Technical information

And now the phase response. Again, the red curve represents standard digital EQ'ing, and
blue curve Pro-Q 2's Zero Latency mode: both have a non-ideal phase response. The green
line illustrates Pro-Q 2's Natural Phase mode response. Apart from a very small, inaudible
deviation above 20 kHz, it perfectly matches the theoretical analog phase response.
Linear Phase
When filtering audio, traditional analog and digital filters not only change the magnitude, but introduce phase changes as well. What
happens is that the phase of different frequencies in the signal is changed in different ways. This can have an audible effect on the
sound, but not necessarily in a bad way. Most of the time, for example for a simple bell or shelving filter, the phase effects are very
subtle and hardly noticable. However, for higher-order filters like steep low cut of high cut filters, the effect can become quite
apparent as the phase distortion starts to affect transients and can make the sound less transparent.
Moreover, problems arise when you mix a filtered and phase-altered signal with another similar signal that has not been filtered, or
that has been filtered in a different way. In this case, it is very likely that the different phase components of both signals won't
match up properly and will cancel each other to some extent. This situation can for example occur when mastering. It is quite
common to apply an equalizer only to a part of the song, using crossfades at the beginning and end of the affected region. Because
the phase information in the original and filtered parts is different, the fades won't work as intended.
Linear-phase processing provides an answer to these problems. Linear-phase filters only change the magnitude of the audio, while
leaving the phase untouched. However, linear-phase filters also have some disadvantages. First of all they introduce latency: the
entire signal is delayed when passing through the plug-in. Higher processing resolution (for better response in the low frequencies),
results in longer latency, but unfortunately this can also introduce 'pre-ring' that can make transients (e.g. a kick drum) lose their
edge.
When Linear Phase processing is selected, a Processing Resolution button becomes available. Choosing the correct resolution is a
compromise depending on the program material and your personal preference. The following resolutions are available:
Low provides linear-phase processing with a minimal latency. Use only with low Q settings, or when only changing the mid-
high part of the spectrum. With a sample rate of 44.1 kHz, it results in a total latency of 3072 samples (about 70 ms).
Medium is a good compromise between low-frequency resolution and latency and we recommend to use this in general for
linear-phase processing. The total latency is 5120 samples at a sample rate of 44.1 kHz (about 116 ms).
High gives great low-frequency resolution. If you need to use high Q settings when changing the low end of the spectrum,
use this mode. The total latency is 9216 samples at a sample rate of 44.1 kHz (about 209 ms).
Very High gives even better low-frequency resolution. The total latency is 17408 samples at a sample rate of 44.1 kHz (about
395 ms).
Maximum results in very high low-frequency resolution at the expense of a very large latency and possible pre-echo
problems. The total latency here is 66560 samples at a sample rate of 44.1 kHz (about 1509 ms).
Note that changing EQ band frequencies in Linear Phase mode sounds just as smooth as when using the other modes, no zipper
effects whatsoever. This might sound trivial, but it's actually quite unique in linear-phase processing!
Choosing a suitable processing mode
As already explained, in almost all normal mixing and mastering situations, Zero Latency mode or Natural Phase mode (with its
even better accuracy and phase response) will be the best choice. It is important to understand that linear-phase processing is not
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