Telex ISP-100 manual LowShelf Filters, HighShelf Filters

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In the ISP-100 highpass filters the passband slope is selectable from 6 dB/octave, 12 dB/octave, 18 dB/octave, and 24 dB/octave for the Bessel and Butterworth types, and from 12 dB/octave and 24 dB/ octave for the Linkwitz-Riley type.

NOTE: Linkwitz-Riley filters are also known as Butterworth-Squared filters because they are the product of two Butterworth filters. A 12 dB/octave Linkwitz-Riley highpass filter may be constructed by cascading two identical 6 dB/octave Butterworth highpass filters; a 24 dB/octave Linkwitz-Riley highpass filter may be constructed by cascading two identical 12 dB/octave Butterworth highpass filters.

LowShelf Filters

LowShelf Filters raise or lower the magnitude response at frequencies below the hinge frequency without altering the response at frequencies above the hinge frequency. Since it is not realistically possible to create a perfect filter that alters only low frequencies without affecting high frequencies, lowshelf filter design involves compromises that allow some rounding of the corner at the filter hinge frequency and some slope in the transition to the unaltered high frequencies.

The hinge frequency is defined as the frequency at which the magnitude of the filter response is changed by (approximately) 3 dB relative to the unfiltered signal. In the ISP-100 lowshelf filters the hinge frequency is continuously variable from 20Hz to 20 kHz.

In the ISP-100 lowshelf filters the transition band slope is selectable from 6 dB/octave and 12 dB/octave.

In the ISP-100 lowshelf filters the boost/cut is continuously variable from -12 dB to +12 dB.

NOTE: When a lowshelf filter with boost is selected, the expected filter frequency response is characterized by boosted low frequencies and unaltered high frequencies. In the ISP-100 this filter will exhibit unaltered response at low frequencies and cut response at high frequencies. The shape of the frequency response will be exactly as expected, but the level of the response will be shifted downward by an amount exactly equal to the selected boost quantity. This is necessary to prevent numerical overflow problems in the DSP filter coefficients, and represents normal operation for the lowshelf filter.

HighShelf Filters

HighShelf Filters raise or lower the magnitude response at frequencies above the hinge frequency without altering the response at frequencies below the hinge frequency. Since it is not realistically possible to create a perfect filter that alters only high frequencies without affecting low frequencies, highshelf filter design involves compromises that allow some rounding of the corner at the filter hinge frequency and some slope in the transition to the unaltered low frequencies.

Processing Components - Filter