On 01/06/2011 17:42, Ralph Glasgal wrote:
In most of the papers on crosstalk cancellation the point is made that at lower
freauencies the power required to cancel the bass becomes prohibitive. I will
make a feeble attempt to explain why this is a fallacy that derives from a
common propensity to rely soly on mathematics when using the HRTF functions
available.
Yes, if you assume that one needs to cancel crosstalk at 20Hz then the usual HRTFs say the attenuation between the ears is negligible and so a high level signal is needed to do any cancellation and then more energy is needed to cancel this cancellation signal and the thing blows up.
But, the ear is not sensitive to crosstalk below say 100Hz so one needs to take this into consideration. On can simply bypass bass frequencies around the HRTF bsased canceller but it turns out this is not really necessary in HRTF-less algorithms.
One basic premise of RACE is that no HRTF functions need be used. But let us just concentrate on the bass region. RACE assumes a constant attenuation for a signal reaching the wrong ear. As the frequency declines this assumption becomes more and more inaccurate. But so what? What this means is that one is not doing much cancellation as the frequency gets down to say 90Hz which is okay since one does not localize well or at all at low bass frequencies anyway. In other words the amount of cancellation automatically declines with frequency so the overhead or power requirement does not change with frequency either. The head room needed is the same at all the normal real localization frequencies. (Very high frequencies are a different problem) You can see this in a brief note by Angelo Farina comparing RACE with other XTC methods. www.ambiophonics.org/papers/CrosstalkFilters.html
Ralph Glasgal
In fact I learned this from RACE. Instead of assuming cancellation at
low frequencies, and then re-introducing large ITDs, either in the
filter or counting on them from binaural recordings, I abandoned
cancellation from 1kHz down and went for ITD directly. However there are
only so many ways to squeeze a balloon without it bulging elsewhere..
BTW RACE has significant ITD, although smaller than mine :-) at lower
frequencies. Perhaps this might explain why it works better than one
might expect.
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