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So, the best approach IMO is to try to get the BFD’s output close to its input. Most response curves have both peaks and valleys, so the most efficient equalizing (i.e., using the fewest filters) means a combination of boost and cut filters applied to them. After EQ you’ll often end up with the BFD’s output close to its input, so you don’t burden your sub with a low-level signal.
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Is "using the fewest filters" our ultimate goal?
Whether you apply gain filters to response dips (and turn the BFD's input level down to allow it), or you apply cut filters to areas around response dips (and setup the input level to maximum), it makes no difference to the output signal level of the BFD. The former example reduces dynamic range and degrades signal to noise ratio, the latter example does not.
Which do you choose?
Setting a best input level maximizes the number of bits used in the BFD. The signal to noise ratio in a digital device is quite sensitive to input level. The BFD is a 24bit device. The ADC has a fixed theoretical noise floor of 147 dB that is a function of bit resolution (6.125x24=~147db). The spec for the BFD is ~94dB, so we know already that not only is the LSB hidden in the noise, but quite a few others. The spec of 94dB equates to between 15 and 16 bits of actual resolution. We have about 8 bits lost in the noise.
As you lower the input signal to the BFD, the noise internally rises exponentially. We know this, since each digital bit we lose drops about 6dB from our noise figure. As we lower the input level, the softest signal we can resolve (and therefore more low order bits) are lost in the noise. The BFD system becomes a 15bit, 14bit, 13bit device. An exponential drop in dynamic range occurs.
Since we know that we can obtain the same output level whether we use cut or gain, why choose gain when it creates a problem as I've described.
brucek