feat: Stub in a first-run commit

Midterm.md

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+# Fun with Signals
+
+## Input / Output
+
+### Input signal: ["Line level"](https://en.wikipedia.org/wiki/Line_level)
+
+First, let’s get a view of the signal we are accepting on the circuit’s input.
+
+#### Magnitude
+
+With some variation, it's common knowledge (by means of human
+experience) that most devices which accept a set of headphones through
+a 3.5mm "TRS" audio jack will output roughly the same maximum volume
+through that set of headphones.
+
+| ![](./images/Typical_TRS_Jack.jpg) | 
+|:--:| 
+| *A typical, well-loved 3.5mm TRS jack besides a laptop line-out port.* |
+
+Don't believe me? Grab a 3.5mm cable meant to connect an output, like
+a phone, to a speaker, and use a multimeter to measure the unloaded
+RMS AC output voltage of a 60Hz tone. Be sure to use the same software
+on multiple devices to get a normalized comparison of different
+hardware.
+    
+| ![](./images/audacity_60_hz_tone.png) | 
+|:--:| 
+| *We generated [this 60Hz tone](./files/60hz.aac) using Audacity, under Generate > Tone > Sine, 60 Hz, Amplitude 1 (100%)* |
+
+  - Experiment pitfalls:
+  
+    - Q: Uhh ... which conductors am I measuring?
+    
+      <details> If you're unsure which conductors to measure between:
+      the sleeve is usually a common reference ground, but in doubt,
+      use your multimeter in continuity mode and see if you can find
+      contuinty to the computer case or, for example, the outer shield
+      of a USB connector.  </details>
+      
+    - Q: Why 60Hz?
+    
+      <details>Not all multimeters are equal, but some things don't
+      change: any portable multimeter equipped to probe the 60Hz,
+      120VAC signal provided by your household electrical socket is
+      also suited to calculate RMS AC voltages for 60Hz sinusoidal
+      signals. The Asian-market Fluke 12E+ used here was able to
+      measure the highest frequencies our sound cards could produce --
+      around 20 kHz -- but a cheaper Centech multimeter struggled past
+      a perfectly audible 1 kHz tone.</details>
+
+  - We tried this experiment on a Thinkpad and a Dell desktop, using
+    the `mpv` media player on two Fedora Workstation systems:
+
+| ![](./images/laptop_mpv_100.jpg) | ![](./images/laptop_mpv_130.jpg) | ![](./images/desktop_mpv_100.jpg) | ![](./images/desktop_mpv_130.jpg) | 
+|:--:|:--:|:--:|:--:|
+| *Laptop, 100%* | *Laptop, 130%* | *Desktop, 100%* | *Desktop, 130%* |
+
+The signal was identical at 100% volume on each system ... but when we
+asked `mpv` to overamplify the volume to "130%", the laptop and
+desktop began to deviate.