User Manual

Page 38 Duende Native User Guide
N
on Linear-phase EQ
P
hase shift in equipment is probably one of the most misunderstood topics in audio. Phase shifts often get blamed for
a
nything that is not classed as typical distortion. The reality is that all analogue EQs produce a deviation from linear phase
w
hich is specifically related to the shape of the EQ curve, and it is precisely these phase changes which produce the required
g
ain reductions. This is a very natural process which can be found in the audio characteristics of rooms, materials and
e
lsewhere.
Though it is not possible for us to detect phase in isolation, the phase shift of an EQ can often be the cause of the ‘magical’
sound of that device once the EQ signal is added back into the mix. For example, the Duende channel EQ derived from the
SSL analogue consoles – plenty of phase shift and plenty of magic!
In contrast, a linear-phase EQ delays all frequencies by the same amount. This can only be achieved in the digital domain by
using special techniques. While this process is very effective in certain situations (eg. ‘surgically’ removing troublesome
frequencies), it should not be considered as a superior tool for the majority of EQ tasks.
Linear-phase equalisers have become popular over the past few years due to the proliferation of powerful digital processors.
We decided not to go down that route with X-EQ and we think you’ll agree with that decision. Maybe linear-phase EQ is
something we will explore in the future but… who knows? An equaliser not only changes the relative frequency balance of a
signal but also its phase. If you EQ a sound until it sounds right or natural, then the chances are that you are EQing to make
it sound in phase with the rest of the mix. We need say no more on this subject.
Parallel Passive EQ
Parallel EQ exhibits quite different sonic properties to the familiar serial parametric EQ. We are generally used to hearing the
effect of one EQ band superimposed on another, as opposed to the band interaction inherent to a parallel EQ. Because the
bands are placed in a parallel configuration, phase cancellations and re-enforcements happen which is not always obvious
when first encountered.
Passive EQ is something that is found in old equaliser units and is generally known for its transparent and natural sound, but
has some problems associated with it. However, in the digital domain these shortcomings do not have such an influence.
A passive EQ does not have any gain elements, but can still have controls to seemingly boost frequencies as well as cut.
What actually happens is that the entire signal is cut by an amount, but the frequencies which are apparently ‘boosted’ are
simply not cut as much. Therefore the unit must attenuate either the input, the output, or both to allow enough headroom.
Unfortunately in the analogue domain, a 20dB reduction in signal level produces a 20dB increase in the noise floor. Luckily,
in the digital domain with a 64-bit floating point DSP, these issues do not remain.
In X-EQ – when the ‘parallel’ button is engaged – you are presented with a parallel passive EQ model which the original
designers of these devices could only have dreamed of. The noise floor can be disregarded due to the huge resolution
available in Duende plug-ins.
You may find yourself entering this mode more and more as you become familiar with the sonic signature. Larger gain changes
are possible without colouration, and boost starts to become something that is useable to a significant degree in a digital EQ!
Parallel EQ does however exhibit asymmetry in its boost and cut characteristics. But this is not such a bad thing as most
engineers would agree that boost is best done with low (wide) Q values and cut with a higher (narrower) Q.