9
Operating Manual - PROTEA SYSTEM II 3.24CL Crossover / System Processor
24dB/oct Linkwitz-Riley filters are the industry standard, the easiest to use, and the filter type recommended by Ashly
.
Other filter types are available, but may require polarity switching or other adjustments for proper results. The following
paragraphs offer a summary of the three filter types as used in the 3.24CL crossovers.
Butterworth
Butterworth filters individually are always -3dB at the displayed crossover frequency and are used because they
have a "maximally flat" passband and sharpest transition to the stopband. When a Butterworth HPF and LPF of the same
crossover frequency are summed, the combined response is 3dB. With 12dB per octave Butterworth crossover
filters, one of the outputs must be inverted or else the combined response will result in a large notch at the crossover
frequency.
Bessel
These filters, as implemented on the 3.24CL, are always -3dB at the displayed crossover frequency. Bessel filters
are used because they have a maximally flat group delay. Stated another way, Bessel filters have the most linear phase
response. When a Bessel HPF and LPF of the same crossover frequency are summed, the combined response is +3dB for
12dB/oct, 18dB/oct, and 48dB/oct Bessel filters, and -2dB for 24dB/oct Bessel filters. One of the outputs must be inverted
when using either 12dB/oct or 18dB/oct Bessel crossover filters or else the combined response will have a large notch.
Linkwitz-Riley
The 12 dB/oct, 24dB/oct and 48dB/oct Linkwitz-Riley filters individually are always -6dB at the displayed cross-
over frequency, however the 18dB/oct Linkwitz filters individually are always -3dB at the displayed crossover frequency.
The reason for this is that Linkwitz-Riley filters are defined in terms of performance criterion on the summing of two
adjacent crossover HPF and LPF filters, rather than defined in terms of the pole-zero characteristics of individual filters.
The 18dB/oct Linkwitz-Riley individually are 18dB/oct Butterworth filters in that they have Butterworth pole-zero charac-
teristics and also satisfy the criterion for Linkwitz-Riley filters.
When a Linkwitz-Riley HPF and LPF of the same displayed crossover frequency are summed, the combined
response is always flat. With 12dB/oct Linkwitz-Riley crossover filters, one of the outputs must be inverted or else the
combined response will have a large notch at the crossover frequency.
4.6e Limit
A full function compressor/
limiter is included on each output chan-
nel. A limiter is commonly used to pre-
vent transient audio signal spikes from
damaging loudspeakers, manage ana-
log and digital recording levels, opti-
mize broadcast levels, or "thicken" the
sound of an audio source (compres-
sion). The adjustable parameters in-
clude Limiter In/Out, Limiter Thresh-
old, Ratio, Attack Time, and Release
Time.
The 3.24CL limiter threshold range is from -20dBu to +20dBu. This setting determines the signal level above
which gain reduction begins, and is indicated by the yellow LED (Lim) in the output meter section. Increases in audio level
above the threshold will be reduced according to the ratio settings.
The ratio control determines the amount of gain reduction above limiter threshold. Ratio ranges from a gentle
1.2:1 to a brick-wall INF:1. To illustrate how the ratio control works, imagine a commonly used loudspeaker protection ratio
of 10:1, which means that for every input signal increase of 10 dB above threshold, the output level will only increase by
1dB. The higher the ratio, the more pronounced the audio effect, so use the lowest ratio possible to sufficiently address
the problem.
