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<div class="refentry">
<a id="k35_lpf"></a>
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<a id="IndexK35_lpf" class="indexterm"></a>
<div class="refnamediv">
<h2>
<span class="refentrytitle">K35_lpf</span>
</h2>
<p>K35_lpf —
Zero-delay feedback implementation of Korg35 resonant low-pass filter.
</p>
</div>
<div class="refsect1">
<a id="idm281472921477992"></a>
<h2>Description</h2>
<p>
Zero-delay feedback implementation of Korg35 resonant low-pass filter. This filter design is found in the Korg MS10, early MS20, and Monotron series.
</p>
</div>
<div class="refsect1">
<a id="idm281472921486696"></a>
<h2>Syntax</h2>
<pre class="synopsis">asig <span class="command"><strong>K35_lpf</strong></span> ain, xcf, xQ [, inlp, isaturation, istor]</pre>
</div>
<div class="refsect1">
<a id="idm281472921484456"></a>
<h2>Initialization</h2>
<p>
<span class="emphasis"><em>istor</em></span> --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.
</p>
</div>
<div class="refsect1">
<a id="idm281472921413144"></a>
<h2>Performance</h2>
<p>
<span class="emphasis"><em>asig</em></span> -- output signal.
</p>
<p>
<span class="emphasis"><em>ain</em></span> -- input signal.
</p>
<p>
<span class="emphasis"><em>xcf</em></span> -- filter cutoff frequency (i-, k-, or a-rate).
</p>
<p>
<span class="emphasis"><em>xQ</em></span> -- filter Q value (i-, k-, or a-rate). Range 1.0-10.0 (clamped by opcode). Self-oscillation occurs at 10.0.
</p>
<p>
<span class="emphasis"><em>knlp</em></span> (optional, default=0) -- Non-linear processing method. 0 = no processing, 1 = non-linear processing. Method 1 uses tanh(ksaturation * input). Enabling NLP may increase the overall output of filter above unity and should be compensated for outside of the filter.
</p>
<p>
<span class="emphasis"><em>ksaturation</em></span> (optional, default=1) -- saturation amount to use for non-linear processing. Values > 1 increase the steepness of the NLP curve.</p>
</div>
<div class="refsect1">
<a id="idm281472921406952"></a>
<h2>Examples</h2>
<p>
Here is an example of the K35_lpf opcode. It uses the file <a class="ulink" href="examples/k35.csd" target="_top"><em class="citetitle">k35.csd</em></a>.
</p>
<div class="example">
<a id="idm281472921405176"></a>
<p class="title">
<strong>Example 451. Example of the K35_lpf opcode.</strong>
</p>
<div class="example-contents">
<div class="refsect1">
<a id="idm281472749031448"></a>
<pre class="programlisting">
<span class="nt"><CsoundSynthesizer></span>
<span class="nt"><CsOptions></span>
<span class="nt"></CsOptions></span>
<span class="c1">; ==============================================</span>
<span class="nt"><CsInstruments></span>
<span class="vg">sr</span> <span class="o">=</span> <span class="mi">48000</span>
<span class="vg">ksmps</span> <span class="o">=</span> <span class="mi">1</span>
<span class="vg">nchnls</span> <span class="o">=</span> <span class="mi">2</span>
<span class="vg">0dbfs</span> <span class="o">=</span> <span class="mi">1</span>
<span class="c1">;; test instruments to demo filter cutoff sweep with high resonance</span>
<span class="kd">instr</span> <span class="nf">1</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">vco2</span><span class="p">(</span><span class="mf">0.5</span><span class="p">,</span> <span class="nb">cps2pch</span><span class="p">(</span><span class="mf">6.00</span><span class="p">,</span> <span class="mi">12</span><span class="p">))</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="n">K35_lpf</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="nb">expseg</span><span class="p">:</span><span class="nb">a</span><span class="p">(</span><span class="mi">10000</span><span class="p">,</span> <span class="nb">p3</span><span class="p">,</span> <span class="mi">30</span><span class="p">),</span> <span class="mf">9.9</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">*=</span> <span class="mf">0.25</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">limit</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.0</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">)</span>
<span class="nb">outc</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> a<span class="n">sig</span><span class="p">)</span>
<span class="kd">endin</span>
