Interpolation is somewhat of an intimidating word. So too is upsampling.
But, at the end of the day, both are just fancy terms for a way of making a digital signal feel smoother and more lifelike by filling in the gaps between the sampled points—a kind of guessing game.
When we take an analog sound and convert it to digital, we’re sampling points along that smooth wave, like taking snapshots along a winding road. But these snapshots alone don’t capture the full curve of the road; they only hint at it. Interpolation steps in to “guess” what’s in between those points, filling in the gaps so the result sounds more like the original, continuous wave.
To help paint a picture of what this means, let’s think about what would happen without interpolation. Imagine our DAC is outputting a signal in steps, jumping from 0.1 volts to 0.15 volts, then to 0.2 volts in each consecutive sample (unlikely but for the purpose of this discussion I am exagerating the size of the steps).
This “stair-stepped” output gives us the basic shape of the sound wave but misses the natural flow we’d get in real life. With interpolation, however, we gain extra points in between. Instead of leaping directly from 0.1 to 0.15 volts, our DAC might now output 0.1, 0.11, 0.13, and then 0.15 volts. By adding these smoother transitions, interpolation gives us a more continuous curve.
Technically, upsampling through interpolation doesn't add "new" information but, in a way, it kind of does, though it's all a guessing game.
