diff --git a/src/dsp/helpers.rs b/src/dsp/helpers.rs
index 7e9e70c..ff9d454 100644
--- a/src/dsp/helpers.rs
+++ b/src/dsp/helpers.rs
@@ -721,11 +721,10 @@ pub fn process_1pole_lowpass(input: f64, freq: f64, israte: f64, z: &mut f64) ->
 pub fn process_1pole_highpass(input: f64, freq: f64, israte: f64, z: &mut f64, y: &mut f64) -> f64 {
     let b  = (-std::f64::consts::TAU * freq * israte).exp();
     let a  = (1.0 + b) / 2.0;
-    let a1 = - a;
 
     let v =
           a  * input
-        + a1 * *z
+        - a  * *z
         + b  * *y;
     *y = v;
     *z = input;
@@ -737,10 +736,10 @@ pub fn process_1pole_highpass(input: f64, freq: f64, israte: f64, z: &mut f64, y
 // (page 5)
 #[inline]
 pub fn process_1pole_tpt_lowpass(input: f64, freq: f64, israte: f64, z: &mut f64) -> f64 {
-    let g  = (std::f64::consts::PI * freq * israte).tan();
-    let a1 = g / (1.0 + g);
+    let g = (std::f64::consts::PI * freq * israte).tan();
+    let a = g / (1.0 + g);
 
-    let v1 = a1 * (input - *z);
+    let v1 = a * (input - *z);
     let v2 = v1 + *z;
     *z = v2 + v1;
 
diff --git a/src/dsp/node_sfilter.rs b/src/dsp/node_sfilter.rs
index 9772b32..5a1da9c 100644
--- a/src/dsp/node_sfilter.rs
+++ b/src/dsp/node_sfilter.rs
@@ -89,7 +89,7 @@ impl DspNode for SFilter {
                 for frame in 0..ctx.nframes() {
                     let input = inp.read(frame) as f64;
                     let freq = denorm::SFilter::freq(freq, frame) as f64;
-                    let freq = freq.clamp(10.0, 22050.0);
+                    let freq = freq.clamp(1.0, 22000.0);
                     out.write(frame,
                         process_1pole_lowpass(
                             input, freq, self.israte, &mut self.z)
@@ -100,7 +100,7 @@ impl DspNode for SFilter {
                 for frame in 0..ctx.nframes() {
                     let input = inp.read(frame) as f64;
                     let freq = denorm::SFilter::freq(freq, frame) as f64;
-                    let freq = freq.clamp(10.0, 18000.0);
+                    let freq = freq.clamp(1.0, 22000.0);
                     out.write(frame,
                         process_1pole_tpt_lowpass(
                             input, freq, self.israte, &mut self.z)
@@ -111,7 +111,7 @@ impl DspNode for SFilter {
                 for frame in 0..ctx.nframes() {
                     let input = inp.read(frame) as f64;
                     let freq = denorm::SFilter::freq(freq, frame) as f64;
-                    let freq = freq.clamp(10.0, 22050.0);
+                    let freq = freq.clamp(1.0, 22000.0);
                     out.write(frame,
                         process_1pole_highpass(
                             input, freq, self.israte, &mut self.z, &mut self.y)
@@ -122,7 +122,7 @@ impl DspNode for SFilter {
                 for frame in 0..ctx.nframes() {
                     let input = inp.read(frame) as f64;
                     let freq = denorm::SFilter::freq(freq, frame) as f64;
-                    let freq = freq.clamp(10.0, 18000.0);
+                    let freq = freq.clamp(1.0, 22000.0);
                     out.write(frame,
                         process_1pole_tpt_highpass(
                             input, freq, self.israte, &mut self.z)
diff --git a/tests/common/mod.rs b/tests/common/mod.rs
index df39c46..5c92124 100644
--- a/tests/common/mod.rs
+++ b/tests/common/mod.rs
@@ -1,6 +1,7 @@
 pub use hexodsp::matrix::*;
 pub use hexodsp::nodes::new_node_engine;
 pub use hexodsp::dsp::*;
+pub use hexodsp::NodeExecutor;
 
 use hound;
 //use num_complex::Complex;
@@ -248,6 +249,20 @@ pub fn pset_d(matrix: &mut Matrix, nid: NodeId, parm: &str, v_denorm: f32) {
     matrix.set_param(p, SAtom::param(p.norm(v_denorm)));
 }
 
