From d845d87e2c7fa7afb2ac0ecb239ae1cc0b341937 Mon Sep 17 00:00:00 2001
From: Robin Haberkorn <robin.haberkorn@googlemail.com>
Date: Tue, 19 Dec 2023 22:19:50 +0300
Subject: added example for pitch tracking via FFT

---
 COPYING             |   3 +++
 examples/fft.ipynb  |  62 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 examples/fft.lua    |  17 ++++++++++++++
 examples/opera.flac | Bin 0 -> 895467 bytes
 4 files changed, 80 insertions(+), 2 deletions(-)
 create mode 100644 examples/opera.flac

diff --git a/COPYING b/COPYING
index 1fb6c6b..a119d7c 100644
--- a/COPYING
+++ b/COPYING
@@ -1,6 +1,9 @@
 examples/haiku.flac is licensed under CC BY-NC 4.0 Deed and based on
 https://freesound.org/people/klankbeeld/sounds/237856/
 
+examples/opera.flac is licensed under CC BY-NC 4.0 Deed and based on
+https://freesound.org/people/Ramston/sounds/262274/
+
 All the other source files are licensed under the GPLv3:
 
                     GNU GENERAL PUBLIC LICENSE
diff --git a/examples/fft.ipynb b/examples/fft.ipynb
index d38cda3..0ac3e96 100644
--- a/examples/fft.ipynb
+++ b/examples/fft.ipynb
@@ -2144,10 +2144,68 @@
     "end):IFFT(1024):play()"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "178a3734-815b-44fa-bd1f-ecc4b45d567b",
+   "metadata": {},
+   "source": [
+    "Pitch Tracking. This however is [not so easy to get right](http://blog.bjornroche.com/2012/07/frequency-detection-using-fft-aka-pitch.html)."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "id": "eb6f6047-c6f0-4155-9029-2f643cdc9c04",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "WARNING: Buffer underrun detected\n"
+     ]
+    }
+   ],
+   "source": [
+    "opera = SndfileStream(\"examples/opera.flac\")\n",
+    "opera:play()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "id": "a11139f5-bb3e-4dc1-aa04-eede12abe0d3",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "WARNING: Buffer underrun detected\n",
+      "WARNING: Buffer underrun detected\n"
+     ]
+    }
+   ],
+   "source": [
+    "opera:LPF(10000):FFT(1024, Hanning):map(function(spectrum)\n",
+    "\tlocal size = (#spectrum-1)*2\n",
+    "\tlocal peak_i, peak_val\n",
+    "\tfor i = 1, #spectrum do\n",
+    "\t\t-- We don't have to take the square root to find the peak\n",
+    "\t\tlocal val = spectrum[i].re^2 + spectrum[i].im^2\n",
+    "\t\tif not peak_val or val > peak_val then\n",
+    "\t\t\tpeak_i, peak_val = i, val\n",
+    "\t\tend\n",
+    "\tend\n",
+    "\t-- Return peak as frequency\n",
+    "\treturn tostream((peak_i-1)*samplerate/size):sub(1, size)\n",
+    "end):ravel():div(10000):crush(7):mul(10000):SqrOsc():crush():play()"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "fc38550e-835c-47a8-a792-7164109dafee",
+   "id": "7f43540f-2b92-4e7f-ac8d-160223518a08",
    "metadata": {},
    "outputs": [],
    "source": []
@@ -2163,7 +2221,7 @@
    "file_extension": ".lua",
    "mimetype": "text/x-lua",
    "name": "lua",
-   "version": "5.1"
+   "version": "n/a"
   }
  },
  "nbformat": 4,
diff --git a/examples/fft.lua b/examples/fft.lua
index 5930f46..1f8687d 100644
--- a/examples/fft.lua
+++ b/examples/fft.lua
@@ -36,3 +36,20 @@ noisy:FFT(1024, Hamming):map(function(spectrum)
 	end
 	return spectrum
 end):IFFT(1024):play()
+
+-- Pitch Tracking
+-- See also: http://blog.bjornroche.com/2012/07/frequency-detection-using-fft-aka-pitch.html
+opera = SndfileStream("examples/opera.flac")
+opera:LPF(10000):FFT(1024, Hanning):map(function(spectrum)
+	local size = (#spectrum-1)*2
+	local peak_i, peak_val
+	for i = 1, #spectrum do
+		-- We don't have to take the square root to find the peak
+		local val = spectrum[i].re^2 + spectrum[i].im^2
+		if not peak_val or val > peak_val then
+			peak_i, peak_val = i, val
+		end
+	end
+	-- Return peak as frequency
+	return tostream((peak_i-1)*samplerate/size):sub(1, size)
+end):ravel():div(10000):crush(7):mul(10000):SqrOsc():crush():play()
diff --git a/examples/opera.flac b/examples/opera.flac
new file mode 100644
index 0000000..e4bce4c
Binary files /dev/null and b/examples/opera.flac differ
-- 
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