<span class="kd">instr</span> <span class="nf">2</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">vco2</span><span class="p">(</span><span class="mf">0.5</span><span class="p">,</span> <span class="nb">cps2pch</span><span class="p">(</span><span class="mf">6.00</span><span class="p">,</span> <span class="mi">12</span><span class="p">))</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="n">K35_lpf</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="nb">expseg</span><span class="p">:</span><span class="nb">k</span><span class="p">(</span><span class="mi">10000</span><span class="p">,</span> <span class="nb">p3</span><span class="p">,</span> <span class="mi">30</span><span class="p">),</span> <span class="mf">9.9</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">*=</span> <span class="mf">0.25</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">limit</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.0</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">)</span>
<span class="nb">outc</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> a<span class="n">sig</span><span class="p">)</span>
<span class="kd">endin</span>
<span class="kd">instr</span> <span class="nf">3</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">vco2</span><span class="p">(</span><span class="mf">0.5</span><span class="p">,</span> <span class="nb">cps2pch</span><span class="p">(</span><span class="mf">6.00</span><span class="p">,</span> <span class="mi">12</span><span class="p">))</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="n">K35_hpf</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="nb">expseg</span><span class="p">:</span><span class="nb">a</span><span class="p">(</span><span class="mi">10000</span><span class="p">,</span> <span class="nb">p3</span><span class="p">,</span> <span class="mi">30</span><span class="p">),</span> <span class="mf">9.9</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">*=</span> <span class="mf">0.25</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">limit</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.0</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">)</span>
<span class="nb">outc</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> a<span class="n">sig</span><span class="p">)</span>
<span class="kd">endin</span>
<span class="kd">instr</span> <span class="nf">4</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">vco2</span><span class="p">(</span><span class="mf">0.5</span><span class="p">,</span> <span class="nb">cps2pch</span><span class="p">(</span><span class="mf">6.00</span><span class="p">,</span> <span class="mi">12</span><span class="p">))</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="n">K35_hpf</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="nb">expseg</span><span class="p">:</span><span class="nb">k</span><span class="p">(</span><span class="mi">10000</span><span class="p">,</span> <span class="nb">p3</span><span class="p">,</span> <span class="mi">30</span><span class="p">),</span> <span class="mf">9.9</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">*=</span> <span class="mf">0.25</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">limit</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.0</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">)</span>
<span class="nb">outc</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> a<span class="n">sig</span><span class="p">)</span>
<span class="kd">endin</span>
<span class="c1">;; beat instruments</span>
<span class="kd">instr</span> <span class="nf">ms20_drum</span>
i<span class="n">pch</span> <span class="o">=</span> <span class="nb">cps2pch</span><span class="p">(</span><span class="nb">p4</span><span class="p">,</span> <span class="mi">12</span><span class="p">)</span>
i<span class="n">amp</span> <span class="o">=</span> <span class="nb">ampdbfs</span><span class="p">(</span><span class="nb">p5</span><span class="p">)</span>
a<span class="n">env</span> <span class="o">=</span> <span class="nb">expseg</span><span class="p">:</span><span class="nb">a</span><span class="p">(</span><span class="mi">10000</span><span class="p">,</span> <span class="mf">0.05</span><span class="p">,</span> i<span class="n">pch</span><span class="p">,</span> <span class="nb">p3</span> <span class="o">-</span> <span class="mf">.05</span><span class="p">,</span> i<span class="n">pch</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">rand</span><span class="p">:</span><span class="nb">a</span><span class="p">(</span><span class="o">-</span><span class="mf">1.0</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="n">K35_hpf</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> <span class="mi">60</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="n">K35_lpf</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> a<span class="n">env</span><span class="p">,</span> <span class="mf">9.8</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">tanh</span><span class="p">(</span>a<span class="n">sig</span> <span class="o">*</span> <span class="mi">16</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">*=</span> <span class="nb">expon</span><span class="p">(</span>i<span class="n">amp</span><span class="p">,</span> <span class="nb">p3</span><span class="p">,</span> <span class="mf">0.0001</span><span class="p">)</span>
