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Amp Sims

September 28, 2009
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HERE’S AN EASY WAY TO IMPROVE AN AMP
simulator’s tone dramatically: increase the sample
rate at which it operates. Now, I’m not some
golden ears guy who insists that recordings
done at 96kHz sound sooo much better than
a CD’s standard 44.1kHz sample rate, but amp
sims are a special case—especially at high-gain
settings.

The sample rate is the rate at which audio
is converted to digital data. The faster the rate,
the more accurate the conversion—just like
how videos that show more frames-per-second
give smoother visual motion.

With most audio
sources, 44.1kHz is an adequate sample rate.
But overdriven/high-gain amp sims generate
harmonics that can extend beyond the highest
audio frequency a digital system was designed
to handle. This causes aliasing—a term for what
happens when the harmonics interact with the
digital system’s sample rate. The audible results
are noise, atonal harmonics, and a blurred,
often harsh sound. Raising the sample rate
helps minimize interaction with the harmonics.

Here’s how…

First, set your audio interface to 88.2kHz
or 96kHz (most current interfaces support
these rates). Then, open an amp sim in standalone
mode, and set its sample rate to match
your interface (Fig. 1). You should hear much
sweeter high-gain sounds. This isn’t just due
to less aliasing. It’s also because the filters that
are an essential part of digital-audio conversion
don’t have to work as hard, and can sound
more natural.

You don’t get something for nothing,
though. Higher sample rates stress your computer
more, so you may have to increase the
system latency to avoid audio dropouts. Fortunately,
with today’s fast multi-processor systems,
this is becoming less of an issue.
When using an amp sim as a DAW plug-in,
it may not be possible to run at higher sample
rates with complex projects. For example, some
other plug-ins might not work properly at
96kHz, and the demands on your computer
might be excessive.

One solution employed by
IK Multimedia and Native Instruments is to
include optional oversampling in their amp sims
(Fig. 2). This runs the amp sim at a higher
internal sampling rate than the project itself,
giving the benefits of a higher sample rate—
even in projects running at 44.1kHz or 48kHz.
Oversampling places more of a load on your
CPU, but far less than doubling the sample rate
of an entire project.

If you can’t raise a project’s
sample rate, check for places in amp sim programs
where you can enable oversampling—
your ears will thank you, because the overall
sound will be much more like a real amp.

Major props to Davide Barbi at IK Multimedia
and Patrick Arp at Native Instruments for providing
helpful background material.

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