Yamaha MG20 - What does the "variable mid frequency" knob actually do?

Question

My mixer has four EQ knobs per channel. Two of them are standard EQ at 10khz and 100hz. The other two are for the mid range, but they're different. The lower knob sets the cut/boost, but the upper knob sets the frequency from 250hz to 5khz. The manual says this knob "sets the variable mid frequency" while the gain knob "sets the amount of attenuation or boost for the range", except the frequency knob isn't a range, it's a discrete number. Hence the confusion. Also, the manual never uses the word "variable" outside of that one sentence.

I ASSUME what that means is that the cut/boost knob creates a normal or near-normal curve centered on where the frequency knob is, but the manual doesn't make it clear and to be honest I can't really hear an obvious, clearly correlated difference when I sweep the frequency knob around.

Answer 1

The EQ system on this device has three filters. Two are 'shelving' filters and one is a 'peaking' filter.

The shelving filters apply a variable cut or boost at a fixed frequency. The lower of these two works to boost or attenuate frequencies below 100Hz and the upper boosts or attenuates frequencies above 10kHz.

The 'peaking' filter allows a 'bell' curve to be applied to a particular 'centre' frequency and then either a boost or attenuation applied to that particular frequency. The 'centre' frequency for the 'peaking' filter can be varied, which is why you will have a control to allow selection of the filter centre frequency.

It is noted that the amount of control over the parameters is quite limited in this scenario however it will give you adequate control over a lot of basic requirements.

You will only hear a clear difference when sweeping the mid frequency when either a gain or attenuation is applied. It is often easier to pick frequencies in this way by applying full gain to the mid frequency control and then identifying the required frequency by sweeping. You can then adjust gain or attenuation to taste once the frequency is picked.