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Originally Posted by Nobody Special  Since you've been talking about allpasses, I'll pass along an observation. Although the traditional view is that allpasses increase reverb density, that's only true up to a point. The number of reflections per second is still limited by the Nyquist theorum (and probably by the auditory cortex as well). With well-chosen allpass values, that limit can be reached quite rapidly. After that point, the significant effects are phase and frequency. |
This is an interesting point. We have been discussing "constant density" algorithms in this thread. If the initial echo density approaches the maximum density supported by the Nyquist theorem, or the maximum density that can be perceived by our ears, then having an echo density that increases over time is a moot point.
From the discussion of the EMT-esque algorithm, and listening to older reverberators, it appears that there were many algorithms that took such a constant density approach. Some of these seem to be improved Schroeder algorithms, others undoubtedly use far different algorithms - there are a lot of ways to get density quickly.
One of the big problems with any algorithmic reverberator, be it constant density or increasing with time, is the ability of the ears to quicky pick up repetitive patterns in seemingly random signals. Most delay based time invariant reverberators will have an audible repetition rate for long decays, where the repetition rate occurs at the sum of all of the delay lengths for a well tuned reverb (and at a much faster rate or series of rates for a poorly tuned reverb). The initial use of randomizers within reverbs seems to have been to reduce or eliminate this repetition artifact. Recent publications have made a convincing argument that real acoustic spaces are time varying, but my guess is that the early time-varying reverbs were not trying to emulate this at all.
The time variation found in most commercial reverbs can go far beyond what would be found in most real-world situations. Boston Symphony Hall may have randomization of the delay times, but an algorithm like the PCM70 Concert Hall, or the Eos Superhall, or the DP/Pro Expert Reverb displays artifacts that would probably not be present without some extensive modifications to that building, such as a series of active bonfires within the structure. This larger-than-life artifact has now become part and parcel of many styles of electronic and ambient music. The Lexicon manuals may warn against pitch warbling on piano with the Concert Hall algorithm, yet many people use this algorithm on piano for this very reason. The Brian Eno / Harold Budd albums display some pitch artifacts that I am now able to hear as part and parcel of the EMT250 used on those songs, and the music would not be nearly as cool without those sidebands decaying into the aether.