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3.3 What is delay compensation by means of digital technology?
3.4 Negative Impedance
Our Digital-Active Loudspeakersystems deliver a lifelike reproduction of music, i.e. all frequencies reach
your ear simultaneously as if you were enjoying a real performance.
The problem is as follows:
Traditional passive and analogue active crossover networks lead to long frequency-
dependant time delays. This means that the different frequencies of a piece
of music leave the crossover network at different times, whereas during a live
performance of a music instrument, all frequencies are emitted simultaneously. The
lower the limit frequency of the network, the worse the time delay. Therefore, the
part of the network which is responsible for the bass range introduces the longest
time delay, and the part responsible for the tweeter the shortest.
In addition, so-called delay distortions occur due to varying driver dimensions.
In a loudspeaker, as in an electromagnet, the voice coil is forced to move by
electric current. Since the tweeter is much smaller than the woofer, the tweeter’s
voice coil is placed nearer to the front of the loudspeaker, which results in an
additional delay.
In consequence of these two delays, high frequencies always reach the human
ear fi rst with conventional loudspeakers. Extensive scientifi c tests have shown
that the human ear is capable of perceiving such differences if they are of a
certain degree.
By means of digital technology, Revox is now in a position to ensure a consistent
signal processing time for all frequencies. When using a digital crossover network,
distortions, which a passive or analogue active crossover network would cause,
are compensated for as well as delays which are due to the loudspeaker drivers.
To achieve this goal, extensive testing and innumerable series of measurements
at our lab have been necessary. However, we think that our efforts have been
worthwhile.
Each loudspeaker driver has a copper wire voice coil which acts as an ohmic
resistor. Since it is a coil, it also constitutes of course a complex AC resistance,
an inductivity. Together with the diaphragm, those two resistors form the
so-called impedance.
If you apply a pulse to such a voice coil, the diaphragm connected to this coil is
defl ected into one direction and afterwards gradually decays. This motion can
be compared to the movement of a pendulum which has been given a push.
However, the motion in the loudspeaker is damped by the magnet. Depending
on the magnet’s strength, the oscillation slowly or quickly ebbs away. Yet, this
post-pulse oscillation is undesirable, because one pulse leads to a series of
oscillations instead of only one, which, in turn, strongly infl uences the musical
reproduction. It leads, for example, to booming bass, overemphasized bass, strange
sound behavior of the medium and high frequencies.
Revox uses amplifi ers with so-called negative impedance for all three ways. This
patented method compensates for the impedance of the loudspeaker driver. To
Digitale LS.indd 25.06.2001, 21:1920