For example, would it look like waveforms printed onto the tape? Or are they fast magnetic pulses of some kind?
In: Engineering
A bit like these, but in a continuous line instead of a series of circles…
https://pbs.twimg.com/media/Dxx4D3JW0AAHMiL.png
https://habrastorage.org/getpro/habr/upload_files/77a/290/da0/77a290da083b113759e70c6976abc33e.png
You can actually get a small device that lets you see the magnetic information stored on a tape.
[It’s made of vertical lines, looks a bit like a bar code](https://youtu.be/aZOxn8ggX8w)
Fast magnetic pulses, basically.
Blank audio tape has metallic particles oriented in random directions. When you record a signal (waveform) onto the tape, the recording head makes many of the particles uniformly point forward or backward, parallel to the direction of the tape’s travel. Roughly speaking, a rising signal orients the particles in one direction, and a falling signal orients them in the opposite direction. The stronger the signal, the more particles are affected.
I like to think of it as being *similar* to [one-bit delta-sigma modulation](https://e2e.ti.com/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/1108.Figure-2.png).
[Techmoan has a demonstration of an interesting old gadget that helps you “see” this magnetic pattern on the tape.](https://www.youtube.com/watch?v=aZOxn8ggX8w) It looks a lot like a 2-D barcode. That device is not a high-precision instrument, though, and the boundaries of the regions of differing polarity are probably not as distinct as what you see.
A similar device ([cmos-magview](https://matesy.de/en/applications/testing-audio-video-tapes)) was used for an image in [the Wikipedia article on cassette tape](https://en.wikipedia.org/wiki/Cassette_tape#Features).
Here are higher-res forensic images of the patterns on a microcassette tape ([click](https://iasforensics.com/09JAN10_OR_1.jpg)) and a VHS tape ([click](https://iasforensics.com/VHS1.jpg)).
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