Linear phase filters and frequency response with the FabFilter Pro Q 2
Filters can be minimum phase or linear phase. But which is best? Should you aim for a perfect phase response, or are the imperfections of minimum phase of little importance? This video demonstrates the differences and shows exactly what happens.
Note on the Bessel filter: This is a filter with a highly linear phase response that can be constructed in the analogue domain. It is commonly used in loudspeaker crossovers. Its frequency response however is not as useful in practice for other purposes and filter designs such as the Butterworth are more often used despite their comparatively poor phase response. https://www.google.com/search?q=bessel+filter
Filters minimum phase or linear phase one of these can seriously mess with your frequency response today you're going to learn which and why learn audio online with audio masterclass audiomasterclass.com in the analog world all we have and have ever had are minimum phase filters as the level changes according to frequency so does the phase change that's the way things are due to the eternal laws of the universe which are very difficult to argue with to create a linear phase filter in the analog domain you'd have to go back in time in the immortal words of dick dastardly drat and double drat but in digital audio we can in effect go back in time simply delay the signal and use that delay to process everything comes out a little later than it should but latency compensation or you can call it delay compensation in the digital audio workstation can fix that with a linear phase filter the level changes according to frequency but the phase stays exactly as it was this seems better and it can be there's a price to pay in terms of pre-ringing but that's another topic for another video i'll put a link in the description so what's the problem with minimum phase usually nothing of any consequence as the history of the last 70 years of analog audio has shown us but there are situations where there can be an issue typically this would be where you're splitting a signal in two then mixing the two parts back together again which would probably be to process them differently an example would be multiband compression you could for instance filter out the low frequencies compress them then mix them back in again that's something you might do in mastering the potential issue is that the filters might cause frequency response problems when you mix the signals back together so let's move on to a demonstration what i have here is a frequency sweep from 100hz to 1600hz and in the following demonstrations i'm going to use a center frequency of 400 hertz here's the sweep with no
filtering i'm going to make two copies of the sweep and apply a low pass filter to one and a high pass filter to the other i'm going to set the cutoff frequencies so that the minus 6 db point of both is 400 hertz this is so when i mix them back together because they are correlated they should add together at the original level here's the low pass filtered version
and here's the high pass version
so when i mix the two filtered versions together we would hope that they sound the same as the original and there are no frequency response problems so let's try this with linear phase i'm using the fabfilter pro q2 set to linear phase
you can hear that the level hardly changes at all and you can see that on the meter let's play it again
so that's all well and good but as it's so much better than minimum phase to find out i'm going to set the filters to minimum phase fabfilter calls it natural phase
before i play the sweep take a moment to imagine what's going to happen is it going to be pretty much the same or will you notice a difference here we go
well i bet that was more of a difference than you thought it was what i expected but a lot more than i expected let's play it again
so going back to my mastering example if you split the low frequencies from a signal to compress them using minimum phase filters then mix them back in again you are going to have a frequency response issue it isn't going to be a disaster but you should consider correcting the problem with eq just make it so it sounds good so this is what happens with linear phase and minimum phase filters but the further question is why does it happen the clue is there already phase i'm going to demonstrate this with a vector scope i'll pan the low pass filter left and the high pass filter right in the vectorscope signal on the left channel causes the trace to tilt to the left signal on the right channel tilts to the right with mono the trace goes vertically up and down if the channels are in phase the trace will be a straight line at whatever angle let's play the linear phase sweep
that is exactly as we should expect the signal pans from left to right and the trace is always a straight line showing that the low pass and high pass filters are exactly in phase all the way through i'm going to play the minimum phase version now like before take a moment to imagine what you're going to see it is going to be different but in what way here we go
that was an experience what it shows is that there's a huge variation in phase as the frequency rises and a huge difference between the low pass and high pass filters when identical signals that are exactly in phase are mixed together they'll add up by six decibels if they're not in phase the level they add up to will be lower so this explains the frequency response issue i demonstrated earlier in summary analog audio has been minimum phase for decades and it works but there are occasions where linear phase is better and you should probably use it i'm david meller course director of audio masterclass thank you for listening.