Multiband-EQ Equivalent to "Low Pass Simple" + "High Pass Simple"?

Hi,

In our project, we have a lot of events where we have both Highpass Simple and a Lowpass Simple effects, and the cutoff frequency has an automation over distance.

I heard that the new Multiband EQ effect is preferred and cheaper (as stated in this question Filters Deprecated?, so we’re thinking of replacing the Highpass + Lowpass effects to a single Multiband EQ.

We find that a Multiband EQ with 12db, Q~=0.41 is kind of equivalent to a single Low/Highpass Single effect. However, if I use two bands for low/high pass respectively, the result sounds quite different ( low+high pass vs. Multiband EQ with 2 bands) when the cutoff frequency get closer, such as both at 1500 Hz. As shown below.

Any advice on how to properly merge a Lowpass Simple plus a Highpass Simple to a Multi-band EQ?

I’ve also been wondering about this. The 6dB slopes in the Simple filters were great for distance cues, and they were easy on the CPU as well.

Firelight, could non-resonant 6dB slopes be introduced to the Multiband EQ as well? Then the deprecated filters could truly be deprecated.

The highpass simple effect is approximately equivalent to a multiband EQ set to Highpass 12 dB with a Q value of 0.707. The lowpass simple effect is approximately equivalent to a multiband EQ set to Lowpass 12 dB with a Q value of 0.707. Note, however, that these will always sound slightly different, as they are implemented using different algorithms.

If you’re not happy with the conversion being inexact, it may be simplest to continue using the highpass simple and lowpass simple effects. As long as your game continues to use a version of FMOD Studio in which they are available, there should be no problem with using them.

Skaven, the 12 dB filters of the multiband EQ actually use less CPU than the 6 dB filters of the deprecated effects.

That said, if you want 6 dB slopes for sound design purposes, we’ll add them to the multiuband EQ in in an upcoming release.

2 Likes

Thank you for the verifications Joseph! The 6db slopes would indeed be useful, and maybe also cheaper on the CPU than the 12dB resonant bands as well.

Skaven, the 12 dB filters of the multiband EQ actually use less CPU than the 6 dB filters of the deprecated effects.

Regarding CPU cost - what is the largest cost in the Multiband EQ, is it memory access and function calls rather than the maths themselves? I used Multiband as a distance cue filter in a category of sounds in a game I’m working on, but these turned out to be a huge cost on the PS4 so I had to replace them with LP/HP Simple filters on the PlayStation.

I found notes from a programmer that had looked into the filter and EQ DSPs code in FMOD Studio and I quote his comments here:

(quote)

I took a quick look at the actual code for the following three dsp’s:

  • Lowpass simple
  • 3-EQ
  • Multiband EQ

My conclusions:

  • There is no explicit SSE code used, nor is the code structured so that the compiler would be able to easily vectorize it (we were speculating earlier that using SSE would give multiple bands for the cost of one).

  • Lowpass/Highpass simple (6dB non-resonant filter) still has by far the lowest computational cost. It may very well be though, that the biggest cost here is in memory access and function call overheads, so that the actual math operations matters little - just trying to imagine any reason for why they have moved these to “deprecated”. A 6 dB filter can sometimes be very useful for more subtle colorations though, so I don’t agree with deprecating them.

  • 3-EQ always calculates all three EQ’s, in serial, so you are paying an unnecessary cost if you are not using all three. The 24 dB version has 2x the computation of the 12 dB version and the 48 dB version similarly has 4x the computational cost.

  • Multiband EQ is a 5 band EQ. Computations are in serial. Like for 3-EQ using 24dB costs 2x and 48dB costs 4x. Unlike 3-EQ however, it will skip any disabled bands. I.e. it will not waste computation on unused bands. The UI is also lightyears better than 3-EQ!

My recommendations:

  • Continue using Lowpass/Highpass simple whenever that makes sense (i.e. when you don’t want a steeper slope).

  • Use the new multiband EQ in all other cases.

  • The 3-EQ should be deprecated as it has no advantages compared to multiband.

  • Be aware that every band you enable has its cost and be aware that 24/48 dB is 2x/4x as costly as 12 dB.

  • Due to the mentioned function call and memory access costs, using a full 5 band 48 dB multiband EQ for sure will not be 20x as expensive as the simple EQ, even though it does 20x more operations. The cost of just “putting in a DSP at all” may well be the dominant cost.

(un-quote)

What is the primary factor for Multiband 12 dB LP being cheaper than LP Simple?

Did 6 dB filters ever get added? Would be very nice for distance filtering, where I feel 12 dB is too steep

As of the time of writing (September of 2022), lowpass and highpass filters with a 6 dB slope have not yet been added to the Multiband Equalizer effect. I’ve added you to the list of people interested in this feature.

1 Like