Originally Posted by walterw
OK, my pea brain is getting me into trouble again;
in our mixing board example, with 5 channels all getting the same signal, how much summing are we getting?
adding a second channel to the first gives us 3dB, right? and doubling that, by adding two more, gets us another 3 for 6dB total, right? and that 5th channel just adds a little more on top of that, right?
those 4 extra channels don't add up to anywhere near a 15dB boost, do they? that means that even cutting one off completely doesn't cut by anywhere near 15dB either!
you can't just cut one band deeply enough to get -15dB!
the only way i can think of that you could actually reduce the signal of the 4 remaining bands by a bigger amount would be to flip the polarity on that 5th fader and start bringing it up, so as to add in a reversed-polarity signal that starts to cancel the others.
is that's what's happening? the "cut" setting is not just ceasing to add to the others, but is actually introducing a flipped signal back into the proceedings?
(forgive me if all this is "filters 101", i never took that class.)
Uh .. .some (depending on filter topology)
.... depends on if they are series or parallel.
.... forgiveness not necessary, because you are not at fault. You simply need the right light shown on the issue. You'll "get it" soon enough .. I barely grajiated from rural public high school in low-desert Arizona with a D-minus average back in the "stoopid 70's" ... so you'll grasp this all very soon, once the light comes on it stays on, and many other things become understood as well.
You'd laugh till you cried if you knew how long it took me to grab ahold of impedance issues!
So ..... let's see if any of this answers The Questions of The Grand Peanut Head.
- From the Rane PE-15 Datasheet:
"Each of the five filters in the PE 15 cover a center Frequency range of four octaves, a Bandwidth of .03 to 1.5 octaves, and a boost/cut range of +15 dB to -20 dB. Bands one and five switch to allow peaking or shelving characteristics. Special attention to the design of the state-variable filters make all of these adjustments possible. All parameters are completely independent. To the user, this means any control may be operated in any order without affecting the others. This flexibility makes the PE 15 ideal for all audio contouring applications. A range that begins with the sharpest of notch filters and continues through broad-band program contouring."
- From the PE-17 Datasheet ....
"There are parallel Parametric designs (PE 15) and there are serial Parametric designs (certain others), then there is the PE 17, which is (of course) a parallel and serial design. (We DON’T like life simple!)
Referring to the block diagram above, you can see that Bands 1, 2 and 3 are arranged in a parallel fashion in series with paralleled Bands 4 and 5. This unique topology offers an optimum combination of the minimum phase shift and delay times of parallel designs with the extreme cut or boost settings available from series designs. So keep in mind that for extreme boost/cut settings use two Bands in series (e.g., Bands 3 and 4), that way, their dB boost/cut values directly add. For example -15 dB on both results in a total of -30 dB. For parallel bands, their boost/cut dB values do not directly add. We know this is confusing, but so is the rest of life!....."
"..... Additionally, you may vary each filter’s bandwidth from as narrow as 1/30 of an octave to as wide as two octaves, and adjust the Level over a boost/cut range of +12 dB to -15 dB. The extra-deep cut range, when used with a narrow bandwidth setting, is effective against troublesome feedback frequencies. Specially designed circuits make all these adjustments completely independent. To you, this means the ability to change all controls in any order without affecting the others. This flexibility makes the PE 17 ideal for all audio contouring applications. Anything from the sharpest notch to widest contouring....."
- From the PE-17 manual .....
"...For extra-deep notching applications, use Bands 3 and 4 and set them exactly the same. Notches as deep as 30 dB are possible, since Bands 3 and 4 are in series while most other bands are in parallel (see the Block Diagram in the Specifications Section)...."
"..... If a -15 dB cut won’t cure the problem, the resonance is extreme and must be dealt with in other ways. For example, move the speakers or the mics; hang some drapes; move some walls; burn down the building; or simply unplug the system and go home—something must give!..."
- From Rane's tech publication titled *Exposing EQ Mythology*
"MYTH #6: An ideal equalizer would add no phase shift when boosting or cutting. Phase shift is not a bad word. It is the glue at the heart of what we do, holding everything together. That it has become a maligned term is most unfortunate. This belief stands in the way of people really understanding the requirements for room equalization. The frequency response of most performing rooms looks like a heart attack victim’s EKG results. Associated with each change in amplitude is a corresponding change in phase response. Describing them as unbelievably jagged is being conservative. Every time the amplitude changes so does the phase shift. In fact, it can be argued that phase shift is the stuff that causes amplitude changes. Amplitude, phase and time are all inextricably mixed by the physics of sound. One does not exist without the others.
An equalizer is a tool. A tool that allows you to correct for a room’s anomalies. It must be capable of reproducing the exact opposite response of the one being connected. This requires precise correction at many neighboring points with the associated phase shift to correct for the room’s opposing phase shift. It takes phase shift to fix phase shift. Simple as that.
One way people get into trouble when equalizing rooms is using the wrong equalizer type. If an equalizer is not capable of adding the correct phase shift amount, it makes equalizing much more difficult than it has to be. The popularity of the many constant-Q designs has come about because of this phenomenon.
Equalizers that produce broad smooth curves for modest boost/cut amounts make poor room equalizers, and good tone modifiers. They lack the ability to make independent amplitude and phase corrections close together with minimal interference to neighboring bands, restricting their usage primarily to giving a shape to an overall response rather than correcting it. Serious correcting requires sharp constant-Q performance, among many other things.
Only by adding many precise narrow phase shift and amplitude corrections do you truly start equalizing a system’s blurred phase response. You do not do it with gentle smooth curves that lack the muscle to tame the peakedness of most rooms. broad smooth curves do not allow you to correct for the existing phase shift. It's just that simple, you must pre-shape the signal in both amplitude and phase. And that requires narrow filters...."
- Walter, he be getting warmer, mon! The lights, they begin to glow in him haid!