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Comment by icy_deadposts | Hacker News Reader

I get that you can monitor phase shift in a signal, if you have knowledge of what the reference is.

If I expect a 1kHz sine wave, and you transmit a signal that sounds imperceptibly no different than a pure 1khz sine wave (to a human ear), you could be encoding data in a signal that actually varies over time between 998hz and 1002 hz which could be extracted with DSP.

But, how would I extract a phase shift from an arbitrary novel music, for example [0], where I don't know what the expected reference is?

[0]https://www.youtube.com/watch?v=yrJYo9d12o8

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Music tones are based on the tuning standard 440 hz and factors from there, that is your reference point. The next tone above and below is so far away that you can make slight alterations and detect how far away from the standard it is by just measuring a single tone.

I believe (but could be wrong) that's only true for western music, tuned to 440hz as an "A" with 12 steps between octaves. Eastern music for example, with stringed instruments without frets, like Sitar for example, sounds very different and I believe its because they don't follow that practice. Highlighting that was my intent between linking a particular piece of music thats well outside the mainstream top 40. (alternatively, consider my tone-deaf self in middle school trying to tune my euphonium by adjusting the slide randomly and pretending i could hear a difference)

If you generate the music yourself you can tune it any way you like. Using 440 hz as the basis lets you still do any kind of music, people wouldn't notice the difference since you mostly hear how it changes and not absolute pitch.

> Sitar for example, sounds very different and I believe its because they don't follow that practice

They still follow the same practice, maybe they don't center it on 440hz but the concept of tones and how far the steps between them are is the same all over the world, they just use different scales.

Off the top of my head, I'd likely try to embed a reference tone. If I couldn't do that imperceptibly I'd then look at what a discontinuous but small phase shift would look like. That would had harmonics and again, it would be a question of how perceptible were the harmonics.

I'm honestly not familiar with audio compression. But if its very low amplitude reference tone, isn't that the sort of thing that would get removed during compression? and if its large amplitude wouldn't it become a matter of "AI music always has a [low|high pitch] hum in the background?

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