Set up one of the filters in a parametric equalizer.
Set the parameters for one equalizer filter. The overall equalizer response will be a product of all its filter responses.
- Returns:
- 0 on success, or one of the HPI Error codes.
- Parameters:
-
phSubSys Pointer to HPI subsystem handle. hControl Equalizer control handle. wIndex Index of band to set. nType The kind of filter that the band will implement has many different options. In the following descriptions, low and high frequencies mean relative to dwBandFrequency. "Elsewhere" means at frequencies different from dwBandFrequency, how different depends on Q (look at the following figures) - HPI_FILTER_TYPE_BYPASS
The filter is bypassed (turned off). Other parameters are ignored - HPI_FILTER_TYPE_LOWPASS
Has unity gain at low frequencies, and attenuation tending towards infinite at high frequencies. nGain0_01dB parameter is ignored. See "lp" in the following diagrams. - HPI_FILTER_TYPE_HIGHPASS
Has unity gain at high frequencies, and attenuation tending towards infinite at low frequencies. nGain0_01dB parameter is ignored. (Not illustrated, basically the opposite of lowpass) - HPI_FILTER_TYPE_BANDPASS
Has unity gain at dwFrequencyHz and tends towards infinite attenuation elsewhere. nGain0_01dB parameter is ignored. See "bp" in the following diagrams. - HPI_FILTER_TYPE_BANDSTOP
Maximum attenuation at dwFrequencyHz, tends towards unity gain elsewhere. nGain0_01dB parameter is ignored. See "bs" in the following diagrams. - HPI_FILTER_TYPE_LOWSHELF
Has gain of nGain0_01dB at low frequencies and unity gain at high frequencies. See "ls" in the following diagrams. - HPI_FILTER_TYPE_HIGHSHELF
Has gain of nGain0_01dB at high frequencies and unity gain at low frequencies. See "hs" in the following diagrams. - HPI_FILTER_TYPE_EQ_BAND
Has gain of nGain0_01dB at dwFrequencyHz and unity gain elsewhere. See "eq" in the following diagrams.
dwFrequencyHz is the defining frequency of the filter. It is the center frequency of types HPI_FILTER_TYPE_ BANDPASS, HPI_FILTER_TYPE_BANDSTOP, HPI_FILTER_TYPE_EQ_BAND. It is the -3dB frequency of HPI_FILTER_TYPE_LOWPASS, HPI_FILTER_TYPE_HIGHPASS when Q=1 or resonant frequency when Q>1 and it is the half gain frequency of HPI_FILTER_TYPE_LOWSHELF, HPI_FILTER_TYPE_HIGHSHELF. The maximum allowable value is half the current adapter samplerate i.e. Fs/2. When the adapter samplerate is changed, the equalizer filters will be recalculated. If this results in the band frequency being greater than Fs/2, then the filter will be turned off. nQ100 controls filter sharpness. To allow the use of an integer parameter, Filter Q = dwQ100/100.
In the following figure, gain is 20dB (10x) (nGain0_01dB=2000) and sampling frequency is normalized to 1Hz (10^0) and nFrequency is 0.1 x sampling frequency. Q=[0.2 0.5 1 2 4 8].
Q can also be thought of as affecting bandwidth or shelf slope of some of these filters.
Bandwidth is measured in octaves (between -3 dB frequencies for BPF and notch or between midpoint (dBgain/2) gain frequencies for peaking EQ).
The relationship between bandwidth and Q is:Shelf slope S, a "shelf slope" parameter (for shelving EQ only). When S = 1, the shelf slope is as steep as it can be and remain monotonically increasing or decreasing gain with frequency. The shelf slope, in dB/octave, remains proportional to S for all other values.1/Q = 2*sinh[ln(2)/2*bandwidth*omega/sin(omega)] (digital filter using BLT) or 1/Q = 2*sinh[ln(2)/2*bandwidth]) (analog filter prototype) Where omega = 2*pi*frequency/sampleRate
The relationship between shelf slope and Q is 1/Q = sqrt[(A + 1/A)*(1/S - 1) + 2]
where A = 10^(dBgain/40)
Effect of Q on EQ filters
nGain0_01dB The gain is expressed in milliBels (100ths of a decibel). Allowable range is -1000 to +1000 mB. Usable range will likely be less than this. This parameter is only applicable to the equalizer filter types HPI_FILTER_TYPE_LOWSHELF,HPI_FILTER_TYPE_HIGHSHELF and HPI_FILTER_TYPE_EQ_BAND. Other filters always have unity gain in the passband.
In the following figure, Q=1.0 and sampling frequency is normalized to 1Hz (10^0) and nFrequency is 0.1 x sampling frequency. dBgain=[-20 -10 0 10 20]
For example, to produce the upper (red) curve in the "Filtertype_eq_band" graph:Effect of gain on EQ filterswHE= HPI_ParametricEQ_SetBand( phSubsys, hMicrophoneControl, HPI_FILTER_TYPE_EQ_BAND, 4410 // 4.41khz 100 // Q=1 20*100, // 20dB );
- HPI_FILTER_TYPE_BYPASS
