     (If you are not using a PC with a PCI VGA accelerator card,
     you need not read any further....)

WARNING: Long post ahead!    updated 8/2/98

In the past this information has been suppressed, now it can be told...

   A good number of VGA card manufacturers are squeezing out a few extra
points on their winbench scores by locking up the PCI bus.  This is fine
for graphics and most systems on the PC (hard disks and such) don't even
notice the problem.... Unfortunately this can hurt the audio system in
a big way.
   Most audio cards use ISA/DMA to trickle samples over the bus one word 
at a time.  Even PCI cards such as the AMIII can be hurt by this problem 
because they trickle the data over the bus in tiny PCI transfers.  When 
another device illegally locks up the bus for more than 1/88200th of a 
second, there's a good chance you will lose audio samples resulting an a 
glitch in the recording or playback.
   How do you know if you're having this problem?  Try opening up your
favorite wave editing program, loading a wave file and hitting the play
button.  While the audio is playing, grab the top of the window (assuming
it is not maximized) and repeatedly pick it up and drag it to another 
location on the desktop.  On a soundblaster compatible card, the audio 
will glitch and pop while you drag the window.  On a ZA2 card, there is
a 50/50 chance the audio will swap L/R channels after such a glitch.  
On an adb card, not only can the right and left channels swap, but there 
is also a good chance the audio will be left in a glitchy mode that makes 
the audio repeatedly jump channels resulting in a high pitch scratchy noise.

   At this point in the discussion, I would like to stress that this is NOT
(I repeat NOT) the fault of the soundcard!  This is not even the fault
of the VGA card... it is in fact the fault of the VGA driver.  If you do 
not experience any glitching or distortion then you can probably ignore 
the rest of this post!  I've heard that the VGA drivers supplied by 
Microsoft (verses the drivers supplied by the VGA manufacturer) do not 
suffer from this problem.

   I think Matrox was the first to play with this, but it doesn't really
matter because most high performance VGA accelerator cards for the PCI
bus are doing the same thing now.  When a number of graphics acceleration
operations need to be performed, these commands are sent from the VGA driver
to the VGA card over the PCI bus.  The VGA chipset has a built in queue
that is capable of holding several accelerator commands.  Normally the driver
checks a status bit on the VGA card to tell if this queue is full or not.
If the queue is full, the driver waits for the queue to have a free space
before sending the next command.  Matrox discovered (and everyone soon
followed) that you could increase VGA performance by NOT CHECKING THIS
STATUS BIT!!!!!  What happens when you write blindly to a full queue of
commands on the VGA card?  The bus hangs... The bus master has started a
PCI transaction, but the target (the VGA card) can't accept the data yet
because it has no place to put it.  As soon as the VGA card has room for the
data, then the transaction can complete... but until that time the PCI bus
is completely locked up.  No PCI or ISA transactions can happen.  This can
take a long time (40 or more audio cycles) if the current VGA operation is
a huge BITBLT on a 24bit screen...  A 256 or 512 bit FIFO just ain't gonna
cut it.
   The only acceptable solution to this problem is to put the queue check 
BACK into the VGA driver.  I have been discussing this problem with some 
of the engineers at Matrox and Tseng labs and have some solutions for you! 
   Tseng labs has released a new version of their ET6000 VGA driver that 
behaves nicer to the PCI bus.. this driver is now trickling down to the 
STB and Hercules products that use the ET6000 chip.  For the Hercules 
Dynamite 128 card, there is a new driver on the Hercules BBS (not web 
page... don't ask me) called DV95112  (Version 1.12)  Using this driver, 
you need to add a special switch in your system.ini file.  Under the 
heading [Hercules] there is a line that reads "Optimization=0"  you will 
need to set this to "Optimization=1"... ta da, problem fixed.
   It turns out that Matrox has ALWAYS had a hidden back door switch to 
enable this check in the VGA driver.  If you are using the Matrox Millennium,
you will need to add the following lines to your system.ini file:

[mga.drv]
PCIChipset=1

This almost fixes the problem completely.... you will also need to disable
the "Use PowerGDI acceleration" feature in the Advanced Matrox setup
(Control Panel->Display Properties->MGA Settings->Advanced->Performance)
ta da... problem fixed.

[update August 1998]  It seems that the generic Trident cards 9685/9682/9680/9440
do NOT suffer any problems using the standard Microsoft Win98 drivers.

[update July 1997]   I've heard from the good folks at S3 that ->ALL<- S3
  drivers for all of their VGA cards (downloaded from www.s3.com) can be
  fixed by adding a line in system.ini under the [display] section... 
  After [display] add the line 'bus-throttle=1' so (for S3 drivers):

  [display]
  busthrottle=1

It seems that this fix also works with the latest Microsoft Win98 drivers for S3 cards.
If your Win98 system.ini file doesn't have a [display] section, you can add it at the
end of the file.

[update 2/11/98]   For owners of newer Matrox cards:
Go get the latest Matrox drivers from www.matrox.com.. (V3.80 drivers)
Go to screen properties (right click in main window)
Go to setting tab, Click on PowerDesk button
On V3.80 there is no longer a 'PCI Bus retries' option.
Uncheck 'Use Bus Mastering' (off)  .. at least this worked for me
On Pentium Pro machines, Uncheck 'Use Write-Combining'
Click on OK

[updated November 1997]  For owners of the #9 Imagine 128 series 2
It seems a new driver (version 4.102.36) is now available directly from
the folks at #9 upon request..


Unfortunately there are other VGA maker who have not
acknowledged this problem (not that Matrox or Tseng has formally done so
either).  And we as users, software manufacturers or hardware 
manufacturers need to get the word out that this is a VGA problem
and not a DMA problem!!!!!!  We need to drill it through the heads of
the VGA card makers that they can't get away with this B.S. without at
least having an OPTION to make the driver behave appropriately.

Have you ever been told to turn the VGA acceleration off???   or to reduce
the size of your VGA screen???   or reduce the color depth??? 

THESE ARE NOT ACCEPTABLE SOLUTIONS!!!

  Call your VGA manufacturer and tell them they need to fix the problem!
(You will probably need to get beyond Joe Tech Support, because he 
probably doesn't know anything about this... please inform him!)  

This message has been brought to you by Greg Hanssen 
copyright 1996 Zefiro Acoustics.   

PLEASE feel free to post this to any forum where the knowledge can be used.

