On Thu, May 14, 2015 at 06:11:49PM -0000, Richard Lee wrote: > Duu.uuh!! http://parole.loria.fr/DEMAND/DEMAND.pdf states > > "the microphones of the array ... are not calibrated with respect to each > other, and so gain variations are to be expected: we found that the energy > in some channels is consistently higher than in other channels. Algorithms > working on this data should compensate for this variation" > > ie they haven't a clue what each capsule is doing. > > This precludes any attempt at conversion to B-format and also of > beamforming.
Indeed. Unless maybe in a very limited frequency range. Once the phase differences between the capsule signals exceed 180 degrees any synthesized polar pattern will break down and be reduced to chaos. This determines the upper limit. On the other side, when wavelenght is much larger than the distance between the mics, creating anything non-omni requires amplification of the small differences between mic signals, and the required gain increases by 6 db times order for each octave down. If the mic gains are not perfectly matched amplified errors will dominate the result. This in practice determines the lower limit of the frequency range. > I was hoping this might lead to a discussion about EigenMike and how it > might be made good enough to record music but this is certainly NOT the > vehicle. The limits pointed out above are something that Gary Elko has clearly understood very well (and some others apparently have not). The beamforming sofware that comes with the Eigenmike will in general not let you do things that depend on unrealistic accuracy of the mic gain calibration. On the upper end the limit for the EM is around 8 kHz. Above that, the SW will just give you the signal of the single capsule that is closest to the intended direction of the beam. The polar pattern above the upper limit will be the one resulting from diffraction caused by the solid spherical body (this becomes quite directional in the frequency range considered). This at least produces a clean signal in the upper octave which is better than the chaotic pattern a beamformer would produce. On the lower end, the limits that can be achieved assuming +/- 0.5 dB gain errors are roughly 1st order: 50 Hz 2nd order: 630 Hz 3rd order: 1.6 kHz 4th order 2.5 kHz The latter has such a limited frequency range that it's probably better to forget about it. The EM software wisely doesn't claim anything above third order. The result of this, in particular of the lower frequency limits, is that any higher order directional pattern will have to be a compromise between on-axis and diffuse-field frequency response. The requirements for this will depend on the application: a spot mic or a set of beams intended for surround reproduction. The tradeoff can be made partly by EQ. So when using the EM for e.g. orchestral recording using a number of beams pointed at the various sections of the orchestra, you will need some rather unconventional EQ for the best results. This will probably surprise most sound engineers used to the more traditional way of using a set of normal mics to cover the sections. It may also put them off. But it is certainly possible to make very good recordings with the EM. Ciao, -- FA A world of exhaustive, reliable metadata would be an utopia. It's also a pipe-dream, founded on self-delusion, nerd hubris and hysterically inflated market opportunities. (Cory Doctorow) _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit account or options, view archives and so on.
