Ashly Protea 4.8SP Betriebshandbuch - Seite 9

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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 always +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 SP processors, 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

crossover 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 characteristics 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

channel. A limiter is commonly used

to prevent transient audio signal spikes

from damaging loudspeakers, manage

analog and digital recording levels,

optimize broadcast levels, or "thicken"

the sound of an audio source (com-

pression). The adjustable parameters

include Limiter In/Out, Limiter

Threshold, Ratio, Attack Time, and

Release Time.

The 4.8SP and 3.6SP 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.

Attack (A__ms) and Release (R__ms) settings adjust the time it takes the limiter to engage and then disengage

when the signal increases above threshold and then subsequently falls back below threshold. Attack time is adjustable

from 0.5ms through 50ms, while release time ranges from 10ms through 1s. A very fast attack time can sound unnatural,

Operating Manual - PROTEA 4.8SP and 3.6SP System Processors

LIMITER 1

INF:1 A05ms

R100ms

0dBu