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