KDFX Reference
KDFX Algorithm Specifications
10-21
13 Stereo Hall
A stereo hall reverberation algorithm.
PAUs: 3
The Stereo Hall reverberation is implemented using a special arrangement of all pass networks and delay
lines which reduces coloration and increases density. The reverberator is inherently stereo with each input
injected into the "room" at multiple locations. To shorten the decay time of low and high frequencies
relative to mid frequencies, bass equalizers and low pass Þlters, controlled by Bass Gain and by HF
Damping, are placed within the network. Room Size scales all the delay times of the network (but not the
Pre Dly or Build Time), to change the simulated room dimension over a range of 10 to 75m. Decay Time
varies the feedback gains to achieve decay times from 0.5 to 100 seconds. The Room Size and Decay Time
controls are interlocked so that a chosen Decay Time will be maintained while Room Size is varied. At
smaller sizes, the reverb becomes quite colored and is useful only for special effects. A two input stereo
mixer, controlled by Wet/Dry and Out Gain, feeds the output. The Lowpass control acts only on the wet
signal and can be used to smooth out the reverb high end without modifying the reverb decay time at high
frequencies.
Figure 10-5 Simplified block diagram of Stereo Hall.
Within the reverberator, certain delays can be put into a time varying motion to break up patterns and to
increase density in the reverb tail. Using the LFO Rate and Depth controls carefully with longer decay
times can be beneÞcial. But beware of the pitch shifting artifacts which can accompany randomization
when it is used in greater amounts. Also within the reverberator, the Diffusion control can reduce the
diffusion provided by some all pass networks. While the reverb will eventually reach full diffusion
regardless of the Diffusion setting, the early reverb diffusion can be reduced, which sometimes is useful to
help keep the dry signal "in the clear".
The reverberator structure is stereo and requires that the dry source be applied to both left and right
inputs. If the source is mono, it should still be applied (pan centered) to both left and right inputs. Failure
to drive both inputs will result in offset initial reverb images and later ping-ponging of the reverberation.
Driving only one input will also increase the time required to build up reverb density.
To gain control over the growth of reverberation, the left and right inputs each are passed through an
"injector" that can extend the source before it drives the reverberator. Only when Build Env is set to 0% is
the reverberator driven in pure stereo by the pure dry signal. For settings of Build Env greater than 0%, the
reverberator is fed multiple times. Build Env controls the injector so that the reverberation begins abruptly
(0%), builds immediately to a sustained level (50%), or builds gradually to a maximum (100%). Build Time
L Input
R Input
PreDelay
PreDelay
Dry
Dry
Out Gain
Wet
Reverb
L Output
R Output
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