Ashly Protea DSP480 Manual de instrucciones - Página 9

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Operating Manual - PROTEA™ DSP480 and DSP360 System Processors

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 DSP 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 tran-

sient audio signal spikes from damaging

loudspeakers, manage analog and digital

recording levels, optimize broadcast

levels, or "thicken" the sound of an

audio source (compression). The adjust-

able parameters include Limiter In/Out,

Limiter Threshold, Ratio, Attack Time,

and Release Time.

The DSP480 and DSP360 lim-

iter 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, while a very long

attack time can miss some of the initial transient. Similarly, a very short release time can make the audio sound uneven, while a

very long release time can create "pumping", or "breathing" characteristics depending on the kind of signal. Experiment to find

the best solution for a given application.