Recording - March 2013 - 24

audio—there could be other factors at work, as we’ll
see a bit later on. For example, a buffer setting of 64
samples on my MOTU 828 MkIII FireWire interface
on a system with an Intel i7 (nicknamed “Nehalem”)
processor produces occasional crackles in my audio.
However, I’ve easily done 64 samples on several PCI
card-based solutions on less powerful machines. For
more information and considerations, read the sidebar on Interfaces and Plug-Ins.

Selecting an appropriate buffer size comes with
tradeoffs, and what works for one musician is out
of the question for another. I know musicians who
say a 512-sample buffer at 44.1 kHz works perfectly fine for them. As a keyboard player, I cannot
tolerate anything over 256 samples before the lag
becomes noticeable. For a drummer, it seems anything higher than 128 samples becomes a bother.
Almost all cards require more CPU grunt for lower
sample buffer settings, so while it’s best to experiment, as a rule use the highest buffer setting that you
or others you’re working with can tolerate. This will
save precious CPU resources for more important
things like being able to process even more plug-ins
in real time. However, if you crank up the buffer to the
highest settings and you still have issues, read on.
The ugly truth about DPC
My experience with Windows 7 (two and a
half years at the time of writing) tells me that 7 is
the best Windows OS to date in regards to stability and “just working” without a lot of fuss.
However, we need to further dig into the machine
to figure out what is causing pops and clicks in
the audio beyond too low of a buffer setting. In
general, my first recommendation these days is to
check on how bad DPC latency is on the system.
Not versed on DPC? Let’s take a look.
DPC is short for Deferred Procedure Calls, and
they are the bane of real-time audio and video
streaming, regardless if you’re a DAW user or not.
A DPC is a function of the Windows operating system that allows for a higher-priority task, such as
an interrupt handler, to make a lower-priority task
wait while the higher-priority task takes over and
24

RECORDING March 2013

completes its job. If handled properly, this is not an issue and allows the higher-priority task to complete without major upsets to the system. However, if your
audio card happens to play second fiddle to your Ethernet adapter, which it just
happens to be interrupting every two seconds because of a poorly programmed driver, this can wreak havoc on a dedicated audio system.
As one of the first things you should do when you get a new machine, pay a visit
to the Thesycon site and download their wonderful (and free) tool called DPC
Latency Checker. (www.thesycon.de/deu/latency_check.shtml). It’s a straight-forward tool that shows DPC spikes on a graph in one-second intervals. See Figure 1.
Hopefully, your machine’s results will look like my screenshot, with fewer
than 1000 µs (1000 microseconds, or 1 millisecond) spikes consistently. If it’s
not, let’s go over some common causes and solutions.
Common causes of DPC spikes
Common causes of DPC spikes are wired and wireless Ethernet adapters,
modems, onboard sound adapters, add-on adapters in the PCI or PCIe slots, and
bad drivers. This is not to say something else might not be causing the spikes, but
it’s the Ethernet adapters that are the most common culprits in my experience.
All flavors of Ethernet adapters, wired or wireless, are generally cheap and
ubiquitous on any current machine. Between network chipsets that won’t behave,
and drivers that are poorly written, it is not uncommon to see DPC spikes occur
every few seconds while the network adapters are active. If, during troubleshooting, you determine the onboard wired or wireless Ethernet is at fault, see if you
can disable it in the BIOS settings. If this is not possible, then attempt to disable it
in the OS. Since many DAW users choose not to have their DAW connected to
the Internet while working with audio, this is possibly a non-issue for many users.
However, if Internet connections are needed, here are a few solutions.
If you only need to be connected to the Internet to do updates of the OS and
get the occasional patches for your DAW and plug-ins, then you can simply turn
the connection on when needed, then disable it when you are ready to record.
need
4 However, if you trying it during your sessions,
I’d recommend
to find a wireless USB
adapter. This is especially useful in the case of
a laptop where it’s not really possible to
replace its guts. It is hard to make direct vendor
recommendations, because chipsets change so
rapidly from day to day even for the same
make and model of cards and adapters. In this
case, it is best to buy local and make sure you
can return the adapter easily if need be.

Driver problems
Another culprit for audio issues is the dreaded case of “the crappy drivers”.
Not all drivers are created equal, and any poorly coded driver for any device
can cause problems with audio playback. The obvious suggestion is to make
sure you have the absolute latest and greatest drivers for everything from the
manufacturer. Even if you have installed the latest, the issues might still persist.
Here are a few things you can try.
1) Roll back to a previous version of the driver. The latest driver might have
introduced a new issue that wasn’t there previously.
2) Try the chipset maker’s driver instead of the driver from the vendor of the
card. For example, even though your video card is made by Vendor X, its
chipset is likely made by Intel, nVidia or ATI/AMD. Try the reference driver
from that chipmaker’s webpage instead to see if it fixes the problem.
3) Roll back to using the driver that came with Windows directly.
4) Use the “generic” driver that’s listed for the device class within Windows.

http://www.thesycon.de/deu/latency_check.shtml

Recording - March 2013

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