can
only chuck pixels out at 24MHz, and with 8MhZ memory has a total
memory bandwidth of about 25Meg/s. So not all existing modes can have
their line rates increased sufficiently. There are hardware add ons
which increase the max pixel rate to 36MHz, but you are still stuck.
With a 540, I understand (check me on this) that it has such a mod
built in, and also has 12MHz memory. So these figures go up, and its
easier to fix things.

Secondly, a lot of games take their timing from the fram rate.  If
your replacement mode uses a different frame rate, games will
(attempt, and with an 540/ARM3 do quite well usually) to run fast.
However, the frame rate can easily be reduced by leaving large gaps
above and below the data, and adjusting the monitor to expand it
vertically - make sure you get a version of the patch module with this
ability.

Basically, for the Taxan795, make sure you see it running before you
buy, switch modes around, find out how to adjust it, and work out
whether you personally can stand the irritation.

But its a lovely picture - I recommend it.
Brian.Brunswick@uk.ac.cam.cl  Disclaimer.  Short sig rules!