If you want to process the bass and low-end of your music production using EQ filters that don't degrade the quality, then this post is for you. We explain the different types of filters and why our plugin BASSROOM delivers more transparent processing than Linear Phase EQs.
Are All Filters Good Enough For Low Frequencies?
We've all seen filters with a logarithmic scale of frequencies whereby a range spanning 100Hz in the low-end is much wider than that same range in say the 10,000Hz to 15,000Hz range. This is a reflection of the perceptual importance of those ranges.
We're going to hear a drastic difference in a track with 6dB taken from between the 100-200Hz range and much less significant difference if 6dB were taken from the 12,000-12,100Hz range.
When designing filters for higher frequencies the range affected is often very large, for example 1000's of Hz for filters working above 10KHz. However for lower frequencies, we will want to work in ranges of 100Hz or even less.
To get the very narrow frequency bands of operation in the low-end, the filters need to be more complex.
Having more complex filters presents us with potential degradation of sound, which is not good for your track.
Finite Impulse Response (FIR) and Infinite Impulse Response (IIR) Filters
Digital filters are limited in that they can only operate on the current sample, past samples or past output samples and then just add multiples of these samples together as a new output sample.
If we just add current and past samples then this is known as a finite impulse response or FIR filter.
If we use the output samples, that is the output of the filter is fed back into the input, then we call these infinite impulse response filters or IIR filters.
The IIR filter is generally much less CPU intensive than the FIR filter so it would seem the obvious choice for most EQ filters, and is often used. However, there are sonic differences between the two that make the choice less clear cut.
What Is Linear Phase?
A Finite Impulse Response filter will have linear phase. You've probably heard 'Linear Phase' being used as a positive feature, but what exactly does it mean?
At any point in time we can break down any digitally sampled sound into it's constituent sine wave components (well, there are a few issues here but we won't go into them) and each of these sine waves will have zero values at particular points in time.
Check out the plots below, we've imagined an original, very simple, signal broken down into it's 2 sine wave components and in each case we plot the constituent sine waves separately.
In plot 1 you could say one sine wave 'starts' before the other - they have different phases.
In plot 2 we have put the signal though a FIR filter. Notice that both sine waves have simply moved along, they have been delayed equally. Ignoring any gain changes from the filter you will essentially hear the input simply delayed.
In plot 3 the signal has been put through an IIR filter. In this filter the sine waves are again delayed BUT by different amounts. This is phase distortion.
We've seen that with a linear phase FIR filter the phase differences between each constituent sine wave in the original signal are unaltered when passed through the filter. So, no phase distortion. Now phase distortion isn't necessarily a bad thing, but it's not what was required of the filtering, we just wanted gain changes for particular frequencies.
Are Linear Phase Filters the Best for Bass and Low-end?
So, it seems that linear phase FIR filters are ideally what we want. Well, not necessarily. As we mentioned FIR filters use the current sample and previous samples mixed together to form a new output sample.
To get accurate filters in the low frequencies these linear phase FIR filters need lots of past samples, perhaps thousands. This means that transients get averaged out or smeared out in time.
These 'long' FIR filters also give rise to a ringing effect, so called because at higher frequencies there is a ringing before and after an attack (such as a snare) sound.
In the case of kick drums it can sound like a sucking reverse effect, more pronounced at the onset but still there in the release of the kick or sharp bass-line note.
Audio Examples
In many ways the sucking effect of a FIR filter is less desirable than the phase distortion effect of an IIR filter. Below is a simple kick drum pulse.
Now listen to the kick after it's been put through a linear phase filter, boosting by 6dB at 240Hz. (Listen for an odd ringing sound before the kick drum).
Now take a listen to the same kick with the same filter settings through BASSROOM.
There are no undesirable effects with BASSROOM's unique filters!
What About Minimum Phase?
You've also probably seen settings for minimum phase or similar. This setting effectively removes the ringing before an attack (pre-ring), but leaves the ringing after (post-ring) intact.
Now, yes the pre-ringing is more evident, but the post-ringing is obviously going to be having an effect, effectively muddying the bass, particularly with the more transient sounds. Exactly what you don't want from an EQ.
With BASSROOM, both pre and post-ringing are dramatically reduced, keeping your transients clear and your processing transparent as can be seen from the graphs below.
In fig. 1 you can see what's known as the step response of a linear phase FIR filter. A step can thought of as a sharp attack sound, say a kick, and this plot is the output of the filter. Now notice on either side of the main body of the 'kick' the pre and post-ringing. A minimum phase filter will simply remove the pre-ringing at the left of the main body.
In fig. 2 you can clearly see a reduction in both pre and post-ringing using the BASSROOM filter.
Conclusion
The low-end is a vitally important part of any mix. The last thing you want is for an unfit-for-purpose filter to subtly muddy your low end and smear the transients. Now you're empowered to make the right decision. Use BASSROOM when EQing your low-end to get the most transparent results that maintain the quality of your production.