+#[allow(unused)]
+pub fn pset_n_wait(matrix: &mut Matrix, ne: &mut NodeExecutor, nid: NodeId, parm: &str, v_norm: f32) {
+    let p = nid.inp_param(parm).unwrap();
+    matrix.set_param(p, SAtom::param(v_norm));
+    run_for_ms(ne, 15.0);
+}
+
+#[allow(unused)]
+pub fn pset_d_wait(matrix: &mut Matrix, ne: &mut NodeExecutor, nid: NodeId, parm: &str, v_denorm: f32) {
+    let p = nid.inp_param(parm).unwrap();
+    matrix.set_param(p, SAtom::param(p.norm(v_denorm)));
+    run_for_ms(ne, 15.0);
+}
+
 #[allow(dead_code)]
 pub fn save_wav(name: &str, buf: &[f32]) {
     let spec = hound::WavSpec {
@@ -407,6 +422,90 @@ pub fn run_and_get_fft4096(
     fft_thres_at_ms(&mut out_l[..], FFT::F4096, thres, offs_ms)
 }
 
+#[allow(unused)]
+pub fn calc_exp_avg_buckets4096(fft: &[(u16, u32)]) -> Vec<(u16, u32)> {
+    let mut avg     = vec![];
+    let mut last_n  = [0; 256];
+    let mut p       = 0;
+    let mut cur_len = 2;
+
+    for (i, (fq, lvl)) in fft.iter().enumerate() {
+        last_n[p] = *lvl;
+        p += 1;
+        if p >= cur_len {
+            avg.push((
+                *fq,
+                last_n
+                    .iter()
+                    .take(cur_len)
+                    .map(|x| *x)
+                    .sum::<u32>()
+                / (cur_len as u32)));
+            p = 0;
+        }
+
+        if i % 16 == 0 {
+            cur_len += 2;
+            if cur_len > last_n.len() {
+                cur_len = last_n.len();
+            }
+            //d// println!("len={}", cur_len);
+        }
+    }
+
+    avg
+}
+
+#[allow(unused)]
+pub fn avg_fft_freqs(round_by: f32, ranges: &[u16], fft: &[(u16, u32)]) -> Vec<(u16, u32)> {
+    let mut from = 0;
+    let mut out = vec![];
+    for rng in ranges.iter() {
+        out.push(
+            (from,
+             ((avg_fft_range(from, *rng, fft)
+               / round_by)
+              .floor() * round_by)
+             as u32));
+        from = *rng;
+    }
+
+    out
+}
+
+#[allow(unused)]
+pub fn avg_fft_range(from_freq: u16, to_freq: u16, fft: &[(u16, u32)]) -> f32 {
+    let mut count = 0;
+    let mut sum   = 0;
+    for (fq, lvl) in fft.iter() {
+        if from_freq <= *fq && *fq < to_freq {
+            sum   += *lvl;
+            count += 1;
+        }
+    }
+
+    sum as f32 / count as f32
+}
+
+
+#[allow(unused)]
+pub fn run_and_get_fft512(
+    node_exec: &mut hexodsp::nodes::NodeExecutor,
+    thres: u32,
+    offs_ms: f32) -> Vec<(u16, u32)>
+{
+    let min_samples_for_fft = 512.0;
+    let offs_samples        = (offs_ms * (SAMPLE_RATE / 1000.0)).ceil();
+    let min_len_samples =
+        offs_samples
+        // 2.0 * for safety margin
+        + 2.0 * min_samples_for_fft;
+    let run_len_s = min_len_samples / SAMPLE_RATE;
+    let (mut out_l, _out_r) = run_no_input(node_exec, run_len_s);
+    fft_thres_at_ms(&mut out_l[..], FFT::F512, thres, offs_ms)
+}
+
+
 #[allow(unused)]
 pub fn run_and_get_fft4096_now(
     node_exec: &mut hexodsp::nodes::NodeExecutor,
diff --git a/tests/node_sfilter.rs b/tests/node_sfilter.rs
index 332287b..1c77193 100644
--- a/tests/node_sfilter.rs
+++ b/tests/node_sfilter.rs
@@ -1,8 +1,7 @@
 mod common;
 use common::*;
 
-#[test]
-fn check_node_sfilter_compare() {
+fn setup_sfilter_matrix() -> (Matrix, NodeExecutor) {
     let (node_conf, mut node_exec) = new_node_engine();
     let mut matrix = Matrix::new(node_conf, 3, 3);
 
@@ -20,52 +19,205 @@ fn check_node_sfilter_compare() {
                        .out(None, None, noise.out("sig")));
     matrix.place(1, 2, Cell::empty(out)
                        .input(out.inp("ch2"), None, None));
-    pset_d(&mut matrix, sf, "freq", 440.0);
     matrix.sync().unwrap();
 
