zdf_2pole_mode — Zero-delay feedback implementation of 2 pole filter with multimode output.
Zero-delay feedback implementation of a 2 pole (12 dB/oct) filter. Offers low-pass, band-pass, and high-pass output.
istor --initial disposition of internal data space. Since filtering incorporates a feedback loop of previous output, the initial status of the storage space used is significant. A zero value will clear the space; a non-zero value will allow previous information to remain. The default value is 0.
alp -- low-pass output signal.
abp -- band-pass output signal.
ahp -- high-pass output signal.
ain -- input signal.
xcf -- filter cutoff frequency (i-, k-, or a-rate).
Q -- filter Q value (i-, k-, or a-rate). Range 0.5-25.0.
Here is an example of the zdf_2pole_mode opcode. It uses the file zdf_2pole_mode.csd.
Example 1238. Example of the zdf_2pole_mode opcode.
<CsoundSynthesizer> <CsOptions> -o dac </CsOptions> <CsInstruments> sr = 44100 ksmps = 1 nchnls = 2 0dbfs = 1 instr 1 asig = random:a(-1.0, 1.0) asigs[] init 3 asigs[0], asigs[1], asigs[2] zdf_2pole_mode asig, line(20, p3, 10000), 4 asig = asigs[p4] outc(asig, asig) endin instr 2 asig = vco2(0.5, 220) asigs[] init 3 asigs[0], asigs[1], asigs[2] zdf_2pole_mode asig, line(20, p3, 10000), 4 outc(asigs[p4], asigs[p4]) endin </CsInstruments> <CsScore> i1 0 4 0 i1 5 4 1 i1 10 4 2 i2 15 4 0 i2 20 4 1 i2 25 4 2 </CsScore> </CsoundSynthesizer>
This filter is based on the work of Will Pirkle that employs Vadim Zavalishin's work with bilinear tranforms to create topology-preserving transform (TPT) implementations of analog filters.
Pirkle, Will. Designing Software Synthesizer Plug-ins in C++: For RackAFX, VST3, and Audio Units. CRC Press, 2014.
Pirkle, Will. AN-4: Virtual Analog (VA) Filter Implementation. 2013.
Zavalishin, Vadim. "The Art of VA filter design." Native Instruments, 2012.