commit 9d254cf8c3b1f7bace434057606994145d2631a9 Author: Martin Kennedy Date: Sun Mar 30 13:40:55 2025 -0400 feat: Stub in a first-run commit diff --git a/Midterm.md b/Midterm.md new file mode 100644 index 0000000..1bfca57 --- /dev/null +++ b/Midterm.md @@ -0,0 +1,60 @@ +# 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? + +
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.
+ + - Q: Why 60Hz? + +
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.
+ + - 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. diff --git a/files/60hz.aac b/files/60hz.aac new file mode 100644 index 0000000..580b1db Binary files /dev/null and b/files/60hz.aac differ diff --git a/images/Typical_TRS_Jack.jpg b/images/Typical_TRS_Jack.jpg new file mode 100644 index 0000000..dd800b8 Binary files /dev/null and b/images/Typical_TRS_Jack.jpg differ diff --git a/images/audacity_60_hz_tone.png b/images/audacity_60_hz_tone.png new file mode 100644 index 0000000..45a0e94 Binary files /dev/null and b/images/audacity_60_hz_tone.png differ diff --git a/images/desktop_mpv_100.jpg b/images/desktop_mpv_100.jpg new file mode 100644 index 0000000..c3bd891 Binary files /dev/null and b/images/desktop_mpv_100.jpg differ diff --git a/images/desktop_mpv_130.jpg b/images/desktop_mpv_130.jpg new file mode 100644 index 0000000..01a3ae4 Binary files /dev/null and b/images/desktop_mpv_130.jpg differ diff --git a/images/laptop_mpv_100.jpg b/images/laptop_mpv_100.jpg new file mode 100644 index 0000000..42ccf39 Binary files /dev/null and b/images/laptop_mpv_100.jpg differ diff --git a/images/laptop_mpv_130.jpg b/images/laptop_mpv_130.jpg new file mode 100644 index 0000000..9e0e5a9 Binary files /dev/null and b/images/laptop_mpv_130.jpg differ