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Quick Start Guide 7
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Quick Start Guide
©2012 PS Audio Inc. All rights reserved.
Owner’s Reference
PerfectWave DAC MKII
arrive at the same time). The issue from an audibility standpoint is then how much group delay
is audible and acceptable? Research has shown the ear is relatively insensitive to group delay
distortion of several milliseconds for low frequencies (less than1kHz) and insensitive to +/-0.5ms
over the 1-5kHz band. Other work shows that the sensitivity to group delay distortion falls after
4kHz and therefore group delay distortion in the upper regions of the audio band is much less
audible. For a typical minimum phase filter designed for 44.1kHz the group delay distortion up
to 10kHz is under 2 samples (less then 46μs) and may be inaudible. The minimum phase filters
used in the PWD have group delay that we feel is inaudible.
Pre-ringing (or Pre-echo) is an interesting type of distortion. It is a type of additive distortion to
the waveform that occurs before the actual event (sound) occurs and is a product of linear phase
FIR digital filters. The ear appears to be very sensitive to this type of pre-echo because re-ringing
rarely occurs naturally. As you can imagine it is very rare to hear the artifacts of the distortion
before the originating sound reaches the listener.
Apodising and slow roll off response
The PWD offers both linear and minimum phase filter choices, along with a Recursive Non-Half
Band type, which is a combination of both. Within each choice of linear and minimum phase
filters the user has the ability to choose either Apodising or Soft Knee choices.
An Apodising filter is one that can be used to control the time smear of a whole recording
and reproducing chain. This type of filter can reduce the pre- and post-ringing of the impulse
response in both linear as well as a minimum phase filter choices.
Soft Knee filters take advantage of the larger transition band to reduce the dispersion and delay
through the filter. Instead of the classic “Brick wall” filter approach of cutting everything above
the passband off very sharply, a soft knee filter does this in a gentler fashion, thus minimizing
group delay problems.
Three of the filters (LP Apod, MP 1/2B, LP Soft) will have a slight high frequency roll off within the
audio band (20kHz) when you are using 44.1kHz modes (Native and 44.1) and playing 44.1kHz
material (CD’s). We have included graphs of the frequency response of every filter for you to see.
The audible effects of this roll off are minimal at best. The most extreme roll off in the series of
filters is LP Apodising, which is -0.2dB down at 19kHz and -3dB down at 20kHz.
All 3 filters will display ruler flat frequency response to 20kHz, while playing 44.1kHz material
if you increase the sample rate to a minimum of 48kHz although we do not believe this is
necessary or warranted.
While no Audiophile wants to hear that there is anything lost in the music, the facts are pretty
clear: these small frequency deviations with different filters at the lower sample rates have very
minimal impact on performance.
The filter choices
1. AUTO. This will automatically choose filter MP Soft for 44.1kHz and LP Soft for any
higher sample rates. These are the choices we believe work best.