-    let ta = std::time::Instant::now();
-    let (out_l, out_r) = run_for_ms(&mut node_exec, 10000.0);
-    let ta = std::time::Instant::now().duration_since(ta);
-    println!("ta1 Elapsed: {:?}", ta);
-
-    save_wav("check_noise_sfilt_in.wav", &out_r);
-    save_wav("check_noise_sfilt1.wav", &out_l);
-
-//    let fft = run_and_get_fft4096(&mut node_exec, 5, 1000.0);
-//    for (fq, lvl) in fft {
-//        println!("{:5}: {}", fq, lvl);
-//    }
-
-    pset_s(&mut matrix, sf, "ftype", 1);
-
-    let ta = std::time::Instant::now();
-    let (out_l, _) = run_for_ms(&mut node_exec, 10000.0);
-    save_wav("check_noise_sfilt2.wav", &out_l);
-    let ta = std::time::Instant::now().duration_since(ta);
-    println!("ta2 Elapsed: {:?}", ta);
-
-    pset_s(&mut matrix, sf, "ftype", 2);
-
-    let ta = std::time::Instant::now();
-    let (out_l, _) = run_for_ms(&mut node_exec, 10000.0);
-    save_wav("check_noise_sfilt3.wav", &out_l);
-    let ta = std::time::Instant::now().duration_since(ta);
-    println!("ta3 Elapsed: {:?}", ta);
-
-    pset_s(&mut matrix, sf, "ftype", 3);
-
-    let ta = std::time::Instant::now();
-    let (out_l, _) = run_for_ms(&mut node_exec, 10000.0);
-    save_wav("check_noise_sfilt4.wav", &out_l);
-    let ta = std::time::Instant::now().duration_since(ta);
-    println!("ta4 Elapsed: {:?}", ta);
-
-//    let fft = run_and_get_fft4096(&mut node_exec, 5, 1000.0);
-//    for (fq, lvl) in fft {
-//        println!("{:5}: {}", fq, lvl);
-//    }
-
-//    let fft = run_and_get_fft4096(&mut node_exec, 50, 1000.0);
-//    assert!(fft.len() == 0);
-    assert!(false);
+    (matrix, node_exec)
+}
+
+fn fft_with_freq_res_type(
+    matrix: &mut Matrix,
+    node_exec: &mut NodeExecutor,
+    ftype: i64, freq: f32, _res: f32) -> Vec<(u16, u32)>
+{
+    let sf = NodeId::SFilter(0);
+    pset_d_wait(matrix, node_exec, sf, "freq", freq);
+//    pset_d_wait(&mut matrix, &mut node_exec, sf, "freq", freq);
+    pset_s(matrix, sf, "ftype", ftype);
+    run_and_get_fft4096(node_exec, 0, 1000.0)
+}
+
+#[test]
+fn check_node_sfilter_lowpass() {
+    let (mut matrix, mut node_exec) = setup_sfilter_matrix();
+
+    // Low Pass @ 1000Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 0, 1000.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 8000, 12000, 16000,
+        ], &fft[..]), vec![
+            (0, 16), (100, 24), (250, 16), (500, 12), (750, 12), (1000, 12),
+            (1500, 8), (2000, 4), (3000, 4), (4000, 0), (8000, 0), (12000, 0),
+        ]);
+
+    // Low Pass @ 4000Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 0, 4000.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 8000, 12000, 16000,
+        ], &fft[..]), vec![
+            (0, 16), (100, 20), (250, 16), (500, 12), (750, 20), (1000, 16),
+            (1500, 16), (2000, 16), (3000, 12), (4000, 8), (8000, 4), (12000, 4),
+        ]);
+
+    // Low Pass @ 22050Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 0, 22050.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 8000, 12000, 16000,
+            22050,
+        ], &fft[..]), vec![
+            (0, 16), (100, 20), (250, 12), (500, 16), (750, 16), (1000, 16),
+            (1500, 16), (2000, 16), (3000, 20), (4000, 16), (8000, 16), (12000, 16),
+            (16000, 16),
+        ]);
+
+    // Low Pass @ 0Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 0, 0.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            500, 1000, 1500, 2000, 8000, 12000, 16000, 20000
+        ], &fft[..]), vec![
+            (0, 0), (500, 0), (1000, 0),
+            (1500, 0), (2000, 0), (8000, 0), (12000, 0), (16000, 0),
+        ]);
+}
+
+#[test]
+fn check_node_sfilter_lowpass_tpt() {
+    let (mut matrix, mut node_exec) = setup_sfilter_matrix();
+
+    // Low Pass TPT @ 1000Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 1, 1000.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 8000, 12000, 16000,
+        ], &fft[..]), vec![
+            (0, 16), (100, 24), (250, 16), (500, 12), (750, 12), (1000, 12),
+            (1500, 8), (2000, 4), (3000, 4), (4000, 0), (8000, 0), (12000, 0),
+        ]);
+
+    // Low Pass TPT @ 4000Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 1, 4000.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 8000, 12000, 16000,
