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Matrixhifi
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Joined: Sep 6 2005 - 12:15am
Is there any chance to understand DRC by reading Stereophile???

The anwser is: NO.

http://www.stereophile.com/reference/108tech/index1.html

What could be seem as a theoretical approach to DRC techniques is, in truth, based on misconceptions and lack of understanding and need some comments about.

Let's look at the figures:

This is a simulation of how the above signal would be measured in a room with a modal node at that frequency, given the signal below. We can appreciate the increase in level with time due to sound reflexions in phase with direct sound, and the tail due to revereberant sound field.

The next figure is wrong:

The first curve is the substraction of the two curves from first figure. Mathematically that's correct, but sometimes, that's not enough. In acoustics this kind of operators hasn't any physical meaning; there is no way to use this signal as feedback in any system. The time delays haven't been included.

If the first curve is wrong, the second goes beyond. It is imposible to use that curve as a "toneburst signal with modal correction applied". If we simulate the acoustics response of this signal as input, it would be worse than without "modal correction". It's imposible to alter the reverberant field of any room by DSP, time delay and phase behavior would kill you. That's out of reach of DRC.

The last mistake, as far as I see, was to talk about:

"An interesting feature of the latter is the sudden polarity reversal at the end of the toneburst, which is shown zoomed along the time axis in fig.6. This sharp discontinuity, although unlikely to occur with music signals, suggests that the speaker must have a wide bandwidth to achieve accurate correction, so generating the necessary anti-resonance isn't an approach that would necessarily work well with a subwoofer rather than with a full-range speaker."

This discontinuity is just a mathematical artifact. It hasn't any physical meaning. Anyone that thinks that "polarity reversal" means something in DRC inversion should go back to basic DSP learning.

DRC techniques are, by far, more complex and, at the same time, rock-solid. I hope in the near future Stereophile gives DRC, and DSP applied to hifi, the analysis they deserve.

A good place to start reading:

http://drc-fir.sourceforge.net/doc/drc.html

About room acoustics behavior under toneburst signals:

http://pcfarina.eng.unipr.it/Public/Papers/153-AES110.PDF

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