Originally Posted by mthomas47
That corresponds to what Keith put in the FAQ. I'm just not sure that it is correct. Of course, depending on how you interpreted what Chris said, several filters (at 4096 control points per filter) would add up to more than 10,000.
I'm not sure how much effort we want to put into this issue, anyway though, when it comes to the subwoofer channel. Even if we assume that XT-32 is EQing every individual frequency from 200Hz (or 400Hz if any subwoofers can make meaningful sounds that high) down to 10Hz, it is difficult for me to understand how the algorithm could use more control points than there are individual frequencies to control. (400 - 10 = 390.) So, it seems to me that for the .1 channel, whether there were 10,000 taps, or "only" 4096, XT-32 would only be able to use a fraction of the number of control points available to it.
I think you're counting 20 Hz, 21 Hz, 22 Hz and up as individual frequencies but those frequencies aren't the only frequencies. 20.1 Hz is a different frequency to 20 Hz, and 20.15 Hz is a different frequency to 20.1 Hz and so on. There's way more frequencies between 20 Hz and 200 Hz than you're thinking of, in fact there's an infinite number of frequencies within the range.
Having said that, there's also only 3 and a half octaves or so between 20 Hz and 200 Hz and that only amounts to 42 or so notes of the musical scale but that number needs to get multiplied a few times since there are several different tuning standards in existence. Everyone thinks of A= 440 Hz for the tuning standard but period instruments for baroque and renaissance classical music use a couple of other standards with A tuned lower and there are also modern orchestras using a couple of higher tuned standards. If there are 181 exact "x.0" frequencies between 20 Hz and 200 Hz, there's probably more than 200 musical note pitches in use between 20 Hz and 200 Hz because of the different tuning standards in use. Of course, when you start to look at instruments like the double bass and cello and trombone which allow the player to glide their pitch smoothly between different notes, and that is actually done at times, you could also say that there are an infinite number of musical pitches between any two adjacent notes of the musical scale, just as there are an infinite number of frequencies.
I don't think counting frequencies or musical pitches within a given frequency range is the way to look at what may be needed in tap points for room correction.
I learnt the start of my very basic understanding of room acoustics from Everest's "Master Handbook of Acoustics". In it he refers to a study which showed that an empty Coke bottle, acting as a Helmholtz resonator, absorbs 5.9 Sabins at its resonant frequency of 185 Hz but that the absorption has a bandwidth between +/- 3 dB points of only 0.67 Hz. If you are going to try to correct for deviations like that in a room response then you may have to have correction points at intervals of less than 1 Hz in order to make a smooth response. Most room problems have much wider bandwidths than that and I also can't see many rooms having several problems as narrowly defined as the absorption suckout from a Coke bottle but correcting for a deviation in room response like that might require several tap points within a fairly narrow frequency band so I suspect that more tap points might be required for low frequency room correction than we think.
Even so, like you I find it difficult to see how as many as 4096 tap points could be used to correct the .1 channel, though I can understand why a lot more tap points may be needed than we might initially think.
Addition: I just realised that someone may read the above and object that we tend not to correct for dips but only for peaks. That's true, but we correct for peaks by adding dips and some physical correction is done using Helmholtz resonators designed and tuned to absorb a peak at a specific frequency. Tailoring an electronic correction that reproduces the correction achieved by a precisely tuned Helmholtz resonator may actually require more than a single tap point. Several tap points may be required to correct for a single peak so looking at a room response curve with x number of points requiring correction may well require the use of several times that number of tap points given that the correction has to be tailored to produce a smooth response and the deviations being corrected may not be symmetrical in behaviour around their nominal frequency.