<span class="nb">outc</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> a<span class="n">sig</span><span class="p">)</span>
<span class="kd">endin</span>
<span class="kd">instr</span> <span class="nf">ms20_bass</span>
i<span class="n">pch</span> <span class="o">=</span> <span class="nb">cps2pch</span><span class="p">(</span><span class="nb">p4</span><span class="p">,</span> <span class="mi">12</span><span class="p">)</span>
i<span class="n">amp</span> <span class="o">=</span> <span class="nb">ampdbfs</span><span class="p">(</span><span class="nb">p5</span><span class="p">)</span>
a<span class="n">env</span> <span class="o">=</span> <span class="nb">expseg</span><span class="p">(</span><span class="mi">1000</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">,</span> i<span class="n">pch</span> <span class="o">*</span> <span class="mi">2</span><span class="p">,</span> <span class="nb">p3</span> <span class="o">-</span> <span class="mf">.05</span><span class="p">,</span> i<span class="n">pch</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="nb">vco2</span><span class="p">(</span><span class="mf">1.0</span><span class="p">,</span> i<span class="n">pch</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="n">K35_hpf</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> i<span class="n">pch</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">=</span> <span class="n">K35_lpf</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> a<span class="n">env</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
a<span class="n">sig</span> <span class="o">*=</span> <span class="nb">expon</span><span class="p">:</span><span class="nb">a</span><span class="p">(</span>i<span class="n">amp</span><span class="p">,</span> <span class="nb">p3</span><span class="p">,</span> <span class="mf">0.0001</span><span class="p">)</span> <span class="o">*</span> <span class="mf">0.8</span>
<span class="nb">outc</span><span class="p">(</span>a<span class="n">sig</span><span class="p">,</span> a<span class="n">sig</span><span class="p">)</span>
<span class="kd">endin</span>
<span class="c1">;; perf code</span>
gk<span class="n">tempo</span> <span class="nb">init</span> <span class="mi">122</span>
<span class="kd">opcode</span> <span class="nf">beat_dur</span><span class="p">,</span><span class="kt">i</span><span class="p">,</span><span class="kt">0</span>
<span class="nb">xout</span> <span class="mi">60</span> <span class="o">/</span> <span class="nb">i</span><span class="p">(</span>gk<span class="n">tempo</span><span class="p">)</span>
<span class="kd">endop</span>
<span class="kd">instr</span> <span class="nf">bass_player</span>
i<span class="n">dur</span> <span class="o">=</span> <span class="nf">beat_dur</span><span class="p">()</span> <span class="o">/</span> <span class="nb">int</span><span class="p">(</span><span class="nb">random</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">))</span>
i<span class="n">pch</span> <span class="o">=</span> <span class="mf">6.00</span> <span class="o">+</span> <span class="nb">int</span><span class="p">(</span><span class="nb">random</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">))</span> <span class="o">+</span> <span class="nb">int</span><span class="p">(</span><span class="nb">random</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">))</span> <span class="o">/</span> <span class="mi">100</span>
<span class="nb">schedule</span><span class="p">(</span><span class="s">"ms20_bass"</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> i<span class="n">dur</span><span class="p">,</span> i<span class="n">pch</span><span class="p">,</span> <span class="o">-</span><span class="mi">11</span><span class="p">)</span>
<span class="k">if</span><span class="p">(</span><span class="nb">p2</span> <span class="o"><</span> <span class="mf">37.5</span><span class="p">)</span> <span class="k">then</span>
<span class="nb">schedule</span><span class="p">(</span><span class="s">"bass_player"</span><span class="p">,</span> i<span class="n">dur</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">)</span>
<span class="k">endif</span>
<span class="nb">turnoff</span>
<span class="kd">endin</span>
<span class="kd">instr</span> <span class="nf">beat_player</span>
i<span class="n">step_total</span> <span class="o">=</span> <span class="nb">p4</span>
