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Q. What's causing our PC audio glitches?

We recently upgraded our studio computer to a Pentium II 400MHz machine with 100MHz buss, 64Mb of RAM and a Yamaha 6416 CD writer. The graphics card is a Verge S3, and on moving the windows' scroll bars, clicks and crackles and pops occur in our audio. Even when these bars are not being moved, the recording suffers from the same problems. We have altered the Graphics Hardware Accelerator to minimum, altered DMA buffer sizes and memory per channel — all the settings recommended in the manual — but with no resolution. If you can be of any help, we would be grateful as we are at our wits' end!

Paul Stokes

Martin Walker replies: While you are on the right track with reducing the Graphics Hardware Acceleration from its default maximum setting, the big clue comes from your make of graphics card. The S3 chipset used in your Verge is well known for causing problems due to 'buss throttling'. Originally brought to musicians' attention by Greg Hanssen of Zefiro Acoustics, this has caused countless problems over the last few years, and happens when graphics‑card drivers are written in an inconsiderate way which allows them to completely take over the PCI buss whenever they feel like it. While this approach gives marginally faster graphic performance, and causes few problems for mainstream users, any interruption of the PCI buss during recording or playback of digital audio files will cause dropouts, giving rise to the clicks and pops that you are experiencing.

The classic test for such problems is to play back a WAV file from your chosen application running in a small (ie. not maximised) window. Then grab the title bar, pick up the window, and drop it elsewhere on the screen. You may get clicks and pops while you drag or when you finally drop the window, and in some cases the left and right channels may also get permanently swapped over after the drop. Although this problem originally seemed insurmountable with some graphics cards, most of the latest drivers allow you to disable this antisocial activity. Where there is a solution (there isn't always one), each make and model of card needs a slightly different approach, ranging from unchecking tick boxes labelled 'PCI bus Retries' to disabling bus Mastering.

S3‑based graphics cards like yours can be fixed by manually adding a line to your System.ini file. You can find this in the Windows folder, and open it using the Notepad utility. Look for a section headed [display], and under this type 'bus‑throttle=1'. Don't worry if your System.ini file doesn't have a [display] section — if there isn't one you can simply add both lines to the end of the file. This fix apparently works with both Windows 95 and Windows 98, though it only works for graphics cards with S3 chipsets, so don't bother adding this otherwise.

Incidentally, you can often narrow down audio glitch problems relating to the graphics card by noticing when and where they happen. As well as on moving the scroll bars, you can sometimes get problems when the screen redraws (when the cursor has scrolled to the right‑hand side of the screen) or even when opening a dialogue box. This may indicate buss throttling, but may also simply mean that you are running 'close to the edge' of your computer's processor capability. At this point, suddenly asking for more graphics to be plotted on screen pushes the overhead over 100 percent; the problem will disappear if you run fewer audio tracks or real‑time plug‑ins.