+        ], &fft[..]), vec![
+            (0, 16), (100, 20), (250, 16), (500, 12), (750, 20), (1000, 16),
+            (1500, 16), (2000, 16), (3000, 12), (4000, 8), (8000, 4), (12000, 0),
+        ]);
+
+    // Low Pass TPT @ 22050Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 1, 22050.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 8000, 12000, 16000,
+            22050,
+        ], &fft[..]), vec![
+            (0, 16), (100, 20), (250, 12), (500, 16), (750, 16), (1000, 16),
+            (1500, 16), (2000, 16), (3000, 20), (4000, 16), (8000, 16), (12000, 16),
+            (16000, 16),
+        ]);
+
+    // Low Pass TPT @ 0Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 1, 0.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            500, 1000, 1500, 2000, 8000, 12000, 16000, 20000
+        ], &fft[..]), vec![
+            (0, 0), (500, 0), (1000, 0),
+            (1500, 0), (2000, 0), (8000, 0), (12000, 0), (16000, 0),
+        ]);
+}
+
+#[test]
+fn check_node_sfilter_highpass() {
+    let (mut matrix, mut node_exec) = setup_sfilter_matrix();
+
+    // High Pass @ 1000Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 2, 1000.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 8000, 12000
+        ], &fft[..]), vec![
+            (0, 0), (100, 4), (250, 4), (500, 8), (750, 8), (1000, 16),
+            (1500, 16), (2000, 16), (3000, 16), (8000, 16),
+        ]);
+
+    // High Pass @ 4000Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 2, 4000.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 8000, 12000
+        ], &fft[..]), vec![
+            (0, 0), (100, 0), (250, 0), (500, 0), (750, 4), (1000, 4),
+            (1500, 4), (2000, 8), (3000, 12), (8000, 16),
+        ]);
+
+    // High Pass @ 22050Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 2, 22050.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            500, 1000, 1500, 2000, 8000, 12000, 16000, 20000
+        ], &fft[..]), vec![
+            (0, 0), (500, 0), (1000, 0),
+            (1500, 0), (2000, 4), (8000, 12), (12000, 12), (16000, 16),
+        ]);
+
+    // High Pass @ 0Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 2, 0.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            10, 50, 100, 250, 500, 750, 1000, 1500, 2000, 3000, 8000, 12000
+        ], &fft[..]), vec![
+            (0, 12), (10, 24), (50, 24), (100, 20), (250, 12),
+            (500, 16), (750, 20), (1000, 16), (1500, 16), (2000, 16),
+            (3000, 20), (8000, 16),
+        ]);
+}
+
+#[test]
+fn check_node_sfilter_highpass_tpt() {
+    let (mut matrix, mut node_exec) = setup_sfilter_matrix();
+
+    // High Pass TPT @ 1000Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 3, 1000.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 8000, 12000
+        ], &fft[..]), vec![
+            (0, 0), (100, 0), (250, 4), (500, 8), (750, 8), (1000, 16),
+            (1500, 16), (2000, 16), (3000, 16), (8000, 16),
+        ]);
+
+    // High Pass TPT @ 4000Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 3, 4000.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            100, 250, 500, 750, 1000, 1500, 2000, 3000, 8000, 12000
+        ], &fft[..]), vec![
+            (0, 0), (100, 0), (250, 0), (500, 0), (750, 4), (1000, 4),
+            (1500, 4), (2000, 8), (3000, 12), (8000, 16),
+        ]);
+
+    // High Pass TPT @ 22050Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 3, 22050.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            500, 1000, 1500, 2000, 8000, 12000, 16000, 20000
+        ], &fft[..]), vec![
+            (0, 0), (500, 0), (1000, 0),
+            (1500, 0), (2000, 0), (8000, 0), (12000, 0), (16000, 0),
+        ]);
+
+    // High Pass TPT @ 0Hz
+    let fft = fft_with_freq_res_type(&mut matrix, &mut node_exec, 3, 0.0, 0.0);
+    assert_eq!(
+        avg_fft_freqs(4.0, &[
+            10, 50, 100, 250, 500, 750, 1000, 1500, 2000, 3000, 8000, 12000
+        ], &fft[..]), vec![
+            (0, 12), (10, 24), (50, 24), (100, 20), (250, 12),
+            (500, 16), (750, 20), (1000, 16), (1500, 16), (2000, 16),
+            (3000, 20), (8000, 16),
+        ]);
 }