# 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.