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Section 16
What is Digital Aliasing?
OK before the noise was identified as aliasing I called it
'YUK Noise'. No it is not an official term. Never has been,
never will be. But it is the best way to sum up this particular
noise in one word. What it is, is a horrible 'someone tuning
a radio' type underlying harmonic type thingyme type
noise—and it is YUK!
In order to define and tell you about digital aliasing, allow
me to quote from an article in the May 1998 issue of Sound
on Sound (SoS) magazine called 'ONE BIT AT A TIME:
All about digital recording'. The full article may be found at
http://www.sospubs.co.uk/sos/may98/articles/digital.html
In
analogue
additional signals, above the original audio
components, which are related harmonically -- as
in second or third harmonic distortion. These may
be undesirable, but being 'musically' related to the
source they tend to be acceptable -- even
beneficial -- if sufficiently mild. However, aliasing
in digital signals produces tones that are not
musically or harmonically related to the source
signals. Instead, they are mathematically related
to the sampling rate and the aliases appear below
the signal which caused them. Since this is a very
unnatural phenomenon, the ear can detect
aliasing even when there is only the smallest
trace of it, and it is particularly unpleasant.
The visual equivalent of aliasing is strobing -- the
classic example being the wheels on the stage
coach in an old western movie which appear to be
going backwards slowly when in reality they are
going forwards very quickly! The true rate of
rotation is too fast for a film camera (with its slow
24 frames-per-second 'sampling rate') to capture,
but the 'alias' of that rate is visible as a much
slower, often reversed, rotation.
As it sounds so unpleasant, it is essential that
aliasing is not allowed to occur in an audio
sampling system, and the original analogue audio
is low-pass filtered to ensure that nothing above
half the sampling frequency can enter the system.
This filter is called an anti-alias filter (since that
is what it filters!), and it prevents the lower image
and the audio signal from overlapping.
The sharpest low-pass audio filters normally
encountered on mixers or synths might have rolloff
slopes of up to 24dB/octave, which means that for
each doubling of frequency (i.e. octave rise in
pitch), the signal is attenuated by a further 24dB.
However, to achieve sufficient isolation between
the wanted audio signal and the unwanted lower
© Copyright 2000-2002 Barry S. Pearce et al.
Digital Aliasing (YUK Noise)
systems,
distortion
produces
image in a sampled signal, we need perhaps 90 or
100dB of attenuation for an anti-alias filter -- and
there is typically only a fraction of an octave to do
it in! Anti-alias filters therefore have to be
extremely steep, with rolloff slopes in the order of
200 or 300dB/octave, and in order to allow them
sufficient space to achieve a useful degree of
attenuation, the Nyquist theorem requires that the
sampling rate is actually around 2.2 times the
highest wanted audio frequency.
The worst thing about aliasing is that you can only hear it in
a particular situation. The rest of the time you cannot hear it
as such. However, this doesn't mean it isn't affecting you—it
is – and it can muddy your sound, making solos and chords
sound bad.
Now the GT-3/5 is not a perfect digital unit. The sampling
rates used just do not give it enough room to sufficiently
remove all aliasing artefacts. However, despite the fact that
the unit will show signs of aliasing in almost every situation,
the artefacts of that aliasing are normally so quiet that they
are masked by the main signal going through the unit.
However, what I am going to show you know is two patches
that highlight digital aliasing (quite audible artefacts
present) and one, which although extreme in its levels, does
NOT produce any additional increase in volume of the
aliasing artefacts. Firstly with a clean preamp we will
increase the affect of aliasing, then we will look at the
OD/DS to try and re-create the problem here and finally
with a dirty preamp.
I will say now that the OD/DS version does not really suffer
from this problem. It is a COSM PREAMP ONLY problem.
The GT-3, GT-5, VF-1 suffer from it, and I've seen reports
on many other Roland COSM based units that suffer
aliasing problems too.
I believe the reason for the problem is that when the
COSM preamps model distortion, they also produce
harmonics outside the range of the normal sampling
frequency. This results in a misread of the frequencies –
digital aliasing. The aliasing artefacts produce here are,
unfortunately, rather audible compared with any other
negligible artefacts produced by the GT-3/5 normally.
The aliasing is most discernible at high gain. Once I have
shown you how to highlight aliasing I shall give you some
recommendations on how to resolve it. To help you identify
the sound there is a very small 10sec .wav file that I have is
available on my bands website called GTx-YUK-
Noise.wav.
Which
http://www.breakingtaboo.com/gear/boss_gt-x.
Increasing Digital Aliasing
To increase aliasing, set-up each of the patches as I show
below and then bend the 2nd string at the 15
can
be
found
th
fret, up one
at
82
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