On Tue, 26 Feb 2013 17:35:56 -0500, James C Chandler Jr wrote: >>The parallel config I'd been thinking about, inspired by your diagrams, would >>be >>slightly different-- >> >>For instance in your figure 8-7 Three-way crossover, substitute an AllPass2 >>for >>your LP2 in the Low output. >> >>And in your figure 8-10 Four-way crossover, substitute an AllPass2 for HP2 in >>the High output, and also substitute an Allpass2 for your LP2 in the Low >>output.
I do not understand why you would want to do this. While it will provide you with your desired "constant transition band slopes", the responses will no longer sum to allpass. >>The preliminary result (barring foolish mistakes)-- A parallel configuration >>using one fourth order highpass and one fourth order linkwitz riley lowpass >>for >>each midband, and one highpass in the high band, and one lowpass in the low >>band-- It does show symmetrical band skirts between the bandpass interior >>sections. They mix surprisingly flat, but not exactly so. There are slight >>deviations. It mixes together unity-gain at very high and very low frequencies >>but gradually accumulates about a half dB gain nearing the center of the audio >>spectrum. What you describe is exactly what I would expect with the LP filters and HP filters replaced by AP filters. >>On the other hand, trying a binary-tree with the same assortment of filters-- >>The binary-tree did not show perfectly symmetrical band skirts (as expected), >>though the skirts are fairly symmetrical. The big surprise-- This binary-tree >>with non-symmetrical band skirts-- It mixes ruler flat over the entire audio >>spectrum! That is non-intuitive and I want to experiment more with the >>spreadsheet to better understand it. It is non-intuitive, but it is mathematically correct. >>With the binary-tree crossover network, the mid-band gain of each bandpass >>section is "almost the same but not quite". With the binary-tree crossover >>network, adjacent bands' gains at each crossover frequency are "almost >>identical >>but not quite". However, the entire tree mixes flat anyway, at least in the >>spreadsheet. Interesting as I didn't expect it to work that way. That might be due to slight differences in the positioning of the cutoff frequencies -- are they *exactly* one octave apart? >>For most people's use of hard limiters as in-line effects, a multiband hard >>limiter might be a weird animal? Yes, very much so. I only used a "limiter" example because it was the simplest way to demonstrate my point about the effects at the crossover frequencies. I don't think I've ever actually seen a multiband limiter in practice. >>Because if a limiter has been inserted to avoid clipping in the broadband >>signal, then merely limiting each band could never guarantee absence of >>clipping >>after the bands are summed back together? Yes, but the same applies to a multiband compressor, to a lesser extent. >>What configuration makes sense to you, which would be "transparent flat" when >>dynamics are not applied, but would do the right thing to signals near >>crossover >>points? You'd have to run two parallel paths -- a traditional crossover that sums to unity gain, to separate the audio signal; and a non-traditional bandsplitter in which the frequency bands overlap, for analysis of the audio signal. The configuration is best described by example. Assume that two of your desired bands extend from 500 Hz to 1000 Hz, and from 1000 Hz to 2000 Hz. Your SEPARATION path might consist of a traditional Linkwitz-Riley crossover, in which case filter spanning 500-1000 Hz would be down 6 dB at 1000 Hz, and the filter spanning 100-2000 Hz would also be down 6 dB at 1000 Hz. But the bands in your ANALYSIS path would overlap, such that the 500-1000 Hz band would be at 0 dB at 1000 Hz and attenuate as steeply as possible above that frequency. Similarly, the 1000-2000 Hz band would be at 0 dB at 1000 Hz and attenuate as steeply as possible below that frequency. You would use the output from the ANALYSIS path as a sidechain to determine gains to be applied to signals in the SEPARATION path. Afterward, you would recombine the signals in the SEPARATION path. >>One alternative that comes to mind is to use a separate "non flat" crossover >>network to drive the side chain, so that levels are sensed in a more realistic >>fashion near the crossover points? Exactly. >>Also, with fourth-order linkwitz riley bands, a crossover network with >>NUMEROUS >>fairly narrow bands would not show as much crossover point amplitude error, >>compared to a multiband compressor with a small number of wide bands? Whether that is troublesome is a judgment call. Greg ========================= Everybody has their moment of great opportunity in life. If you happen to miss the one you care about, then everything else in life becomes eerily easy. -- Douglas Adams -- dupswapdrop -- the music-dsp mailing list and website: subscription info, FAQ, source code archive, list archive, book reviews, dsp links http://music.columbia.edu/cmc/music-dsp http://music.columbia.edu/mailman/listinfo/music-dsp
