EQ filters

Discussion in 'Effects and the DSP' started by WxMan1, Mar 20, 2009.

  1. WxMan1

    WxMan1 Active Member

    Joined:
    Dec 26, 2007
    Messages:
    307
    Likes Received:
    11
    Trophy Points:
    28
    How does one properly set Q for "X"-pass filters ("X" being either high or low)?

    I understand that for band-pass filters, F_center = SQR [ F_min * F_max ], i.e., the square root of the product of the -3dB cutoff thresholds for the specified bandwidth. Q, then, is F_center / [ F_max - F_min ], where the denominator is the bandwidth of the filter.

    It is my understanding that the "X"-pass filters are of second order type, i.e., implying -12dB/octave attenuation. So what does Q do with respect to "X"-pass filters?

    What would be the most effective means to eliminate any signal <50Hz? On the one hand, the 4th order cross-over suggests and inherent -24dB/octave attenuation at the crossover point, but uses significantly more resources than a notch filter. While the notch filter uses as much resource as a high-pass filter, and I'm clear on how to configure Q for a notch filter, I'm unsure whether implementing a high-pass filter instead wouldn't be more appropriate. Perhaps its 6 of one, and 1/2 dozen of either the other two?

    :confused:

    Right now I'm running a notch filter with F_center at 20 Hz, and Q = 0.5 so that should get the job done.

    FWIW, the whole cause of my consterntion was noticing distortion in the sub-woofer when the EQ 10-band 32Hz slider had a bit too much gain. Digging deep into the rules I discovered the freq. response for my Monsoon MM-700 speaker system runs 50Hz - 20kHz. Drilling deep into the rules for the CT4780 soundcard suggests that the 10k1 DSP has a freq. response between 20Hz - 20kHz. There's an equation that describes power with respect to frequency, not putting the energy the woofer can't handle into the active 3rd-order cross-over would be conducive to minimizing unecessary stress on said components and retain fidelity when taching the system to the red-line (if you know what I mean).
     
    Last edited: Mar 20, 2009
  2. WxMan1

    WxMan1 Active Member

    Joined:
    Dec 26, 2007
    Messages:
    307
    Likes Received:
    11
    Trophy Points:
    28
    Fingered out what the deal is is concerning Q for the x-pass filters.

    Q defines the resonance of the transfer function at the cutoff frequency.

    At Q = 0.75 defines maximally flat response, i.e., -3 dB @ cut-off freq. Q < 0.75 defines maximal flat time delay (i.e., -3 dB progressively beyond cut-off freq), and Q > 0.75 creates resonant peak at cut-off freq (w/associated progressive gain increase beyond cut-off).

    The frequency domains defined by all values of Q coincide with roll-off rate attributable to 2nd order filters (12 dB / Oct) at ~0.3 of cuf-off freq.

    http://i.cmpnet.com/planetanalog/2008/04/C0289pt1-Figure4.gif

    The graphic iluminates what I'm describing: Q>0.75 and Q=0.75. The curve for Q<0.75 is not shown, however, the freq response is below the lower curve - the -3 dB level farther to the right - and all three curves coincide at about the same point and roll-off w/the same slope.

    Roger that?
     
    Last edited: Jul 24, 2010

Share This Page

visited