i<span class="n">step</span> <span class="o">=</span> i<span class="n">step_total</span> <span class="o">%</span> <span class="mi">16</span>
<span class="k">if</span><span class="p">(</span>i<span class="n">step</span> <span class="o">%</span> <span class="mi">4</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span> <span class="k">then</span>
i<span class="n">pch</span> <span class="o">=</span> <span class="p">((</span>i<span class="n">step_total</span> <span class="o">%</span> <span class="mi">128</span><span class="p">)</span> <span class="o"><</span> <span class="mi">112</span><span class="p">)</span> <span class="p">?</span> <span class="mf">4.00</span> <span class="p">:</span> <span class="mf">8.00</span>
i<span class="n">amp</span> <span class="o">=</span> <span class="p">(</span>i<span class="n">step</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span> <span class="p">?</span> <span class="o">-</span><span class="mi">9</span> <span class="p">:</span> <span class="o">-</span><span class="mi">12</span>
<span class="nb">schedule</span><span class="p">(</span><span class="s">"ms20_drum"</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mf">0.5</span><span class="p">,</span> i<span class="n">pch</span><span class="p">,</span> i<span class="n">amp</span><span class="p">)</span>
<span class="k">endif</span>
<span class="nb">schedule</span><span class="p">(</span><span class="s">"ms20_drum"</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mf">0.125</span><span class="p">,</span> <span class="mf">14.00</span><span class="p">,</span>
<span class="p">(</span>i<span class="n">step</span> <span class="o">%</span> <span class="mi">4</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span> <span class="p">?</span> <span class="o">-</span><span class="mi">12</span> <span class="p">:</span> <span class="o">-</span><span class="mi">18</span><span class="p">)</span>
<span class="k">if</span><span class="p">(</span><span class="nb">p2</span> <span class="o"><</span> <span class="mf">37.5</span><span class="p">)</span> <span class="k">then</span>
<span class="nb">schedule</span><span class="p">(</span><span class="s">"beat_player"</span><span class="p">,</span> <span class="nf">beat_dur</span><span class="p">()</span> <span class="o">/</span> <span class="mi">4</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">,</span> i<span class="n">step_total</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span>
<span class="k">endif</span>
<span class="nb">turnoff</span>
<span class="kd">endin</span>
<span class="c1">;; start play of beats</span>
<span class="kd">instr</span> <span class="nf">start_beats</span>
<span class="nb">schedule</span><span class="p">(</span><span class="s">"beat_player"</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="nb">schedule</span><span class="p">(</span><span class="s">"bass_player"</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">)</span>
<span class="kd">endin</span>
<span class="nt"></CsInstruments></span>
<span class="c1">; ==============================================</span>
<span class="nt"><CsScore></span>
<span class="nb">i</span><span class="mi">1</span> <span class="mi">0</span> <span class="mf">5.0</span>
<span class="nb">i</span><span class="mi">2</span> <span class="mi">5</span> <span class="mf">5.0</span>
<span class="nb">i</span><span class="mi">3</span> <span class="mi">10</span> <span class="mf">5.0</span>
<span class="nb">i</span><span class="mi">4</span> <span class="mi">15</span> <span class="mf">5.0</span>
<span class="nb">i</span> <span class="s">"start_beats"</span> <span class="mi">22</span> <span class="mf">0.5</span> <span class="mi">0</span>
<span class="nt"></CsScore></span>
<span class="nt"></CsoundSynthesizer></span>
</pre>
</div>
</div>
</div>
<p><br class="example-break" />
</p>
</div>
<div class="refsect1">
<a id="idm281472921403256"></a>
<h2>References</h2>
<p> 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.</p>
<p>
</p>
<div class="orderedlist">
<ol class="orderedlist" type="1">
<li class="listitem">
<p>
Pirkle, Will. Designing Software Synthesizer Plug-ins in C++: For
RackAFX, VST3, and Audio Units. CRC Press, 2014.
</p>
</li>
<li class="listitem">
<p>
Pirkle, Will. AN-5: Virtual Analog (VA) Korg35 Lowpass Filter v3.5. 2013.
</p>
</li>
<li class="listitem">
<p>
Zavalishin, Vadim. "The Art of VA filter design." Native Instruments, 2012.
</p>
</li>
</ol>
</div>
<p>
</p>
</div>
<div class="refsect1">
<a id="idm281472921397816"></a>
<h2>Credits</h2>
<p>
</p>
<table border="0" summary="Simple list" class="simplelist">
<tr>
<td>Author: Steven Yi</td>
</tr>
<tr>
<td>April 2017</td>
</tr>
</table>
<p>
</p>
<p>New in Csound 6.09.0</p>
</div>
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