notes

20210219114633-digital_audio_processing.org

@@ -1,4 +1,7 @@
 #+title: Digital Audio Processing
+
+Analysis and manipulation of [[file:20210219234904-pcm_audio.org][PCM Audio]].
+
 * Sine Wave
 #+name: sine-wave
 #+begin_src python :exports code :results silent
@@ -19,9 +22,47 @@
       <<sine-wave>>
 
   plt.plot(wave()[:100])
+  plt.title("Sine Wave")
   plt.savefig("dap-sinewave.png")
   return "dap-sinewave.png"
 #+end_src
 
 #+RESULTS:
 [[file:dap-sinewave.png]]
+* Getting volume with audioop
+The power of an audio signal is computed using the *root mean square* of the
+fragment:
+
+\begin{equation}
+{\displaystyle x_{\text{RMS}}={\sqrt {{\frac {1}{n}}\left(x_{1}^{2}+x_{2}^{2}+\cdots +x_{n}^{2}\right)}}.}
+\end{equation}
+
+#+begin_src python :noweb yes :results file
+  import audioop
+  import struct
+
+  from matplotlib import pyplot as plt
+
+
+  def sinewave():
+      <<sine-wave>>
+
+  wave = sinewave()
+  chunksize = 441
+  chunks = [wave[i : i + chunksize] for i in range(0, len(wave), chunksize)]
+  raw_chunks = [struct.pack(f"{chunksize}h", *map(int, chunk)) for chunk in chunks]
+
+  plt.plot(
+      [audioop.max(c, 2) for c in raw_chunks],
+      label="peak")
+  plt.plot(
+      [audioop.rms(c, 2) for c in raw_chunks],
+      label="rms")
+  plt.legend()
+  plt.title("Volume")
+  plt.savefig("dap-volume.png")
+  return "dap-volume.png"
+#+end_src
+
+#+RESULTS:
+[[file:dap-volume.png]]

20210219234904-pcm_audio.org

@@ -0,0 +1,17 @@
+#+title: PCM Audio
+
+Puse-code modulation digitally represents analog audio signals as a stream of
+measurements of the amplitude of the signal sampled at uniform intervals (the
+sampling rate) and quanitized to the nearest value within the range of available
+steps (limited by the bit depth). Multiple audio channels are commonly
+interleaved.
+
+* Sampling Rate
+Digital audio is typically sampled at 48 kHz (DVD video) or 44.1 kHz (CD audio).
+* Bit Depth
+| Bit Depth | Minimum Value | Maximum Value |
+|-----------+---------------+---------------|
+|         8 |          -128 |           127 |
+|        16 |        -32768 |         32767 |
+|        24 |      -8388608 |       8388607 |
+