ee2135-midterm/Midterm.md

Fun with Signals

Input / Output

Input signal: "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.

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.

We generated this 60Hz tone 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:

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.